Compression ratio, DCR and pump gas (merged threads)

[novaguy73]

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

[new2novas]

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.

[ryan63]

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

[DriveWFO]

Originally Posted by ryan63

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.

[ill deuce]

Originally Posted by DriveWFO

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

[new2novas]

http://www.chevytalk.org/threads/sho...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

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

[Mike Goble]

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

[Paul Wright]

Originally Posted by ill deuce

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.

Originally Posted by ryan63

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".

Originally Posted by new2novas

http://www.chevytalk.org/threads/sho...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

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.

[Bob T]

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.

[Paul Wright]

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.

[Paul Wright]

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!

[stock z/28]

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

[Bob T]

Originally Posted by Paul Wright

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?

[Paul Wright]

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.

[Paul Wright]

Originally Posted by Bob T

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

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

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
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?