Compression ratio, DCR and pump gas (merged threads)

Ok, another stupid question, I know, I know I am full of them :D But anywho....What is the max compression ratio you can use and still run pump gas? Lets say 92 octane. BTW is it the dynamic compression ratio that you have to be concerned with? If so what is the max Dynamic compression ratio you can run and still use pump gas?

Typically 8:1 dynamic compression is acceptable with iron heads and 8.5:1 with aluminum heads.

Kev

David Vizard recommends a DCR of 7.5:1 - 8.5:1 for pump gas.

Correct me if I may be wrong but I've also heard it's also in the cam profile that determines what octane you'll need.

The cam is factored into dynamic compression.

Kev

David Vizard recommends a DCR of 7.5:1 - 8.5:1 for pump gas.


When they say pump gas which octane gas are they talking about? :confused:

Dawg I posted a link the other day in the weblink library that has a calculator that helps you determine both your dynamic and static compression ratios.

I thought we had a post about this somewhere. Need to find it and put it in the FAQ before it gets deleted.

I'm reluctant to draw a line to the limits of compression vs octane. A long time ago 6:1 was considered High Compression.

I was talking the other day to Scott at King motorsports and he claims to have street Honda's running 12:1 :eek: with 93 octane. These have VTEC so the DCR is variable as well.

There is much more to octane tolerance than DCR, which btw most of those online free calculators don't figure it correctly or even the same way. If the calculation isn't correct then comparing DCR numbers will be misleading.

More pressure = more power but also puts you closer to detonation. The closer you stand to the brink the better your setup has to be.

Minimum squish is essential. Spark curve must be custom tailored. Air fuel ratio must be smack on and even on the rich side. Air inlet temperature must be minimized or at least controlled. Piston speed rate of change near TDC can make or break a borderline detonator. Atmospheric pressure changes will effect an engines octane tolerance.

The last Engine Masters challenge it was noted that a lot of the competitors who pushed it to the edge were pinging in the dyno cells.

I am not sure if the calculator is a good one or not. I found it on the Keith Black website.

http://kb-silvolite.com/calc.php?action=comp

Paul are there any books on engine building that you would possibly recomend?

Just a quick question, can you run pump gas on 10.5 to 11:1 Compression with Brodix Aluminum Heads?

well yes and no. i have 11.1 with some vic jr heads. depends on how much timing you go with. i have had problems at 32* but i add ocatne in a bottle and can squeeze a little mo timing.
i switched over to the aluminum heads to run pump gas i dont think it was worth the effort.

10.5 compression on pump gas is easily done with the right cam, quench, combustion chamber, etc.

Do a net search for "dynamic compression ratios". Your static compression ratio is only part of the equation for your engines ability to run on lower octain fuels.

absolutly....i run 11.1:1 compression on 89 octane. And with the Race Rite heads. I get my compression from 5cc valve reliefs in my SRP'S, 60 cc chambers {cnc option from brodix} a 4.040 bore and a .040 gasket, now this is actually a .033 quench because my machinist ran my 0 deck a little past 0 and my pistons actually stick out about .007. I also dont run over 165 degree temps. I run a stock car Howe aluminum radiator, aluminum intake, water pump, and heads. Now for the kicker i have WAYYYYYY too much cam at 268/278@.050. Im switching to somthing in the 250's @.050 and im still confident i can run 91 octane dependably.
Eric68 runs 11.3:1 on pump gas with a comp Street roller that specs at somthing like 248/254@ .050. It can be done fairly easliy, but not with a small cam, the correct cam choice is CRITICAL!!

I am running a comp cams xtreme energy.507/.510 lift with 1.6 rockers which bumps it up to around .540 lift

Lift dosent really matter in this situation. What is your advertised duration?

chevytalk.com is down until monday morning, but if you go to their performance section there is an awesome link called "understanding dynamic compression" stickied there.

due to the site being down i can also not post the link to pat kelleys awesome compression ratio calculator which a very good description on compression ratios as well as a sweet downloadable calc.

Yeah, lot's of duration will bleed of cylinder pressure enabling a high static compression motor to run on lower octane.

Yeah, lot's of duration will bleed of cylinder pressure enabling a high static compression motor to run on lower octane.

Duration doesn't "bleed off" cylinder pressure. The cylinder doesn't begin to build pressure until the intake valve closes.

Duration doesn't "bleed off" cylinder pressure. The cylinder doesn't begin to build pressure until the intake valve closes.
it has to do with the overlap,something long duration cams usually have a lot of :)

http://www.chevytalk.org/threads/showflat.php/Cat/0/Number/1137028/an/0/page/0#1137028

READ THIS!!!!!!!! and all the links provided!

this link is for the calc and description, also in the post, but figured i would link it here for ease of use :cool:

http://cochise.uia.net/pkelley2/DynamicCR.html

I just used Pat Kelleys DCR calculator and it has the Durango Deuce DCR at 8.7. We have no problems at all running on 91 octane California fuel.

EA3.0 puts it at about 7.71 with 192# cranking compression.

The RSR Advanced Dynamic Compression Ratio Calculator puts the DCR on the same engine at 9.57:1 with 200.06# of cranking compression.

http://www.rbracing-rsr.com/comprAdvHD.htm

it has to do with the overlap,something long duration cams usually have a lot of :)

Wrong. It has nothing to do with overlap. Overlap occurs after the power stroke and before the intake stroke. It's 360 degrees away from compression.

Yeah, lot's of duration will bleed of cylinder pressure enabling a high static compression motor to run on lower octane.
Wrong. DCR is effected by the intake valve closing. Pressure doesn't begin to build until the valve closes. There is no pressure "bleeding off".

http://www.chevytalk.org/threads/showflat.php/Cat/0/Number/1137028/an/0/page/0#1137028

READ THIS!!!!!!!! and all the links provided!

this link is for the calc and description, also in the post, but figured i would link it here for ease of use :cool:

http://cochise.uia.net/pkelley2/DynamicCR.html

The internet has the uncanny ability to disseminate misleading information at the speed of light.
Like Mike has pointed out those online calculators don't figure DCR correctly. If you compare the output of the equations to reality you'll find significant discrepancies..... something that the Chevytalk "experts" haven't figured out. The reason is they don't use the true intake closing and when figuring cranking compression they don't take into account heating from compression.
A stopped clock is right twice a day. Kelley's program is never right.

I've seen the effect of advancing a cam (intake valve closing earlier) and how it typically raises the cranking compression and the DCR, but doesn't the intake opening point also affect the DCR?

If you start the intake opening point sooner isn't there more time/duration for the gas/air mixture to fill the cylinder, causing the DCR to be higher? I realize this is over simplifying things because of intake/exhaust overlap, rpm and who knows how many other factors to take into consideration, but I would think the intake opening point is also a factor in the actual DCR the engine sees.

That's a good question. The short answer is no.

Here's why:
At cranking speed the cylinder is filled by atmospheric pressure trying to fill the vacuum caused by the decending piston. Opening the valve earlier increases overlap which at low rpm draws exhaust gas into the cylinder. This causes the choppy idle and lowers vacuum which hurts intake velocity...not help it. Below about 3,000 rpm the VE is very poor on typical "Big Cam" engines.

Earlier Intake opening and increased overlap helps once the rpm increases to the point that air inertia can fill the cylinder more effectively. The ram effect can overfill the cylinders (over 100%VE) and is not considered in DCR. In some DCR situations if the VE is over 100% detonation is most likely at the point of peak torque WOT.

Just as a supercharged engine requires a lower static compression ratio to prevent detonation under boost, some normally aspirated engines have to consider what happens when the ram effect "boost" comes in.

So if someone says the acceptable DCR for a certain octane is "X.X" it may only be true for low VE engines. That same DCR would not be acceptable for over 100% VE engines. This is why I prefer monitoring cranking compression as opposed to DCR numbers. If your cranking compression is too high then it's only going to get worse when the ram effect comes in. If your DCR is incorrectly calculated and/or you use the wrong rule of thumb you're asking for trouble.

Now of course someone will ask "what cranking compression is ok for XX octane?"
I hate defining what cranking compression is ok for any octane because of all the variables involved. The combustion chamber shape and material; the air inlet temp, spark advance, rpm and engine load all are factors.

Altitude changes and ambient air pressure can cause an engine to detonate one day and run great on another.

A 10.5:1 350 towing a trailer up a hill is in a much different situation then in a V-8 Vega doing the 1/4 mile.

Generally, a SBC with bathtub heads and 150-160 psi will run on 87 octane. With optimum squish and the right spark advance you can run higher. Anything over 200 psi is scary even on 93.


The closer you get to the detonation edge the more important you have ALL your facts correct. My advice is to choose wisely and don't get greedy. Otherwise it really hurts when your "rule of thumb" gets smashed.

As soon as everyone is done posting, I'm going to merge the previous posts on this subject and put this into FAQ section.

I've got some additional comments based on stuff I've learned from on this Honda project I'm working on. The big overlap cams that gives the choppy idle and helps the ram effect are very inefficient at low rpm. Unless you have a converter and gears to get the engine into the power band, all you have is a 3 legged dog that's all bark and no bite. If your engine can only turn to 6,000 rpm you have a very narrow powerband. Only useful for short 1/4 mile runs.

I've been very impressed with the specific output of the Honda engine. 125hp per liter normally aspirated, idles smooth and runs on pump gas is fantastic accomplishment.

If a 350 made that kind of power you'd have 712 HP!

Already the smooth idling 500 hp Z06 is redefining what's fast and "cool".
I think you'll see the day soon when a production GM Corvette V-8 has a VTEC like system. Everyone will have to adjust their old school thinking when there's a 5 liter Vette that makes 625hp and still gets 27 mpg!

Hi Paul,

Just a couple questions.
Whats your opinion of swept volume verses compression gage readings?

Ive seen stock (and I mean untouched) 305s with over 200 psi cranking pressure, that had no detonation problems at all.


And Ive seen 455s with around 155 psi cranking and serious detonation problems.

What are you getting specifically out of the Honda, at what rpm?
If its improper of me to ask, I understand if you dont want to provide details.

Thanks

Jeff

That's a good question. The short answer is no.

Here's why:
At cranking speed the cylinder is filled by atmospheric pressure trying to fill the vacuum caused by the decending piston. Opening the valve earlier increases overlap which at low rpm draws exhaust gas into the cylinder. This causes the choppy idle and lowers vacuum which hurts intake velocity...not help it. Below about 3,000 rpm the VE is very poor on typical "Big Cam" engines.

Earlier Intake opening and increased overlap helps once the rpm increases to the point that air inertia can fill the cylinder more effectively. The ram effect can overfill the cylinders (over 100%VE) and is not considered in DCR. In some DCR situations if the VE is over 100% detonation is most likely at the point of peak torque WOT.



Paul, I'm closer to understanding but not quite there.
The short answer says no but the detailed answer seems to say yes, the intake opening point does affect the DCR. If I understand you correctly, in this example with a typical "Big Cam" at lower rpm's the earlier intake opening would lower the DCR and at higher rpm's the earlier intake opening would raise the DCR. Correct?

For example if you had a race car that ran fine on a certain octane fuel and you changed only the intake opening point of the cam timing, maybe the cranking compression would be about the same but now when you race the car you discover signs of detonation at high rpm because of the ram effect of the air inertia is increasing the cylinder filling/DCR from the previous cam timing?

Jeff:
My list of variables was by no means complete. Bore and stroke, rod length Quench heads or non quench, etc. Every engine design is different.

I remember an engine builder here that milled the heads on a 460 going into our car hauler ("for a little better torque"). That thing detonated so bad the boss personally wanted to know who built the engine. The compression wasn't that much higher than stock but the 460 was obviously more sensitive to compression than a SBC.

That's why it's impossible to make blanket statements that a certain DCR is ok for a certain octane for every engine. You can make general statements and observe trends and results but you can't flatly state the highest SCR or even DCR an engine won't detonate in all circumstances. I thought I made that clear in my comments about "rules of thumb".


I worked in the Chevy dealership when 305's were new. I worked on many 305 engines but never saw 200 psi. I maybe saw 165 psi maybe 170 if it was carboned up. If you got 200 psi then I'm sure there is an explaination.
If you are cranking until you get the highest reading then you aren't doing it right. It's 4 puffs then stop. I explained why in a previous post.
As a side note on cranking compression, if the engine is in poor condition you'll get misleading readings also. It's very important to understand what influences air pressure readings.

I might add that stock 305's would detonate like crazy if the EGR was deactivated unless you ran 93 octane. The fuel economy savings of blocking off the EGR was offset by the price of fuel. I know this because I tried everything I could think of to increase fuel economy in my 305 during the 78-79 oil embargo.

:chev: Paul, I'm closer to understanding but not quite there.
The short answer says no but the detailed answer seems to say yes, the intake opening point does affect the DCR. If I understand you correctly, in this example with a typical "Big Cam" at lower rpm's the earlier intake opening would lower the DCR and at higher rpm's the earlier intake opening would raise the DCR. Correct?

For example if you had a race car that ran fine on a certain octane fuel and you changed only the intake opening point of the cam timing, maybe the cranking compression would be about the same but now when you race the car you discover signs of detonation at high rpm because of the ram effect of the air inertia is increasing the cylinder filling/DCR from the previous cam timing?

Bob,

Technically the DCR does not change since it only figures the swept volume when the intake closes not opens. Therfore the DCR value is NOT effected by intake opening.

MEP is Mean Effective Pressure and that is what changes dynamically from VE changes. There's a ton of engineering information on the subject of MEP.Downloadable PDF file 134kb (http://www.seas.upenn.edu/~meam100/handouts/Thermo.pdf)

You are correct that a cam change could have the same DCR but detonate due to increased VE (cylinder filling) increasing MEP past octane tolerance.

Calculated cranking compression is determined by DCR multiplied by atmospheric pressure and adjusted for heating caused by compression.
1st law of thermodynamics (http://physics.bu.edu/~duffy/py105/Firstlaw.html)

Observed cranking compression is influenced by starter speed, carbon build up, ring and valve seal, timing chain slop and probably a number of other factors.

If my observed deviates from my predicted pressure I'll want to know why.

That's why I test for pressure on the run in stand and keep track of leak down percentages. To be accurate you must keep the # of cranks the same and you must know % leakage.

Let me also add that OEM's use a pressure transducer and a crank angle encoder to measure and plot cylinder pressure vs cylinder volume. This is the best way to determine what's going on but it's very expensive.
Internal combustion engine PV graph animation (http://230nsc1.phy-astr.gsu.edu/hbase/thermo/otto.html#c1)
I have heard of people using a compression gauge on one cylinder of a running engine to observe IMEP but I'm not sure if this is a valid test since the cylinder is not firing and you can't measure vacuum.

For the average joe in his garage, the cranking pressure test is a low cost tool that if done correctly can give you useful information you otherwise might not have. It's better than scratching your head and guessing, right?

Hi paul,

Boy you are putting in a lot of time on this one!

I am aware that you were not covering every factor involved with compression and detonation. The reason that I brought the swept volume up,is that in my experience that makes more difference than just about anything when looking at a static compression ratio for potential detonation risks.

I have seen many small engines that can get by with what would appear to be a risky compression ratio while larger motors experience problems.

As far as the 305 compression (cranking) im pretty framiliar with compression tests. Maybe its the atmosphere around here but I have seen several that would pump 200+, and I dont think they were severly carboned. I always attributed this to valve timming. I have put 305 cams in 350s just for very low rpm torque and found similar pressures.

The 305s go have egr and knock sensors for that mater, but my point was that I have seen larger motors (with the same devices) that were much more sensitive.

I get real confused on the max compression for the street deal anyway.
I have several 11 to 1 cars I drive on the street with pump gas (I had a 375 396 chevelle that I put about 40,000 miles on in the mid eighties), but I NEVER run them hard (with pump gas).

My customers say one thing and do another. When your buddy is with you and you REALLY want to kick it, some reasoning goes away.

Bad gas is common, weather changes etc, I just dont see it real world terms.


Thanks

Jeff

Thanks Paul,
Your time and effort are greatly appreciated!
Any time you want to give away any of your automotive secrets feel free to do so, we promise we won't tell.
:shh:

I have a svt focus it has a lil more bang then the regular focus but it has 10:8:1 compression and it has to use 91 octaine. Is there something alot different with v8's cuz it sounds like the compression ratios you guys are talking about would be something ide here a 4 banger need for some forced induction. Did i miss something or am I just an iddiot when it comes to v8's.

I have a svt focus it has a lil more bang then the regular focus but it has 10:8:1 compression and it has to use 91 octaine. Is there something alot different with v8's cuz it sounds like the compression ratios you guys are talking about would be something ide here a 4 banger need for some forced induction. Did i miss something or am I just an iddiot when it comes to v8's.


I am sure this wont be the best answer to this but I suspect part of the reason for this is most of the cars you were talking about (4 banger cars) are largely computer controlled (timming, fuel mixture, etc) which can make minute changes on the fly to eliminte detination almost before it even happens.

As has been pointed out by some of our more knowledgable members(which I am NOT one of), in many posts, combustion chamber shape, amount of quench, camshaft design (specs, lobe shape, etc), piston design, timming, air fuel mixture, atmosphic pressure, ambeiant air temp and a whole host of other issues come into play and which effect the compression ratio, whether we are talking dynamic or static, you are able to run on a specific car and not have problems. I am sure I have not done a good job explaining this largely cause I am tying to understand this very issue myself. :)

Really i was under the impression that most v8's had 8:1 to 9:1 compression. With that low compression is the cam open for a long period of time cuz it seems like if the compression was that low and all that air fitting in there and the cam not open for a long while you would run lean and burn the tops of the piston? Remember I only know bout 4's so if im wrong let me know.

Really i was under the impression that most v8's had 8:1 to 9:1 compression.


Well it will run with just about any compression you want to run the real limiting factor is the quality of the gas. The L48 in my Nova came from the factory (1969) with 10.25 to 1 static compression ratio. The higher the compression the more likely you will have to run preimium gas to avoid detenation. And after a certian point you will need to run racing gas. Very expensive. Also your 10.8 to one compression ratio is that static or dynamic. I suspect it is the static compression ratio and not the dynamic ratio. I am sure, but could be wrong, that your dynamic compression ratio is much lower than the 10.8 you quoted.


BTW there are lots of V8 drag cars running well over 12-1 compression just not with 91 octain fuel

My BMW 325i has these specs. I wonder what the DCR is :confused: :

Liter/type: 2.5/inline 6
Bore/stroke: 3.31/2.95 inch
Nominal output/rpm: 184/6000 hp
Max. torque/rpm: 175/3500 lb-ft
Compression ratio: 10.5 :1
Fuel grade: 91 or better

Sorry but I really dont know the differences between the two (dynamic or static). I know about the higher the compression the higher the octaine. I just thawt it was weird to here 8:1 or even 6:1.

Sorry but I really dont know the differences between the two (dynamic or static). I know about the higher the compression the higher the octaine. I just thawt it was weird to here 8:1 or even 6:1.

Take a gander here:
http://www.rehermorrison.com/techTalk/08b.htm

I have a goodwrench 350 crate engine in my nova. It says it has 8:5:1 compression with 76cc heads. Now im wondering wut headgasket i should go with to raise my CR. I mean, which kind of gasket should i get, what thickness, what bore size? Im all clueless on this. I know i have a 4.000" bore now, but will a 4.060" bore head gasket work? ANy links or part numbers to head gaskets that will work with my motor and rasie my CR would be great. Thanks

These gaskets from Summit have a 4.100" bore and a .015" compressed thickness:
http://store.summitracing.com/partdetail.asp?part=FPP%2D1094&N=4294925237+4294908216+4294925236+4294891421+4294 908395+4294840140+4294889107+4294877281&autoview=sku

Those are the ones you want.Your head gasket size right now is .039.With the steel shim head gaskets you will up your compression some.You should definitley consider getting some larger(smaller) cc heads if you really want to raise your compression.Maybe something like 180/72cc or 180/64cc heads.bm

Thanks DriveWFO!

Batman, i planned on upgraded heads but not any time soon as there are other stuff on the car tht i need to attend to. Mostly bodywork. I just thought that i could do this is maybe a day or two and have it back together with more compression...Does anyone know what my compression should be? Thanks

I know GM says that engine has 8.5 CR but if you plug the numbers into a calculator, the actual comes in more like 7.90 to one. I believe a few of the car magazines confirmed this.

My "as is" assumptions
Bore: 4.000"
Stroke: 3.48"
Piston dish: 14cc
depth in the hole: .028"
compressed gasket thickness: .039"
Gasket Hole diameter: 4.03"
Squish: .067"
compression Ratio: 7.9 to 1

By modifying only the gasket, the following changes:
compressed gasket thickness: .015"
gasket hole diameeter: 4.100"
Squish: .043"
Compression ratio: 8.24 to 1

Paul Wright suggested in another post that 1 point in compression was worth about 25-30 HP, that would theoretically make this swap worth about 10 HP.
I'm not sure I have the piston dish size correct, or the "in-the-hole" number, so changing a variable changes the calculation.

In all honesty, this sounds like a lot of work for 10 HP. If you change the heads to ones with a smaller combustion chamber at the same time , like vortecs or iron eagles, you'd get the improvement of the compression ratio (to 9.24 to one) plus the better breathing. 50 HP would not be out of the question, and it would be the same amount of work. What happens if you postpone the change by 2 or three months while you get a line on a better set of heads?

In all honesty, this sounds like a lot of work for 10 HP You hit the nail on the head there. Save up for a head and / or cam up-grade and continue on with the body and paint work, is my thought. ;)

You use different thickness of head gasket to control the quench distance. This is the distance between the top of the piston at TDC (Top Dead Center) of the stroke and the cylinder head.

"The quench distance (piston/head clearance) should always be set between .035" and .045" with the lower limit giving the best performance and detonation resistance." You can find this quote or variations of it on several web sites and in a some engine building books.

The pistons on a small block Chevy normally sit .025" down in the cylinder hole at TDC. So you would want around .015" of head gasket to give you a quench distance of .040" (.025 + .015). If the block has been decked then the pistons would not be as far down in the hole and you would use a thicker head gasket.

If you want to change the compression ratio, the correct method would be to change the cylinder head volume or go to a different shaped piston.

However, never take any of these measurements for granted and always measure them.

In general the easiest and cheapest performance mods you can do are a better intake, 4BBL carb if you don't have one, headers and free flowing exhaust system.

Here's some actual numbers using Kev's engine:

4" bore 3.48" stroke 5.7" rod length.

Piston volume -10.2cc (4 eyebrows dished stock piston)
Deck clearance .038"
Stock Gasket .039"
Head volume 76cc
Stock cam

SCR= 8.01:1 DCR =6.44:1 cranking compression =150psi
A stock '78 L-48 4 barrel engine is rated at 180hp so VE is .6 at 5,800 rpm.

Changing the head gasket to a .015" thin shim gasket:

SCR =8.39:1 DCR =6.72 cranking compression= 160 psi. With the same VE the increase in horsepower is 188.5 or a modest 8.5Hp gain

Swapping on 67cc heads and same .015" gaskets.

SCR = 9.13 DCR = 7.30 cranking compression = 180 psi, too high for regular gas but 25-30 hp possible on premium with correct fuel and timing. Not taking into account any VE increases due to improved head flow, a stock L-48 will increase to 205 HP just from the compression bump (25hp).

Swap cam for 110 LSA, 106 Intake CL, 224@ .050" duration.

SCR 9.13 DCR = 6.48 Cranking pressure 152 psi. Now the cranking pressure is back to 87 octane levels but due to VE gains from the cam the power increased by maybe 40-50+ hp over stock, assuming fuel mixture and timing are correct. This is with apples to apples port flow. A better flowing port will net additional gains.
With VE increased to a mild .8 the hp of the L-48 is 275Hp, a whopping 95 hp gain.

Removing the entire top end of an engine just to change gaskets in an attempt to gain noticeable power is too much work for the trouble. Remove the heads only when ready to install better ones. That's where you'll get noticeable power increases with a matching cam , intake and carb.

Hi guys,

Jeeze thats a lot of numbers. Im not sure what head gasket is on the crate motor now, but the 78 engine should have came stock with a steel shim head gasket, a lot thinner than .039.

Its real important if you change to the shim type gasket (or any gasket for that matter) on the later Gm 350s (early 90s up) to carefully check the sealing bead on the bore of the gasket where it gets close to the spark plug cooling passages.

These heads have the passage cast very large and at an angle that makes it worse if you mill the surface. I try and find the smallest bore 4.00 gasket for theses heads. I have had to weld some of these heads to prevent them from leaking compression into the cooling system (I tig weld the edge of the passage with a nickle rod and then mill the head) this is a big problem with these heads and various gaskets.

Im not sure about the crate motor castings, but I would sure check that closely. The larger the bore on the gasket the worse the problem.

Just my opinions and observations.

Good luck
Jeff

Good points Jeff! Some engines had thin shim gaskets from the factory. If you have one of these it will be a waste of time swap gaskets or if you use the thicker gaskets included in many gasket kits you'll actually lose compression and horsepower.

I agree with max turbo, Re doing the entire top end will be a much better solution. To do the head gasket it all basically has to come apart anyways. You can still use the dished pistons, some good head work, performance cam and thinner head gasket. All this can be done at a very reasonable cost, and the gains will be way more than 10 hp. Good posts on this site, its my favorite. Good luck with the project.

If the cam is big enough duration wise, then the ?DCR will be low enough to start the engine easily.

To test this, block vacuum advance, start engine, set to the desired initial, try to restart the engine. This is a temporary test to see if the engine will start with the high initial. If it will, then make a lockout plate that goes where the weights/springs go under the rotor and go racing.

If it won't, you will need to make/use some sort of defeat device or retard system for starting, just depends on the DCR.

Big cam? Aren't all Chevy cams the same size? :D

63II, good to hear.

If the duration is big enough, and the intake valve closes late enough, the DCR will be tolerable even with more static comp ratio. It all boils down to just how much compressible mixture is blown back up the inlet port to kill off compression resistance against the starter motor's torque ability to spin the engine over. Sounds like it is OK on your setup.

According to the cam card mine closes at 66 deg after bdc, so it bleeds off a big chunk of swept volume. Guess i'll know for sure next summer, but it runs great right now. Hoping to get the car consistent enough to run (i mean win) a bracket event lol

Where that intake valve closes starts the real world DCR, or dynamic compression ratio, the actual volume that gets compressed, discounting the volume that gets back past the inlet valve and back up the port before the valve closes. You may have 10.50:1 static c/r, but at idle, the DCR may well be right down there at 8.00:1.

Ain't engines fun!

Where that intake valve closes starts the real world DCR, or dynamic compression ratio, the actual volume that gets compressed, discounting the volume that gets back past the inlet valve and back up the port before the valve closes. You may have 10.50:1 static c/r, but at idle, the DCR may well be right down there at 8.00:1.

Ain't engines fun!

I did a calc a while back and I think the DCR was 8.5 or 8.7, anyway it was close enough for pump gas! But right now all the top engine builders are calling DCR a myth, but I wont open that can of worms here lol

That is because they don't understand engines well at all, and they didn't think of it first.

Think of DCR being active, ever changing. Ever see the vacuum actually rise as the rpms are raised? That is because the DCR changes as the engine is slow revved, allowing for less reversion because of the time between intake events speeding up. Less time for the mixture to travel up the inlet port, higher DCR.

DCR will actually rise as the rpms do, until the SCR is met when the cam "hits". Ever feel the cam come on at say, the cam's design 3,000 rpms in an engine? This is the time the DCR matches the SCR, and reversion is at its lowest, capturing the most mixture it will catch.

To really understand DCR, a person has to know two stroke engines and their theory as well as how all that interreacts to four stroke cams, mixture (NOT just air) flow works, and most of these four stroke Gurus just don't get it, so they berate it and say it isn't relevant.

DCR is vitally relevent, but some just don't want you to question them about it. Too bad for them misleading others.

But right now all the top engine builders are calling DCR a myth, but I wont open that can of worms here lol

Sorry, you can't pass along a dumb *** internet rumor like that and not expect me to respond.

What "top" engine builders say DCR is a myth? You'd better have facts and references to back your statement up.

Intake valve closing has a definite impact on lower speed pumping ability and is described in engineering books going at least as far back as World War 2.

You have to agree that compression pressure will go up or down depending on the static ratio, right? Change the ratio and you will change the compression pressure. A 12:1 engine will squeeze a mixture to a higher pressure than an 8:1 engine would...IF all things are equal.

It's been scientifically observed that MEP (Mean Effective Pressure) is influenced by the Intake valve closing. Keep the static ratio fixed but vary the intake valve closing and the compression pressure will change which, in turn helps MEP.

Keep in mind that a ratio, even though it is expressed with numerals, is not a number. This is what goofs up many "experts" since a ratio means nothing without a reference. It's like you can't predict MPH solely from axle ratio. You have to know tire diameter and driveshaft rpm. A DCR value is only useful if you know volume and pressure. Even some of the DCR internet guru's don't do the calculations correctly or mistakenly explain how overlap "bleeds off" compression.

Builders that use big cams rely on the high rpm inertial ram effect to overcome the late closing so I can see how they could dismiss it as nonsense, however, this same kind of "common sense" once determined a round earth was nonsense too.

Among other things, DCR is useful to predict cranking pressure, which has importance to octane sensitivity. It's not a myth.

I have to agree with IgnitionMan's comment about 2 strokes.

It's late and my text books are at work. I may add/edit this tomorrow.

A DCR value is only useful if you know volume and pressure.

Paul, with my cam's valve timing event specs and compression test results, do I have the needed criteria for calculating my DCR?

Paul, with my cam's valve timing event specs and compression test results, do I have the needed criteria for calculating my DCR?
There is a link in the weblink library to the silvolite/Keith Black site where there is a Calculator to figure out SCR and DCR. I know Paul says a lot of the sites do it wrong and maybe they do too but I would assume a company that sells pistons would have a decent calculator on their site

Hope this helps ya out:)

EDIT: here is the link;
http://kb-silvolite.com/calc.php?action=comp

There is a link in the weblink library to the silvolite/Keith Black site where there is a Calculator to figure out SCR and DCR. I know Paul says a lot of the sites do it wrong and maybe they do too but I would assume a company that sells pistons would have a decent calculator on their site

Hope this helps ya out:)

EDIT: here is the link;
http://kb-silvolite.com/calc.php?action=comp

I believe I used that one before. I guess I'm curious as to which online DCR calculators (if any) are accurate?

I believe I used that one before. I guess I'm curious as to which online DCR calculators (if any) are accurate?


Ya me too...now I could be wrong but I would assume, ya I know there is that word again, that the sites put up by actual companies like Silvolite etc are likely to be more accurate then say a site like BillyBobs automotive calculator site...or SpeedFreaks drag racing site that has a few calculators tossed into the site...I dunno if my logic is correct but that is the theory behind it :)

Ya me too...now I could be wrong but I would assume, ya I know there is that word again, that the sites put up by actual companies like Silvolite etc are likely to be more accurate then say a site like BillyBobs automotive calculator site...or SpeedFreaks drag racing site that has a few calculators tossed into the site...I dunno if my logic is correct but that is the theory behind it :)

I'm really starting to dislike that word. "assume" Never realized how much I used it until recently. :D

Here's something I found. If it's bogus someone feel free to delete it.

http://cochise.uia.net/pkelley2/DynamicCR.html

The trouble with online calculators is most don't show how they arrive at the number. Enter in the same numbers into various programs and you'll get a different answer.

As near as I can figure they are wrong because they use the wrong value for IVC. (Intake Valve Close) True IVC for DCR purposes is not on the cam card. "Advertised Duration" is not accurate and varies by manufacturer and cam type. Inlet air flow is negligible at lifts under .050". That's why they went with the .050" standard to eliminate ramp variability.
...however.. a valve that is open .050" doesn't seal pressure. A valve that is open .010" doesn't seal, either. It has to be closed tight.

To get true IVC you have to get the degree wheel out or better yet use a cam profile checker (like Cam Doctor).


Remember DCR is not a number. It is not "the answer"... it's a variable. It has little value by itself.

I may split this discussion into a separate thread. There is more DCR info in Best of Tech

Sorry, you can't pass along a dumb *** internet rumor like that and not expect me to respond.

What "top" engine builders say DCR is a myth? You'd better have facts and references to back your statement up.

Intake valve closing has a definite impact on lower speed pumping ability and is described in engineering books going at least as far back as World War 2.

You have to agree that compression pressure will go up or down depending on the static ratio, right? Change the ratio and you will change the compression pressure. A 12:1 engine will squeeze a mixture to a higher pressure than an 8:1 engine would...IF all things are equal.

It's been scientifically observed that MEP (Mean Effective Pressure) is influenced by the Intake valve closing. Keep the static ratio fixed but vary the intake valve closing and the compression pressure will change which, in turn helps MEP.

Keep in mind that a ratio, even though it is expressed with numerals, is not a number. This is what goofs up many "experts" since a ratio means nothing without a reference. It's like you can't predict MPH solely from axle ratio. You have to know tire diameter and driveshaft rpm. A DCR value is only useful if you know volume and pressure. Even some of the DCR internet guru's don't do the calculations correctly or mistakenly explain how overlap "bleeds off" compression.

Builders that use big cams rely on the high rpm inertial ram effect to overcome the late closing so I can see how they could dismiss it as nonsense, however, this same kind of "common sense" once determined a round earth was nonsense too.

Among other things, DCR is useful to predict cranking pressure, which has importance to octane sensitivity. It's not a myth.

I have to agree with IgnitionMan's comment about 2 strokes.

It's late and my text books are at work. I may add/edit this tomorrow.

Paul - do you ever frequent the speedtalk forums? I tried to copy a link to the discussion (still had it in my links!)

http://speedtalk.com/forum/viewtopic.php?t=1372&highlight=dcr

Hope that works, scroll down you will see Darin Morgan's comments.. Now I respect the Morgan brothers because alot of my deep engine building theory comes from them. But im not one to get into arguments, i will still build my motors the same way, just found it interesting.. (just in case the link didn't work here is a snippet:

--------------------------------------

All this stuff is out of text books and is probably 20-30 possibly 45 years old and in my personal opinion its wrong! I get what they are trying to do, but I dont think it works.


Let me put it this way.

You have an engine with 15:1 static CR.

the engine operates at a peak VE of 124% with a minimum trapped volume of 118%. Now what is the dynamic CR again???????

That same engines VE ranges from 118% to 124%

The Dynamic CR doesnt change??? Come agian?


If I am all washed up on this please educate me becuse what they are saying does not ring true. Are they refering to somthing differant?
_________________
Darin Morgan
R&D-Cylinder Head Dept.
Reher-Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
817-467-7171
FAX-468-3147

http://www.rehermorrison.com

Wow i didn't know DCR was such a hot topic!? Hope im not getting hijacked, but does MSD make just a retard module for cranking? I dont see a problem, but in case I do... Thanks guys

Derek

That is because they don't understand engines well at all, and they didn't think of it first.

Think of DCR being active, ever changing. Ever see the vacuum actually rise as the rpms are raised? That is because the DCR changes as the engine is slow revved, allowing for less reversion because of the time between intake events speeding up. Less time for the mixture to travel up the inlet port, higher DCR.

DCR will actually rise as the rpms do, until the SCR is met when the cam "hits". Ever feel the cam come on at say, the cam's design 3,000 rpms in an engine? This is the time the DCR matches the SCR, and reversion is at its lowest, capturing the most mixture it will catch.

To really understand DCR, a person has to know two stroke engines and their theory as well as how all that interreacts to four stroke cams, mixture (NOT just air) flow works, and most of these four stroke Gurus just don't get it, so they berate it and say it isn't relevant.

DCR is vitally relevent, but some just don't want you to question them about it. Too bad for them misleading others.

Yes ignitionman - I have done alot of crazy R&D work on 2-strokes years ago and learned all about swept volumes and port timing, duration, port-time-area, etc..! (anyone remember Dr. Gordon Blair??) Same principle for 4-strokes, the 'swept volume' is only when the valves are closed and the cylinder is actualy building pressure, but that stuff makes my head hurt. lol

Paul - do you ever frequent the speedtalk forums? I tried to copy a link to the discussion (still had it in my links!)

http://speedtalk.com/forum/viewtopic.php?t=1372&highlight=dcr

Hope that works, scroll down you will see Darin Morgan's comments.. Now I respect the Morgan brothers because alot of my deep engine building theory comes from them. But im not one to get into arguments, i will still build my motors the same way, just found it interesting..

No, I do not go there or Chevytalk or all the million others. I don't have that kind of time or inclination. I read the discussion and it's typical internet gobbledy gook.

I'm surprised that someone at Reher Morrison doesn't understand basic engine design better. I don't know Darin Morgan but I gather from his comments that his knowledge is not book based but derived from Superflow observations. Meaning he knows that a big cam with late intake timing makes more power and has high VE in the upper RPM range.
In his comments he's refering to VE's over 100%. He's right about one thing.
DCR and low rpm cylinder pressure gets shoved aside by inertial ram effect which increases high rpm cylinder presssure dramatically. This is more important than DCR for the kind of engines that R-M build so in his world he's right.

The original question in the thread was what DCR can run on 87 octane.
I go back to my statement that DCR is not an answer to a question. It's a variable. Nobody, especially the DCR proponents seem to get that.

Use Pat Kelley's calculator, it works correctly. He is a personal friend as well.

I have spent a few sessions with both Kevin Cameron and Dr. Gordon Blair, both are friends.

I used to work on the older DuHamel's F9R, H1R and H2R's at both Team Hansen and Team Kawasaki Road Racing in the early 1970's. Kevin worked with us then as well.

I also curently work on an F9R, two H2R's, and an H2R for both Dave Crussell, who is the last year and now, current AHRMA vintage Champion, and former factory Kawasaki rider Hurley Wilvert. Crussell just won again this last Sunday on the H2R at Sear's Point.

Use Pat Kelley's calculator, it works correctly. He is a personal friend as well.

I have spent a few sessions with both Kevin Cameron and Dr. Gordon Blair, both are friends.

I used to work on the older DuHamel's F9R, H1R and H2R's at both Team Hansen and Team Kawasaki Road Racing in the early 1970's. Kevin worked with us then as well.

I also curently work on an F9R, two H2R's, and an H2R for both Dave Crussell, who is the last year and now, current AHRMA vintage Champion, and former factory Kawasaki rider Hurley Wilvert. Crussell just won again this last Sunday on the H2R at Sear's Point.


Ok ok, im not worthy now! lol But as always I appreaciate your input Dave! It's great to have such knowledgable folks here! Ok anyone really up on fuel curve, emulsion design, and booster curves?? - Just kidding! lol

Yes, fairly current. I also worked at Holley in the 'bandsaw your Dominator in half, widen" department.

Ain't nothin' better than a spinner design drop leg booster.

Dave, I really like Kevin Cameron. He really knows his stuff and has a knack for explaining things.
Can you have Pat Kelley contact me with his equations? Last time I ran his calculator it came out wrong. I tried emailing him with my concerns but got no response. He may have updated it since then.

I posted extensively on this subject and put it in Best of Tech (http://stevesnovasite.com/forums/showthread.php?t=16771). Hey, I'm always open to new ideas. If I'm wrong then someone tell me why.

63IIpost:

Interesting page by David Reher of Reher-Morrison on DCR (http://www.rehermorrison.com/techTalk/08b.htm). I guess Darin Morgan doesn't talk to the boss much.

Interesting page by David Reher of Reher-Morrison on DCR (http://www.rehermorrison.com/techTalk/08b.htm). I guess Darin Morgan doesn't talk to the boss much.

Aparently not, six of one half a dozen of the other!

In his comments he's refering to VE's over 100%. He's right about one thing.
DCR and low rpm cylinder pressure gets shoved aside by inertial ram effect.

That is the part that is missed many people who assume DCR is "Final word". Cylinder filling at different RPMs is a big factor. I'll propose that Cam timing selection and a restrictive intake tract can change things, even if the intake valve closing is in the "right" spot.

Edit: I just followed the link..."Wolfplace" on the speedtalk link said this better than me.

DCR or more correctly "effective compression ratio as Dynamic implies something in motion or transition as used in Pat's DCR calculator is a very important part of the equation when it come to cylinder filing,, or lack of cylinder filling but,,,.
There is a bit more to cylinder pressure than just when the intake valve closes.

At low RPM, DCR as it is being used here will suffice when trying to run crap gas with high compression.

But, when you start getting up near peak torque & the VE is starting to climb everything changes & you better damn well have fuel with enough octane to support the cylinder pressures you are making.

This is not as simple as where the intake valve closes, it now becomes, what I feel is truly what Dynamic compression ratio is all about & includes where the intake valve closes ALONG WITH how much cylinder filling you have done taking into account all the other factors like how efficient the intake & exhaust system is at a particular RPM, which is essentially what volumetric efficiency or VE is about.

VE is a ratio of how much you filled the cylinder verses what you could have filled the cylinder (100%) at atmospheric pressure & in a well tuned system (or forced induction) can reach well over 100%.
IE: "overfilling"

I finally found the graph I was looking for.

It shows that IVC and therefore DCR is not a linear function. Just by changing the inlet air speed mach index (Z) you get different VE and therefore different cylinder pressure....with the same IVC.
I've got other graphs that show the effect of other variables.

DCR is not a myth but it clearly is misunderstood by some. As mentioned above, the cylinder pressure changes from other factors such as rpm, air velocity, throttle opening, temperature, atmospheric pressure etc.

You cannot predict octane sensitivity by SCR or even DCR alone. You at least need to know air pressure, temperature and rate of change.

One more time. DCR is not a number. DCR is not directly equal to an Octane value.

I did a calc a while back and I think the DCR was 8.5 or 8.7, anyway it was close enough for pump gas! But right now all the top engine builders are calling DCR a myth, but I wont open that can of worms here lol

Interesting page by David Reher of Reher-Morrison on DCR (http://www.rehermorrison.com/techTalk/08b.htm). I guess Darin Morgan doesn't talk to the boss much.


Aparently not, six of one half a dozen of the other!

Apparently, you don't have enough eggs to make an omlete out of your wildly eggagerated statement..

Apparently, you don't have enough eggs to make an omlete out of your wildly eggagerated statement..


I like donuts better anyway!

What gasket bore size is best to use on a 4.00" bore? I'm running 4.10" bore gaskets now with a compressed thickness of .028".

I recall that it has been some discussions about DCR on this board and some people claimed that Pat Kelleys calculator & other stuff said about this topic at Chevy talk were not correct. I cannot remember exactly what was said but it would be nice if someone here could refresh my memory...
Matt

http://www.stevesnovasite.com/forums/showthread.php?t=16771&highlight=dynamic

thanks drive,
that was exactly what I was looking for!
Matt

"Some people" may just be me. Keep in mind, I'm a big proponent of the relationship between intake closing and engine performance, though I go farther and use DCR to calculate compression pressure.

However, I think the Chevy Talk DCR bandwagon is missing the big picture.
First of all DCR is not a number. It's a ratio just like SCR. Because ratio's are expressed with numbers many people confuse it with a number and ask "what DCR will run 87 octane?"
It's like saying: "what gear ratio is needed to go 60 mph?"

I'm not the only one that has noticed different results from the online calculators. I think Pat Kelley has improved his calculator since I first looked at years ago, but I haven't analyzed it because I have my own programmed into my HP-48.

General public statement: I recommend that anyone serious about building engines ought to learn the math. You'll never learn anything by using online calculators that may be giving wrong or misleading answers. To all the young people still in school: Math and Science classes are the key to performance.

The discrepancies in the results from the different online calculators appear to stem from the method for determining the exact point of intake closure.
Most cam cards don't post the intake close, so it has to be calculated from .050" numbers.
This adds to the confusion and the inability to get a precise answer. It think if you call the cam company they should be able to tell you true intake close degrees.
I think the best way is to actually measure the intake closure point yourself. Then you can calculate the piston position. Once you have the piston position where the intake valve closes you can determine the effective volume which helps determine cylinder compression pressure which is more useful than DCR.

As a general rule of thumb as cranking pressure goes up so does octane requirements.

Another mistake I see is when calculating cranking compression is to simply multiply DCR by air pressure. This gives erroneous results because it doesn't take into account the self heating effect of compression.


The DCR calculation doesn't take into account intake air inertia or exhaust scavenging or VE or any number of factors. Try figuring DCR on a Honda VTEC engine! The DCR varies so which one is right? Answer: They all are.
Variable Valve timing is marvelous improvement over the conventional fixed cam compromise between good low rpm performance and good high speed power.

How high of a compression ratio can I go up to before I cant use 85 octane? What compression will 90 or 93 octane support?

Hmmm...you're kidding, right?

Best of tech has a thread on the subject (http://stevesnovasite.com/forums/showthread.php?t=16771). If that doesn't help you can ask over at Chevy Talk. If they can't answer your question then just hit it with a sledge hammer. Just kidding!

in my opinion if you are worried about running on pump gas then dont go over 9-1 comp. i personaly dont worry about it, you just mix your fuel. my current 355 in my nova runs 10.5-1 comp. i use 92 or 93 at the pump and add a couple gallons of racing fuel or another type of octane boost. in iowa the advertised octane rating number on the pump is the minimum rating. it is really 3-8 points higher depending on the brand of gas you are using. at the station i worked at for years the 92 rated fuel tested out at 98 octane. build the motor you want and play with the fuel mixture till you get it right and there you go. ive had great luck with an octane booster called Turbo 108. i ran a 396 with 12.7-1 compresion on 92 octane and 4 bottles for every tank full of gas, never had a pinging problem at all..

Don't add jet fuel, that is just a purified kerosene that will lower the octane rating by a bunch. Unless that is some name brand for octane booster, stay away from jet fuel, you may as well fill up on diesel, it will run about the same. I don't know where people get the idea that jet fuel is a good motor fuel, it isn't.

here's another one

http://www.badasscars.com/techtips.html#Anchor8

here's another one

http://www.badasscars.com/techtips.html#Anchor8

The trouble with whoever wrote that tech tip for badasscars.com is they don't know what they are talking about.
Valve overlap doesn't have any effect on compression. Overlap occurs a full revolution after compression. It's a fallicy that too much overlap "bleeds" compression. The intake valve closing point is the cam function that has an effect on compression, not overlap. They sort of got the pressure thing right but must have missed the basic cam timing 101 lesson in shop class. Must have been doodling pictures of badass cars.

As far as the octane booster comment above, I think that gets very expensive to keep a case of booster handy. Not very practical for a street car. Jet fuel? Yeah, right.
If you are talking about JP5 Jet fuel (as in airplanes) here's a quote I found:
Diesel and Jet fuel (along with kerosene) have, indeed, terrible octane numbers; typically about 15-25 "octane". They tend to ignite easily from high compression. Their use in a gasoline engine will quickly destroy the engine.

I did another search and found that STP is marketing an octane booster that claims to be made with 'Jet Fuel'. Apparently they use the kerosene as a base or it's something they call 'Jet Fuel' but it's foolish to think that real Jet fuel will do anything good for your engine.

funny thing is i know of a couple of guys that have used jet fuel. with success or so it seemed.
one night street racing in brooklynn my buddy had mixed vpc-16, airplane fuel and hit the nitrous.
it exploded right off the top off the carb and took out the air cleaner , leaving just the stud and wing nut. no damage to the big block

we found the air cleaner base about 100 feet away.

i mix non-oxygenated premium (rated at 92 octane) with VP Red race fuel (105 octane).


Eric

I run pump gas in my 14.5-1 small block when driving on the street(all the way from Calif. to Tenn.) I found octane boosters to be completely useless for anything but draining your wallet. With a huge cam and loose converter you can get away with a lot. When at the track I use race gas, but the quickest passes are usually with a blend of pump gas(Arco's finest) and race gas.(VP C-12 or lately 110, since Irwindale has it at the pumps)
Where the heck do they run 85 octane?

Jet fuel????:rolleyes: :rolleyes: or small propeller airplane fuel. I wouldn't recommend running kerosine based fuels in a car..lol.:eek: There is a huge difference. I had a friend who worked at the small airport in Memphis TN and we used to always go down and fill our jugs with "Blue gas" (high octane) airplane fuel. He drove the fuel truck that filled the planes. There is absolutley no difference in blue gas you pump in small propeller airplanes and the blue gas you buy at the drag strip. Gasoline engines do not run on jet fuel.

"Small, propeller-powered, two-passenger airplanes can still use leaded gasoline. That’s because they have engines that are similar to those found in cars. The only difference is that the airplane engine has a higher compression level than a car engine. This means they need fuels with octane levels over 100. For this reason, they are still using leaded gasoline, even though it causes air pollution."

...you can however use aviation fuel (AV gas), not to be confused with jet fuel.

AV gas, that i have used often....

i have heard that av fuel is rated at a certain elavation and not at sea level?
does anyone know that to be true?

i cant see how that makes a difference.

There is absolutley no difference in blue gas you pump in small propeller airplanes and the blue gas you buy at the drag strip.

This is quite true. Neither has had state nor federal gasoline tax paid on them and although highly unlikely that you would ever be caught, there is always the chance you could be charged with tax evasion if you used this fuel on the street. It is fine as far as I know to use it off road without it being taxed, that is why it can be sold at the strip but if airport fuel services get caught selling it to someone that is using it on road they can be in a heap of dodo with the feds and be fined and put out of business.
?

The trouble with whoever wrote that tech tip for badasscars.com is they don't know what they are talking about.
Valve overlap doesn't have any effect on compression. Overlap occurs a full revolution after compression. It's a fallicy that too much overlap "bleeds" compression. The intake valve closing point is the cam function that has an effect on compression, not overlap. They sort of got the pressure thing right but must have missed the basic cam timing 101 lesson in shop class. Must have been doodling pictures of badass cars.

I asked him to clarify for me a bit because i was lost here, this is what his reply was.

Yes he's right. You can't explain everything perfectly every time. Overlap affects the velocity and "reversion" of the incoming / outgoing charges, and can affect the sluggishness at low RPM's, such as when an engine becomes a blooming turd because it is "over cammed". The intake valve dictates how much static cylinder pressure you have. if it has a short amount of timing, it won't allow the piston to suck-in much air, so theye isn;t much to compress. You can't compress a 'vacuum" environment, so you have to let something in to be compressed. This is all dictated by the intake valve timing (opens at 2degrees BTDC, closes at 6 degrees after BDC, etc.). These timing events can vary greatly, as do the "temperment" of cams and engines. "Scavenging" durning the overlap cycle dictates how much effective cylinder pressure you will have. This is why cylinder pressures increase as RPM's increase. Typically, the more overlap you have, the more scavenging effect you'll have AS the RPM comes-up. The problem with an "overcammed" engine is, you have trouble getting TO that RPM, such as town driving, which is why engines become complete turds when they are overcammed and don;t "come alive" untila certain RPM (usually above 3,000 RPM or so). That is when the scavenging effect starts working and cylinder pressures increase, which in turn makes the power. Horsepower is a no brainer. There is no magic to it at all. All an engine is, is an air pump. You simply meet that air witha given amount of fuel and you have internal combustion. The key to a good running engine is to make the "airpump" work at it's best. The key to horsepower is cylinder presure, period! raise the cylinder pressure and you get more power AS LONG as you keep things under control, such as A/F ratios, detonation, timing, etc.. The tech tip on my web site is one of those, "I know what I meant to say" things, but didn't say it very well. Hey, I'm not a "writer" and what do you expect for free :-)

i've run the "blue gas or av gas" in my car alot and it really likes it.
i get it straight from the pump but used to get it from the truck into
some fuel jugs you do have to tell them it's for off-road use though.

i have heard that av fuel is rated at a certain elavation and not at sea level?
does anyone know that to be true?

i cant see how that makes a difference.


I dont know if it is true or not but I have heard the same thing...something like 10,000 feet or there abouts rings a bell...:confused:

This is quite true. Neither has had state nor federal gasoline tax paid on them and although highly unlikely that you would ever be caught, there is always the chance you could be charged with tax evasion if you used this fuel on the street. It is fine as far as I know to use it off road without it being taxed, that is why it can be sold at the strip but if airport fuel services get caught selling it to someone that is using it on road they can be in a heap of dodo with the feds and be fined and put out of business.
?We are talking the physics and chemistry of the gas not about the taxation and laws regarding it. Interesting points though. uhhhh..I used it in my Chevelle to go off roading.:D

BTW- I can drive down to my local gas station here and pump 100 octaine (which has been taxed) straight into my car.

BTW- I can drive down to my local gas station here and pump 100 octaine (which has been taxed) straight into my car.

Same here (Sunoco 100).

Same here (Sunoco 100).
Yep...you gotta love Sunoco.:cool: ..thats what I used to run all the time when my car was last on the road...never had a problem...of course its not always available when you are on road trips...this topic has been discussed many times and the last time it was I did some research and found some info on the Sunoco site (it was them or one of the other oil compaines site) that listed the various fuels they offered and what octain each had and what CR that each would run correctly...I looked yesterday to see if I could find it but was not able too:(

We are talking the physics and chemistry of the gas not about the taxation and laws regarding it. Interesting points though. uhhhh..I used it in my Chevelle to go off roading.:D

BTW- I can drive down to my local gas station here and pump 100 octaine (which has been taxed) straight into my car.

Sorry if I ventured from where you wanted the post to go. I just wanted to point out that there can be some stiff penalties that can be filed on the use of untaxed aviation gas on the street.

here is an interesting link....go down the page until about 1/3 or so down and there is an article from Popular Hot Rodding Magazine, January 1998 By Scott Parkhurst It is titled "Petroleum Proof, High-Performance Gasolines". This article addresss things such as Avgas and its uses in a car...anyway I'm not sure how good the rest of the page is but that article seems good


http://www.nsxprime.com/FAQ/Miscellaneous/FuelAdditives.htm

Sorry if I ventured from where you wanted the post to go. I just wanted to point out that there can be some stiff penalties that can be filed on the use of untaxed aviation gas on the street.

No problem..interesting to say the least.

Good thing they don't have "race fuel" police or exhuastilizers...lol.http://i5.photobucket.com/albums/y178/FunkyuNova66/smileys/rollinglaugh.gif

No problem..interesting to say the least.

Good thing they don't have "race fuel" police or exhuastilizers...lol.http://i5.photobucket.com/albums/y178/FunkyuNova66/smileys/rollinglaugh.gif


As stupid as it may sound ,Travis county Texas has just started emission testing at inspection time but has also set up mobile roadside testing for emissions and can tailpipe you just from you driving by where they are set up and will mail a ticket to you if your vehicle exceeds the emission standards so I guess there is kind of an exhaust police here. I dont know if they can detect leaded fuel or not.

As stupid as it may sound ,Travis county Texas has just started emission testing at inspection time but has also set up mobile roadside testing for emissions and can tailpipe you just from you driving by where they are set up and will mail a ticket to you if your vehicle exceeds the emission standards so I guess there is kind of an exhaust police here. I dont know if they can detect leaded fuel or not.

Have you seen any of these mobile roadside testing stations? And whereabouts?

Yes I have seen 2 recently, one at Mopac and Braker on the southbound side and once at Research and Burnet near Pep Boys. They move around a lot, everything is self contained in their truck, they just pull up on the shoulder and set out the probes and sensors. They have signs they set out that show what they are and what they are doing.

set up here in Houston as well

They should come up here and test all the dump trucks, tractor-trailers and buses :rolleyes:

The reason the race fuel is different color like "Turbo Blue" is so you can tell that the taxes have been paid on it. Semi-trucks pay road tax on diesel fuel, farmers don't on farm diesel. They are different colors, if you get caught with the farm fuel in a semi-truck, you will pay the price. They have a cup on a stick that will fit in the filler neck to check.

from http://www.badasscars.com/techtips.html#Anchor7:
"Q: How much compression can I run and still use pump gas?

A: This is a good one with no single answer. The key here is cylinder pressure, not compression! Cylinder pressure is controlled by valve timing (overlap), not the piston. Although bore, stroke, compression height, deck height and combustion chamber size, among others, have a great deal to do with the whole ball of wax, they don't dictate the actual working compression or cylinder pressure of an engine. Example: two identical engines with 9:1 compression. The only difference is in the cam profiles. One has no overlap and the other has a lot. Overlap is the time in which both valves are open. When the exhaust valve is closing and the intake valve begins to open, there is a time (on high performance and race cams) where both valves are actually open at the same time. A piston can not make cylinder pressure if a valve (or valves) are open. As the piston comes up and starts to squeeze the air and fuel in the cylinder, some of the air and fuel (charge) is being pushed out of the open valves, thus creating that "rumpity bump" sound everyone likes so much, but by letting some of the charge escape, there is less there to squeeze thus creating less pressure. It also decreases the velocity of air feeding the engine as well as decreases manifold vacuum. When you have too much overlap, you end-up with a dog for an engine that has no throttle response. This is what's known as being over-cammed.

If you did a compression test on these two engines, the engine with no overlap would probably have about 140 -150psi or so in the cylinders. The engine with the overlap may only have 100 psi or so, depending on how much overlap the cam has. Some racing engines with 13:1 compression only have 125psi or so of cylinder pressure when a compression test is done. It just means that the cam has a ton of overlap. Once the engine gets spinning, the cylinder pressure will greatly increase and this is why hot cams "come-in" or "start breathing" at higher RPM's. So, you can run 92 octane pump gas on 11:1 compression with the right cam but 10:1 could ping (detonate) with the wrong cam. Way back in the early 80's, Crower had a fuel economy kit that was designed to run on pump gas (for gas mileage) yet had a compression ratio of 14:1!! Really! It's all in the cam profile. It worked but never caught-on so they dumped the idea. Bottom line, cylinder pressure, not compression dictates what kind of fuel you'll need to run."

I know this have been discussed earlier and people have had different opinions on this subject.

Matt

We already discussed Mr. Baddass's theory and he admitting to being wrong about overlap.
The trouble with whoever wrote that tech tip for badasscars.com is they don't know what they are talking about.
Valve overlap doesn't have any effect on compression. Overlap occurs a full revolution after compression. It's a fallicy that too much overlap "bleeds" compression. The intake valve closing point is the cam function that has an effect on compression, not overlap. They sort of got the pressure thing right but must have missed the basic cam timing 101 lesson in shop class. Must have been doodling pictures of badass cars.

I asked him to clarify for me a bit because i was lost here, this is what his reply was.

Yes he's right. You can't explain everything perfectly every time. Overlap affects the velocity and "reversion" of the incoming / outgoing charges, and can affect the sluggishness at low RPM's, such as when an engine becomes a blooming turd because it is "over cammed". The intake valve dictates how much static cylinder pressure you have. if it has a short amount of timing, it won't allow the piston to suck-in much air, so theye isn;t much to compress. You can't compress a 'vacuum" environment, so you have to let something in to be compressed. This is all dictated by the intake valve timing (opens at 2degrees BTDC, closes at 6 degrees after BDC, etc.). These timing events can vary greatly, as do the "temperment" of cams and engines. "Scavenging" durning the overlap cycle dictates how much effective cylinder pressure you will have. This is why cylinder pressures increase as RPM's increase. Typically, the more overlap you have, the more scavenging effect you'll have AS the RPM comes-up. The problem with an "overcammed" engine is, you have trouble getting TO that RPM, such as town driving, which is why engines become complete turds when they are overcammed and don;t "come alive" untila certain RPM (usually above 3,000 RPM or so). That is when the scavenging effect starts working and cylinder pressures increase, which in turn makes the power. Horsepower is a no brainer. There is no magic to it at all. All an engine is, is an air pump. You simply meet that air witha given amount of fuel and you have internal combustion. The key to a good running engine is to make the "airpump" work at it's best. The key to horsepower is cylinder presure, period! raise the cylinder pressure and you get more power AS LONG as you keep things under control, such as A/F ratios, detonation, timing, etc.. The tech tip on my web site is one of those, "I know what I meant to say" things, but didn't say it very well. Hey, I'm not a "writer" and what do you expect for free :-)

Overlap causes the rumpity idle not because it's pumping air out the intake but because exhaust gas is contaminating the intake charge. The relationship of cam timing to compression is caused by when the intake valve closes. He's still a little off the mark when talking about how long the valve remains open. He may be able to build engines but I don't think he fully understands how they work. A lot of what he posts is regurgitated information.

Look in Best of Tech for more information.

Does anyone know the true formula for compression? I had it but I cant find it and even looking through the past threads here I see things that arent complete. I know you can figure compression by head cc, deck height, piston cc, or what have you. The only thing missing off of every calculator is compression height. I know not every piston will give you the same compression height and this will affect your compression. Its difficult to shop for pistons if you dont have all the variables added and then match the pistons compression height to your application.

take a scroll through the Link-Library.

lots of formula links ;)

take a scroll through the Link-Library.

lots of formula links ;)

Yeah I didnt see a formula that includes compression height to calculate compression ratio just like every calculator on the web.

http://kb-silvolite.com/calc.php?action=comp

The only thing missing off of every calculator is compression height.

Compression height is just a measurement of where the pin is in relation to the top of the piston. No calculator can decide what your engine combo is based on compression height; that's determined by crankshaft stroke length and rod length. If you have the deck height of YOUR combo, that's what you need-not compression height. Some manufacturers measurements might vary slightly, but you would have to adjust(mill) the block to alter deck height. Those calculators are only accurate if you already have an assembled engine to get REAL numbers from. You can use them to estimate what you're looking for, but until you have the actual numbers, you're guessing.
Does that make any sense, or are you looking for something else that I'm missing?

http://i56.photobucket.com/albums/g180/1969NovaSS/CRformula.jpg

You can use them to estimate what you're looking for, but until you have the actual numbers, you're guessing.

Everything you said makes perfect sense just wished there was a way to calculate rather than a guess.

Some manufacturers measurements might vary slightly, but you would have to adjust(mill) the block to alter deck height.

Most manufactures will give you compression height but varies between piston to piston. If I have all other data like gasket thickness, bore, stroke, head cc, and rod length it would be nice if their was a calculator or calculation that could figure that out.

You can use them to estimate what you're looking for, but until you have the actual numbers, you're guessing.

Everything you said makes perfect sense just wished there was a way to calculate rather than a guess.


you will need to actually CC your chambers, and piston dome volumes. You will also have to know your deck height and calculate the CC's for that too. Your gasket CC's you should be able to get that from your gasket manu.

Then you will punch those numbers into the formula and that will give you your CR not a guess...but your actual CR.

BUT the only way to get that is to measure and get the actual CC's of the items listed...

Are you looking for your Compression ratio or your Piston pin height?

Reason I ask is because I read that some pistons will require milling while others dont. And compression height for any specific piston will dictate that. In other words advertised compression could be for a zero deck height while others are at a standard deck height for specific pistons. I mean if you want to shop for pistons the manufacture seem to never tell you this.

hmmm....well normally wouldnt you select a specific pin height that will work the the stroke and rod length that you are going to run...I have heard of milling pistons but in the instances I have heard of it they did it to lower the compression of a combo down to a more streetable level...such as milling the domes off of dome pistons...or cutting the dome in half or something like this....I would think if you selected the correct pin height to match the combo you are building you shouldnt have to mill them to get the pin height right...BUT hey...what do I know...If you ask my wife she'll say "Sweet FA thats what he knows".....lol:D

I got this info off a different engine board that not all pistons are created equal because some "rebuilder pistons" will actually have the impression that you will deck the block anyhow. The reasoning from what I understand is that they feel if your rebuilding an engine that the block be decked in all cases. But not all pistons are made this way.

Reason I ask is because I read that some pistons will require milling while others dont. And compression height for any specific piston will dictate that. In other words advertised compression could be for a zero deck height while others are at a standard deck height for specific pistons. I mean if you want to shop for pistons the manufacture seem to never tell you this.

Well the books I have read said you cannot take the dome volumes that the manufactures state as fact...often they do not take the valve reliefs into account and normally when CC'd many pistons domes will CC smaller then what the manufacture stated them to be......they say the only way to truely figure out your actual compression is to actually CC everything....PLUS the CC of combustion chambers are rarely as stated...my heads are supposedly 64CC but again I have read where Visard says this head routinely CC's out to 68 CC's NOT 64 CC's as GM says they should be...all of theses little differences add up and will give you a false compresion ratio if you just use the numbers supplied by the various manufactures...to get an accurate CR for your motor you will have to measure your motor...:)

See thats the problem is if the manufacture doesnt divulge that wether the compression piston height is for a zero or standard block then it makes it difficult to buy the pistons either requiring you to deck the block or not. And of course you cant cc anything without buying the pistons first. Seems like catch22 if you ask me.

See thats the problem is if the manufacture doesnt divulge that wether the compression piston height is for a zero or standard block then it makes it difficult to buy the pistons either requiring you to deck the block or not. And of course you cant cc anything without buying the pistons first. Seems like catch22 if you ask me.

I do see what your saying but if your going to try to zero your block you will need the pistons before so you can mock it up (after the bores have been machined of course) and measure how much the block will need to be cut to get to zero with this combo...SO you should be able to buy the pistons that you want to run and then if decking is required do that afterwards as you will need the pistons to figure out deck height anyway...:)

Compression height is listed by piston manufacturers but if what you are looking for is how to determine what CH will work with your combo it's this:

(You need to know the following)
Block height (this is crank center line to deck).
Stroke
Rod length
Deck clearance you want (for figuring quench it's simply DC + GASKET)

CH= BH - ((stroke/2) + RL + DC)

Stick 350 example: 9.025" -( 1.74" + 5.7" + .025") = 1.56 CH

Once you've figured what CH you need then you can plug it into the CR calculation.

I do see what your saying but if your going to try to zero your block you will need the pistons before so you can mock it up (after the bores have been machined of course) and measure how much the block will need to be cut to get to zero with this combo...SO you should be able to buy the pistons that you want to run and then if decking is required do that afterwards as you will need the pistons to figure out deck height anyway...:)

I guess my beef is with the manufacturers for not advertising wether the pistons are for 0 or standard deck height as im sure they have this type of information. It could also save time and money for shadetree mechanics who would rather just use a ridge reamer and be done with it. Sometimes you buy pistons and they are setup for a standard deck and then the next time you get pistons for a zero deck you will look at them and say wow those pistons are sunk down more than I thought. Or how about you buy standard deck height pistons and your deck height is already decked. I found out its because piston manufacturers dont reveal this info.

Now this was on the Keith Black website as to how they calculate their advertised compresion ratio:


All weights and compression ratios are based on a .030" oversize piston for that listing. The compression ratios in each listing are calculated using a combined deck clearance and gasket thickness of .040" and bore diameter of .030" oversize, unless otherwise noted in the listing under the notes.


So depending on the gasket you picked the piston would either be about .020"-.025" in the hole or nearly at zero deck...it would all depend on the thickness of your gasket...Possibly if you looked around other pistons manufactures website you might find simular information that would be helpful....

http://kb-silvolite.com/article.php?action=read&A_id=15

That still leaves you guessing wether the compression height they advertise is for 9.025 or a zero deck.

combined deck clearance and gasket thickness of .040"


That still leaves you guessing wether the compression height they advertise is for 9.025 or a zero deck.


NO...it doesnt leave you guessing....now it might requre that you figure a few things out but you shouldnt have to guess....If you use a thick composite gasket which are in the .039" compressed range you will have to deck the block so the piston is only .001" in the hole (so you end up with the .040" total)...basicly zeroing the block..if you use a steel shim gasket...they are about .015" thick when compressed so then the piston would need to be .025" in the hole (again so you would end up with the .040" total)

You can buy a set of pistons...mock it up to see what your actual deck clearance is and then decide on the gasket you want to use...then you will know if you have to deck the block or not to end up with the combined thickness of .040" that KB uses to calculate their Compression ratio.....fairly straight forward it would seem:)

BTW the gasket manufactures will be able to tell you the COMPRESSED thickness of their various gaskets so you wont have to guess on this part either......

check Speed Pro's site for example... they show the figure for deck at a measured spec for their posted compression rations...

Makes sense now thanks.

Makes sense now thanks.


Cool...NPHNP....:)

That still leaves you guessing wether the compression height they advertise is for 9.025 or a zero deck.

It doesn't matter what your deck is. The CH is the CH. It's the distance from the center line of the piston pin to the top of the piston.
The deck clearance depends on the block height. Don't assume any block is 9.025" That's a nominal dimension. It's the Actual block height that you may mess you up. It will vary and it will also vary front to rear. Assume you'll need to square your block so allow some milling wiggle room. Don't try to buy your way to a zero deck.

I guess my beef is with the manufacturers for not advertising wether the pistons are for 0 or standard deck height as im sure they have this type of information. It could also save time and money for shadetree mechanics who would rather just use a ridge reamer and be done with it. Sometimes you buy pistons and they are setup for a standard deck and then the next time you get pistons for a zero deck you will look at them and say wow those pistons are sunk down more than I thought. Or how about you buy standard deck height pistons and your deck height is already decked. I found out its because piston manufacturers dont reveal this info.

If you need a ridge reamer to get the pistons out, your bore is worn. Zero decking is worthless with excessive blowby and slapping pistons. The purpose of getting the piston flush with the top of the block is with a std. .039" gasket gives a tight quench distance.
Minimum quench is a NEAR collision of the piston to the head. .039" is not a lot of room! A rocking piston in a loose bore will smack the head if you don't maintain precision dimensions and tolerances.
Zero decking for minimum quench is something you have to sneak up on by test fitting and carefully removing material from the deck. You are committed to that gasket size. You certainly can't install a .015" gasket if it's set up for .039".
It has to be done at a competant machine shop. If you think otherwise you are kidding yourself.

I never decked my block so im not sure of the deck height. It was bored and honed out to .030 with a 3.75 stroke and 5.7 rods. I was scared I would run to close like you stated and ran a 0.041 compressed gasket. I have this with 76cc heads and flat top pistons with 7cc valve reliefs. In the calculator it says im about 9:1 compression just right for pump gas i guess. I just wasnt sure if maybe my compression was even lower. The pistons I got were 9.5 with 76 cc heads but Im not sure if they were for a 0 deck or not and the manufacture of the pistons no longer exist.

it may very well be lower... might be higher too.

that's one of the reasons for checking ;)

The pistons I got were 9.5 with 76 cc heads but Im not sure if they were for a 0 deck or not and the manufacture of the pistons no longer exist.

OK let me repeat one more time. Pistons aren't FOR any paticular deck clearance. That's why the manufacturers don't list deck height. It's for you to calculate from the stack up dimensions you have.

They do have a compression height which is:

the distance from the wrist pin centerline to the top flat surface of the piston.

Got it? it's not the distance to the deck. They don't know or care what deck clearance you may have or want. It's the responsibility of the engine builder to measure everything and figure it out.

For example:
If you have .025" deck clearance with 1.55 CH and want "zero deck" (pistons flush with deck), then you'd need either remove .025" from the deck or order pistons with a CH of 1.575" (1.55 + .025")

To calculate estimates from nominal dimension figures still all depends on the true block height dimension (centerline of crank to deck). You don't know if your block was ever machined. Since you don't know your actual block height you'll have to measure it.

Three ways.
One is to buy a special tool for doing so.
Or you can use a 12" caliper and measure the saddle to deck dimension and add half the main bore.
Or you can mock up known components and measure the clearance and calculate the BH using the formula I posted above.

9:1 is very low compression. Probably not worth the trouble and risk to try for minimum quench if it can't be done properly.

I was aiming for a pump gas friendly engine with fairly low compression in the first place. Im running iron heads also and really would like to do a mild supercharge if possible. I really didnt want a high compression race engine. My fear was more of having too much compression.

This is where the piston information came from. It discusses how the piston manufactures dont talk about these certain "sucker pistons" or "rebuilder pistons" and how they are difficult to identify.

I`m positive it means .020 off the tops, the silvolite jobs are one in the same, it shows it on there site, however, speed pro has went out of there way to hide it, as there is very little info on any of there piston or parts lines period. sterling, badger and other companies do this as well, there called "rebuilder" pistons, these things are all over Ebay at steal prices, and they advertise them as "10:1 flat top pistons" they don`t mention there`s .020 off the tops, hence the name "sucker" comes in. I didn`t know mine were like this when I got them, all I knew was the engine didn`t have no where near the same power as compared to the TRW forged flat tops I had in there before, what gave me the first clue was when I took the heads off and looked in the bore, I was making sure my timing marks were lined up correctly, so I brought it to TDC and noticed the piston was way down in the bore, and I thought "that don`t look right, what`s going on here?" so I came inside and looked at silvolite`s web site and that`s where it said 1.540 and I said "man, they cut .020 off the tops,,, why? no wonder it hasn`t got any power" and since then I looked at other pistons, the 345NP was one of them as replacements, and found they too had .020 whacked off the tops, all the cheapy replacement "flat tops" are like this, as are the dish top pistons that we recently took out of my brothers 85 vette, they had .030 dish top jobs that had .020 off the tops, that killed the compression to about 7.9:1, the only reason the car had any throttle response was due to the TPI, had it had a carb, it would have been a complete slug. the piston companies are taking people out to lunch with these pistons, and they try to keep it a big secret as if they don`t want us to know, because they don`t want us to know! they don`t even put it on the piston box, and it`s not mentioned anywhere in the instructions. I don`t think there made this way because of a cylinder ridge, anyone can buy a ridge reamer, I think there made this way to get money out of something that`s not worth it, it kills compression and quench, and anyone who knows performance knows these are 2 prime ingrediants in any performance rebuild, so the piston company don`t mention it cause they hope the builder don`t know, and what the builder don`t know won`t hurt`em right?"]I`m positive it means .020 off the tops, the silvolite jobs are one in the same, it shows it on there site, however, speed pro has went out of there way to hide it, as there is very little info on any of there piston or parts lines period. sterling, badger and other companies do this as well, there called "rebuilder" pistons, these things are all over Ebay at steal prices, and they advertise them as "10:1 flat top pistons" they don`t mention there`s .020 off the tops, hence the name "sucker" comes in. I didn`t know mine were like this when I got them, all I knew was the engine didn`t have no where near the same power as compared to the TRW forged flat tops I had in there before, what gave me the first clue was when I took the heads off and looked in the bore, I was making sure my timing marks were lined up correctly, so I brought it to TDC and noticed the piston was way down in the bore, and I thought "that don`t look right, what`s going on here?" so I came inside and looked at silvolite`s web site and that`s where it said 1.540 and I said "man, they cut .020 off the tops,,, why? no wonder it hasn`t got any power" and since then I looked at other pistons, the 345NP was one of them as replacements, and found they too had .020 whacked off the tops, all the cheapy replacement "flat tops" are like this, as are the dish top pistons that we recently took out of my brothers 85 vette, they had .030 dish top jobs that had .020 off the tops, that killed the compression to about 7.9:1, the only reason the car had any throttle response was due to the TPI, had it had a carb, it would have been a complete slug. the piston companies are taking people out to lunch with these pistons, and they try to keep it a big secret as if they don`t want us to know, because they don`t want us to know! they don`t even put it on the piston box, and it`s not mentioned anywhere in the instructions. I don`t think there made this way because of a cylinder ridge, anyone can buy a ridge reamer, I think there made this way to get money out of something that`s not worth it, it kills compression and quench, and anyone who knows performance knows these are 2 prime ingrediants in any performance rebuild, so the piston company don`t mention it cause they hope the builder don`t know, and what the builder don`t know won`t hurt`em right?

http://www.hotrodders.com/forum/heads-21002.html

It figures. Why do you bother going to "other" web sites for information?
That guy is a flaming internet idiot.
His explanation that .020" after the piston number means that these are "sucker" piston shows his total ignorance and lack of experience.
He makes up a wild conspiracy story to cover the fact that he stupidly installed 1.540" CH pistons.

These would work perfectly with a 9" block with a 3.5" stroke and 5.7" rods and a .038-042 gasket OR a 9.010" block and a .015" gasket or any combination of gasket/quench possibilities.

If he installed them in a 9.025" block a 3.48" stroke and 5.7" rods his compression would indeed be in the toliet...which is where his head ought to be.

Always beware of internet "experts" that talk about their older brother's 'Vette.

It figures. Why do you bother going to "other" web sites for information?
That guy is a flaming internet idiot.
His explanation that .020" after the piston number means that these are "sucker" piston shows his total ignorance and lack of experience.


The only reason it seemed plausible is because of this remark.

A guy at Silvolite told me their cast (not KB) pistons for a Chevy 307 were .013 too short. The Federal Moguls were .020 too short.

I dont know why Silvolite would lie about such a thing.

no wonder it hasn`t got any power" and since then I looked at other pistons, the 345NP was one of them as replacements, and found they too had .020 whacked off the tops, all the cheapy replacement "flat tops" are like this

The hyper pistons are advertised as 1.560 compression height and teh as cast are 1.540 compression height piston.

It's a boo-boo. I have a brand new set here and from the center of the pin bore to the crown it measures 1.541"


http://www.hotrodders.com/forum/compression-ratio-64-cc-heads-and-federal-mogul-sterling-345np-61914.html

The 345NP piston the guy talks about is Federal Mogul Sealed Power piston.
The 345NP piston is on the NHRA list of accepted replacement pistons.
On the FM web site it clearly shows the piston CH is 1.540.
They also have a piston available in sets that's 345NCP with a 1.56" CH. It's no conspiracy, though it's possible to get the two confused.
Just a quick check of available piston CH showed:
1.54"
1.55"
1.56"
1.563" (the L2256F piston)
1.57"

They make pistons with different CH's for a reason and it's not to "sucker" ignorant unsuspecting engine assemblers. Some classes allow tolerances to factory specs. This means that a stroke that's nominal 3.48" could be within tolerance if it was +-.020". A smart racer would grind the crank so it was on the high side of that tolerance or 3.5".
Same is true of decking. Since they allow some machining it's better to whack the block to 9". That way you have minimum quench with a .039" gasket, a few more cubes, higher compression and still factory "stock". A 1.54" piston is also useful for the rebuilder that has milled the block out of neccessity but doesn't want to raise the compression.

If your engine requires a 1.56" CH don't buy a piston with 1.54". I do agree that buying pistons on Ebay is a crap shoot. I'm finding a lot of the "new" stuff must be seconds or poor quality counterfeits.
I had to buy more than a couple Ebay sets to find ones that were acceptable for Kev's engine. The best thing to do is buy good pistons from a reputable dealer that stands behind the sale. Yes, it will cost more that way but if for some reason it's wrong you can return them.

If a piston is listed as 1.56" and it comes as 1.54" then it's machined wrongly or wrongly labeled or the web site/catalog is wrong. Either way send it back and tell them they have a problem. Make sure you've measured it right or you'll have egg on your face.


My point in all this is I've posted everything you need to know. Formulas, how to do it, what to use.... and for some reason some idiot on another site still has more credibility than I do.
I may be thought of as mean and rude but I know more than that guy does.
Hey, I don't care. Really.

You are the one ultimately responsible for your engine. You need to learn how to measure and calculate stackup, compression, quench and clearance. If you won't do that and instead prefer to look for shortcuts from the internet BS monkey's then I can't help you.
Good luck.

Can I hear and Amen from the congregation?

AMEN!

Kev

Im just saying compare the compression heights to the advertised compression each piston gives you.

I think it's the other way round, because if you look in Jeg's, h345 np which is the Hypereutectic is said to produce 9.5 with 64cc heads and 345npstd is said to produce 10.2.

They're just advertised compressions for people that are too lazy to measure. It's so some silly guy on the hotrodders forum can say, "I've got an 11 and a half to one Chevy 302 with camel humps that will rev to 9000 rpm and eat big blocks for lunch." If you don't measure, it's a crapshoot.

Kev

A piston manufacturer can't compare CH to CR because they wouldn't know what the block is decked at. Also, they wouldn't know how big the chamber of the head is, gasket thickness, etc. There are many factors that go into CR, not just the pistons. Piston, as well head manufactures may list a possible CR, but they can't state it for certainty. They are general numbers, not exact. If you want to achieve exact numbers, it has to be measured!

Well ok thats a better explanation since this was just an observation. I know for my situation I will eventually check for piston depth at tdc and go from there since I have no info on my pistons what so ever. Oh and of course thanks because i feel educated now.

My point in all this is I've posted everything you need to know. Formulas, how to do it, what to use.... and for some reason some idiot on another site still has more credibility than I do.
I may be thought of as mean and rude but I know more than that guy does.
Hey, I don't care. Really.

I took all you had to say whole heartedly and was just trying to understand why the compression heights differed so much from the advertised compression. I havent sensed you or anyone in this thread to be rude but just offering good info to learn off of. If I dont ask I wont learn.

I get frustrated sometimes because it takes time to draft a response, type it up and edited to where I think it gives the correct answer to a question.

I don't mind a student who asks a question but a student that doesn't listen to the answer is irritating.

You seem to have difficulty understanding the difference between piston advertised compression ratio vs the compression height. Then this guy got you convinced that piston makers are lying to people (he doesn't explain why they would do that) about it.

The key thing to remember is there are people that observe and make up theory's about everything. It's a faulty leap of logic that piston manufacturers are going to the trouble of milling off exactly .020" ( a crazed and yet still accurate machinist?) and purposely selling them as 1.56" CH pistons to "suckers".

Just because someone says he talked to a guy at Silvolite about making "short" pistons doesn't mean he did.

What if I said I heard from Santa Claus that the Easter Bunny is handing out eggs infected with bird flu? At least I've named my source!

How about if I said those evil Chinese are copying Silvolite pistons so well, the only way you can tell is the CH is a metric 40 mm or 1.544"?

Hey, I just made that up but it sounds plausible!

The internet is full of this kind of hearsay, rumor and innuendo. That's why this site is better than most because the moderators diligently monitor for BS and slander.

I realize that even my credibility has to be earned. I can't just say I'm an expert. I have credentials (ASE master machinist is one, 21 years at Roush might count)
Hopefully my answers have been right a lot more times than wrong but there's a "check and balance" built in.
This forum uses peer review just like the scientific community does. If I say something that is false, misleading or flat out wrong, it will be caught by one or many of the other smart people in our 4,000 and growing members.

Go ahead and ask a question. Then all you have to do is listen to the answer.

but Paul... it's no fun if ya don't give us the answer we wanna hear !!! :rolleyes: :D :D :D

hehehe :D

but Paul... it's no fun if ya don't give us the answer we wanna hear !!! :rolleyes: :D :D :D

hehehe :D

I want a lumpy idle :eek: :D :shh:

If Dave's is lumpy, I want mine SUPER lumpy!

Kev ;) :D

If Dave's is lumpy, I want mine SUPER lumpy!

Kev ;) :D

Would you like one lump or two? :D I'm still pissed that PRP didn't open today....***, too much sunshine today or what :mad: They got a freakin' track dryer. USE IT!!! :mad:

Doesn't bother me one bit. The Nova's at home, I'm finishing up a paper at Pitt and the Rex still needs a patch panel or two before I start driving it everywhere.

Next weekend perhaps? I was thinkin' of maybe running the little rotary thing for sh**s and giggles. Or maybe I'll bring the Nova down and see how it runs with the weight reduction and new wheels, just to isolate some variables for when the big PW350 hammer goes in. It's been feeling a little sluggish lately, though. I think it might be dying. I'll run a compression check before I take it out to make sure it's not gonna blow up on me.

Kev

I'll run a compression check before I take it out to make sure it's not gonna blow up on me.
Kev

Let me get a few passes in before you oil down the track :o :D

They say engines alwyas run best just before they blow. You might get your 13.99 or get into triple digit mph.

Paul Wright suggested in another post that 1 point in compression was worth about 25-30 HP

Paul, is this fairly accurate?

Paul must be heads-down workin on the PW350 :)

I have about 350 hp,355 ci sb,9:8:1 compression and it runs on 87 without run on's.It hase run on a mixture of 87,93,111,and 116:eek: :eek: :eek: :eek: :eek: :eek: with no promblems.This is what a chevy can do not f@rds.:D :D :D Hope this helps.BTW i'm not lying eather.

Yes, I am trying real hard to get Kev's engine done by the end of the month.

anyway that rule of thumb applies to a typical 350 SBC with quench heads.

Car Craft did a test with a 440 Chrysler (non squish heads) where they went from 8:1 to 11:1 and recorded an 8% increase per point of CR, but the engine was "overcammed" at 8:1, just right at 10:1 and under cammed at 11:1. There's an important interaction between the cam specs and CR so you can't just raise the CR without changing other factors.

It isn't a one to one increase, meaning if you go from 10:1 to 14:1 it isn't automatically worth 100 hp. The increase in power is not linear.
Car Craft also did a test by changing head gaskets on a 355 and the hp chart shows a non linear rise, dip and rise in HP as the quench changed along with the CR. The got about 30 hp per point at power peak but the average gains were:
9.62:1 avg 390.5hp avg VE 99.6
9.86:1 avg 396.1hp avg VE 100.1
10.14:1 avg 394.6hp avg VE 100.3
10.49:1 avg 400.4hp avg VE 100.5

The dip in power at 10.14 is strange. the VE peak was higher than peak torque on this test run also. I'm not sure why this test was different but it goes to show that you can't count on anything to happen the way you think it will.

You also can't raise CR forever even if you could get octane high enough. HP gains fall off as you get near 15:1, though some restrictor engines have run CR's in the 16-17:1 range because the manifold air density is very low.


Here's an example of predicting HP increases just from CR changes:

HP= Intake pressure x SCR x VE X CID x rpm/5252/150.8

A 350 with 9:1 can make 350.79 Hp at 6,000 rpm assuming 100% VE at sea level.
Same 350 350 with 10:1 can make 389.8Hp (+39),
11:1 can make 428.74 hp (+38.94)
12:1 can make 467.72 (+38.98)

This is just a mathematical equation using best case parameters. In the real world it's more like 25-30 hp for every point of CR increase, however the cam must be compatible with the CR. As noted, on a SBC the squish distance is important.

You mentioned milling your heads. If you mill more than .010" you'll have intake manifild fit and port alingment problems. It's important to mill the intake to fit the new stack up height.
There are benefits to angle milling the heads. There are airflow gains possible from rolling the heads to a lower angle than 23 degrees but the cost and skill level for this project is "expert" category.

This is just a mathematical equation using best case parameters. In the real world it's more like 25-30 hp for every point of CR increase, however the cam must be compatible with the CR. As noted, on a SBC the squish distance is important.


I don't have much quench right now at .053".


You mentioned milling your heads. If you mill more than .010" you'll have intake manifild fit and port alingment problems. It's important to mill the intake to fit the new stack up height.
There are benefits to angle milling the heads. There are airflow gains possible from rolling the heads to a lower angle than 23 degrees but the cost and skill level for this project is "expert" category.

Ok, good to know. Money may be better spent on a new short-block.

427 or 396 small block:) Yum, big small blocks for Dave!

Kev

As far as the octane booster comment above, I think that gets very expensive to keep a case of booster handy. Not very practical for a street car. Jet fuel? Yeah, right.
If you are talking about JP5 Jet fuel (as in airplanes) here's a quote I found:


Diesel and Jet fuel (along with kerosene) have, indeed, terrible octane numbers; typically about 15-25 "octane". They tend to ignite easily from high compression. Their use in a gasoline engine will quickly destroy the engine.


Paul; I was just rereading this tread today and came across the above quote you had posted(pg. 6 post #88) that I find very interesting.


Not trying to be a jerk here but I'm just wondering how this can be true? I'm confused because as far as I know most, if not all, diesels have very high compression ratios as that is what causes the fuel to combust in the cylinder (as they dont have spark plugs though they do have glow plugs but they serve a different purpose than spark plugs)

Most diesels, I believe, have like 16:1cr or better. Very few cars will ever have compression ratios in that range. You can actually run diesel in a gas engine..it wont run well and will smoke like crazy but it will work...you cannot however run gas in a diesel motor...you get massive detenation going on if you do that...or at least that is my understanding...

Just trying to understand things:p

Hmmm, that does sound backwards, doesn't it!

Diesel fuel is rated with a Cetane number not Octane. Centane is backwards from Octane. The higher the Cetane #, the less resistant it is to detonation. However, Diesel fuel isn't just a low grade gasoline and it won't self ignite with the relatively low compression of a gas engine. Diesel simply isn't a good fuel for gas engines.

Just goes to show when you cut and paste from the internet, you don't always get a good answer.

Added:
Found a tech article on Keith Black Silvolite site:

Compression Ratio vs compression pressure (http://kb-silvolite.com/article.php?action=read&A_id=36)