DIY Port Matching and Porting (combined thread)

Anyone ever do this? I'm thinking about tackling the project after I'm done with my rear end and tranny swap.

http://www.sa-motorsports.com/diyport.shtm

http://www.valvoline.com/carcare/articleviewer.asp?pg=res20040801pm&cccid=5&scccid=9

http://www.chevyhiperformance.com/techarticles/95518/

Anyone ever do this? I'm thinking about tackling the project after I'm done with my rear end and tranny swap.

http://www.sa-motorsports.com/diyport.shtm

http://www.valvoline.com/carcare/articleviewer.asp?pg=res20040801pm&cccid=5&scccid=9


I like those links...thanks for posting them :)

no problem! Has anyone done this (thats not a professional)? or will I be the first? :eek:

Not me. I guess it depends on what kind of heads your gonna try and do it on. Some cheap GM casting's, why not try it. For an expensive set of heads, personally I would leave it up to someone who knows what they are doing and has access to a flow bench.
Just my 02.

Yepp, did it to a Mu$tang I had when I was younger. the heads and lower intake were aluminum and you have to be real careful if you're using a peanut grinder on such a soft material. If you're grinding on cast iron then you take less material off at once. Just take your time...it's not really that hard. I didn't read the articles you posted links to but you can match the intake to the heads and the gaskets also.


Good luck! Take some pictures and let us see your progress.

P.S. If you get feeling comfortable you can try to polish the exhaust ports on the heads....I did that one too.

The best advice I can give you is find some cheapo crappy heads to practice on before you try and grind away some nice trick flows or something. Also A while back I had purchased a gasket intake template kit that came with just about every gasket ever made which is good for matching ports made by MR Gasket part #73074G. Its awesome for figuring out exactly how far you can go with the ports. Mr gasket also offers this part # 4362

http://static.summitracing.com/global/images/prod/large/mrg-4362.jpg

Here is a link to thr port gauge i mentioned so you can see what it looks like.

http://go.mrgasket.com/pdf/promotional.pdf

thanks for the info!

I will be port matching my Victor Jr. intake and my GM high compression heads.

I was going to try to polish the exhaust ports.. one of the links details it.

Could we get these links sticky'd for future reference? or moved somewhere that they dont get lost?

Thanks,
Chris

I'll put this in Best of Tech.

Thanks! Much appreciated!

What gains could I exspect over what I have now If I ported my Heads.
I mean bowl blending port matching everything With out going over board.
Also including polishing the exhaust ports.

The reason i ask if I should bother doing it is the motor is in the car and will take a while to do. I figure 50+ hours on the port work its self. Would the gains ..all be it free... be worth the time?

These are G2 twisted wedge heads on a 355.Dont know what power is at this point but I ran a 12.77 at 106mph..

That's a hard question to quantify. Your results are going to be directly tied to the quality of the porting. Small changes can make differences both positive and negative. Without a flow bench how will you know if you made an improvement or not? It's also possible to have inconsistant results port to port.
I'll tell you porting is an art and requires good hand to eye co-ordination. It's real easy to get wavy surfaces, inconsistant radii, take off too much (especially on aluminum) Once the material is gone it's gone.

My advice is to practice porting on heads you can afford to mess up on. I wouldn't use expensive aluminum heads as your first try.

True story:
I had a co-worker who wanted his $25 heads ported. Of course I think he wanted me to do it for free. He seemed skeptical it would take so much time and it "wasn't that hard to do". It got to the point where I finally had to invoke the "first rule of lifesaving".
I told him he was welcome to try it himself using all my expensive equipment.
I set him up on the bench and showed him what to do. He quit after about an hour and a half. His 1st port had so many ripples, waves and dips it was basically ruined along with about $25 worth of my grinding bits. I think it was a fair trade.
He got an hour and a half lesson in porting and I saved two weeks of my time.

Here's a simple formula for potential HP from head flow.

Port CFM (at 28") x .25714 x # cylinders = HP potential (at peak power rpm)

Use 0.43 for flow taken at 10" and 0.27 at 25"

How many cfm's can an "expert" head porter usually get out of a set of heads?

How about a "beginner," with just enough experience and guidance to not screw it up?

Kev

Maybe a Good First Attempt would be just a Simple Gasket Match.

Joe

I saw a set of 441 SBC heads ported by a beginner that didn't know what he was doing. They looked awesome huge ports smooth as glass they shined like mirrors. He had them pressure tested for water leaks to see if he had hit the water jackets. The heads leaked water everywhere and were worthless when they were done.

Port CFM x 0.43 x # cylinders = HP potential (at peak power rpm)

Wouldn't cubic inches play a large part in using this?

Wouldn't cubic inches play a large part in using this?

Also need to factor in cubic ****$ too :eek:

I just got Info that the twisted wedge heads have very thin cast ports and bowls with 360* water jacketing around them So I think I will stay away from porting them...

Wouldn't cubic inches play a large part in using this?

It's important to note that I said power potential, not how much power you will get.

(sorry I got called away before I could finish)

200 cfm SBC heads could be bolted on a 283 or a 400. The actual results will depend on many factors.

At an efficiency of 1hp per CI you could get 283hp or 400hp but the same heads won't flow enough AIR to support 450 HP even if they're on a 500 CID engine.

Best Hp is the result of burning 1 part fuel to 12.7 parts of Air (by weight).

You need to supply about 1/2lb of fuel per hour for every HP. For 450 hp, that's 225 lbs of fuel per hour or which is only .008 gallons per second (2 table spoons).

Getting enough fuel into an engine is easy. You can get it rich enough to blow black chunks out the tailpipe.
However, you also need 12.7 lbs of air for every pound of fuel. For a 450 hp engine you need 2,857 pounds of air per hour. That's a lot of air!

Getting a lot of air to go in by itself is hard unless you force it in with a supercharger or introduce it chemically with Nitrous Oxide. It takes about 60 hp to run the supercharger on a 450hp Ford Lightning. Cramming over a ton of air is a lot of work.
A tank of nitrous is cheaper than a supercharger but it doesn't last very long. An hours worth of continuous spray would be very expensive, indeed.

In a normally aspirated engine, heads are the key to power production. The bowl and valve is the most restrictive part of the path so port matching often doesn't give the best gains.
If you have a lumpy idle cam with lot of overlap then it may be more productive to leave the manifold smaller than the port to act as an anti-reversion ledge.
The actual benefit depends on how bad the before was and how good the after gets.

Heads, intakes and carburators are not superchargers. A bigger CFM rating doesn't mean the engine will get more air, but generally speaking improvements in head flow improve power output.

Here's one final comment on head porting.

The 5 axis CNC machine has revolutionized head porting. Porting heads is nasty, grueling, exacting work. It's also very difficult for even the best head porters to duplicate good ports consistantly. It's also hard to train and keep head porters. Most quit to do something else. I'd bet there are less than 100 expert head porters in the country and surprisingly, the demand for more isn't really growing.

These days once a good port is developed it's mapped and programmed into a CNC machine. A head is done overnight instead of weeks later and very little hand finishing is required. It really cuts down on labor intensive skilled work.
Aftermarket CNC ported heads are a bargain compared to expertly hand ported heads.
I think the new 505hp Z06 even has CNC ported heads from the factory! A big change from the old crappy heads from the last century.

start hogging out exhaust port passages and if yer not carefull, you'll bust through to water passage...


pocket porting is one thing (matching gaskets) but a full on port job requires lots of experience....

If anyone is wanting to try porting for the first time, Standard Abrasive has a really good video. It shows the step-by-step for their porting kits.

http://www.sa-motorsports.com/diyport.shtm

Ok I was corrected by a NEW member hope he sticks around.




Whomever told you that our heads are thin is misinformed. All of our port walls are a minimum of .250" thick (The only exception to this statement is the pushrod pinch point, Which is very thin...)
The deck thickness is 9/16".
While it is true we have a 360* water jacket around the sparkplug, That does not mean the head is thin or fragile.
If you have any questions on this head, Please feel free to call me at work ***********ext ++++, Or email me at **BLOCKED**@trickflow.com...
Artie:chev:

And yes I have little porting experience I do have alot of metal finishing experience including lead (PB) which goes away faster than Aluminum..

I was really just going to work on the bowl, Bowl bias, Short side radius and guides. do some slight work on the roof,divider wall and polish the exhaust side.

If I remember correctly the valves are pretty unshrouded by design and the chamber was pretty clean so i may leave them alone or give'em a good polishing too.



Paul as always a lot of info

On a 400hp 350ci, what kind of hp/tq gains (on average) could be expected from port matching the intake manifold?

Hey DriveWFO ... One of the many experienced "engine gurus" :D will have a much better qualified response ... but I will go out on a limb :eek: and say no more than 3% - 5% improvement. But my response is based on a 3/4 to 1 inch clean-up into the runners to the selected gasket combination.

I am only basing this on reading material and theory - since I just started my first real adventure into porting (really only very careful clean-up work to stock parts). Just completed a gasket match on Victor Oval Port intake ... getting set-up to start some mild clean-up on the '781 BBC heads.

I think the real answer is a bit more complicated and depends on a few more variables - but I am anxiously waiting responses to this thread.

... now we can wait for the better qualified to respond and see how wrong I am ... wouldn't be the first time though :rolleyes: :D :D

Im not all that experienced in this myself but I have researched it for my own knowledge. Generally you want your intake and heads to be port matched so to elliminate any turbulence through the intake path that would hurt flow. For instance you want it to be matched depending on whatever the head port size is. Say you have a stock head matched to a felpro 1204 gasket. You will want your intake to be the same so the intake charge is evenly passed through. Increasing the port to say a 1205 will probably gain very little or even hurt flow and be pretty pointless. Unless you actually do a full out port to the entire intake path of your heads or go with stock race port heads to really see any gains. Im not so sure it would be gaining flow but more so getting your heads flowing optimal to whatever your heads should be flowing without any obstructions. I guess it all depends on what exactly your trying to match. I mean if your trying to put a 1205 port intake on a 1204 port head then I would think it would be needed. Hogging out a 1204 intake to a 1205 fully ported or stock race port head would probably give you some good HP gains but probably affect torque in some way. There may be lots of variables depending on intake design like runner length or intake volume. I would imagine some porting actually may not gain anything but actually move the power band in different rpm areas. Ive actually been comparing a couple of intakes one was a 1205 intake the other a stock 1204 intake. The 1205 intake actually created slightly more horsepower and levels that were farther up on the rpm scale while the 1204 had slightly less but similar power levels but lower on the rpm scale. Even though the intakes had similar power levels the 1204 intake created lots of low rpm torque and power that was better for the street while the 1205 made better high rpm power more suited for the track. Both intakes seemed to do similar 1/4 mile times which is kind of shocking because you would assume the bigger intake would totally outrun the smaller intake no matter what. Im sure the bigger intake would eventually out flow and do better than the smaller intake when head flow increased but this is what helped me decide what I wanted.

Ive also heard that if you have an intake that has a large plenum you can create a reversion step by making the head port one size larger than the intake manifold.I guess this helps with long duration cams.

On a 400hp 350ci, what kind of hp/tq gains (on average) could be expected from port matching the intake manifold?
Don't think you'll feel the change from the seat. Just match them up exactly or make the intake very slighly smaller is ok, on a single plane you can look down in an see the match, if they misalign an there is a step up to the head or the head is smaller now ya messed up, that will cause bad turbulance an flow in that port an may cause the fuel to puddle there an hurt power. I'd let those small vortec's alone an get some good heads on that 400 if power is what your after, much easier!!!

Port matching arguably yields the least amount of gains vs other areas but it's the easiest thing for a novice to do. My first attempt at head porting as a kid was gasket matching a 283 with a dremel tool. Took forever and the results were less than spectacular but it didn't seem to hurt the performance.
I've read a few online blogs where gasket matching was attempted, but never found any scientific results to prove the gains.

Here's a quote from the FAQ archive section of Port Matching
For many years I've thought intake port matching was not worth the trouble. In fact, for some intake manifolds you can end up ruining the "aim" of the intake runner design and negatively affect cylinder fill. Some years ago I read that it was definitely not recommended to port match the Edelbrock Torker for this very reason. Looking at a Torker it is obvious that some of the runners are shaped such that a large portion of the intake port on the head is not being fed. Why would they design it that way when they can do most anything they want with a new design? To positively affect flow efficiency is my guess.

Lo and behold there is an article in this months Car Craft (see, that C*evy biased rag is good for something) where they dyno tested a big block engine before and after port matching. Bottom line - no significant difference. Something like 2 HP and a couple lbs-ft of torque. Glad I haven't wasted all that time and money over the years.

Forget about port/gasket matching your intake/heads UNLESS the intake has larger passages than the heads. Then, and only then, is it worth it to even consider this expensive (if you pay someone else) and time consuming exercise. And I don't care how careful your are, how do you know that after everything is torqued down, run and re-run a couple dozen times, that your "port match" is still perfect. You could easliy introduce edges that can mess up the flow as much as help it. Waste of time for sure on a street driven car and probably also for 99% of race engines.

[ Thanks to Bob Handren for this information. ]



Here's an online article (http://circletrack.com/techarticles/general/139_0305_intake_flow/)from Circle Track.

and one from Chevy High Performance (http://www.chevyhiperformance.com/techarticles/95518/)

Note that they also say the gains are "insignificant" in the CHP article.

However, I know pro race teams spend a lot of time lining up intake runners with head ports to make sure the flow vectors are aligned. They don't go to all that trouble because it's a waste of time.

Stock rules often allow port matching within 1" deep of the opening. There's a good reason for that rule restriction. True port alignment needs to extend all the way to the valve. If you just match the opening to the gasket hole, without considering the flow alignment you probably haven't really helped the flow. I use special aluminum templates from Brysinski Racing instead of a sloppy fitting gasket and bolts. They have close tolerance alignment dowels and are double sided to get a precision anti-reversion step, if required.


I searched out and merged together some of our threads on DIY porting subject and put it in Best of Tech.

I've read a few online blogs where gasket matching was attempted, but never found any scientific results to prove the gains.

I searched out and merged together some of the threads on the subject and put it in Best of Tech.

Here's an online article (http://circletrack.com/techarticles/general/139_0305_intake_flow/)from Circle Track.

and one from Chevy High Performance (http://www.chevyhiperformance.com/techarticles/95518/)

Note that they say the gains are "insignificant" in the CHP article.

Thanks Paul - great information (as usual) ... back to reading and glad to have some practice at least :D - and hopefully don't hurt anything by staying really conservative.

I'm not done yet. The reason why port matching on a Gen 1 SBC Chevy doesn't do much is the placement of the pushrod. The port wall curves inwards and creates a narrow spot. Opening up the port entrance doesn't help because the air still has to funnel down. In fact making the entrance larger on some heads just increases the problem. The bottom line is flow wants to be continuous not just here and there.

Ohhh man ?! ... uh-oh ... now I am just hoping I haven't screwed anything up :eek: ... really, I was conservative and have only completed the Intake ... stayed within the lines, left a 1/16 outside the gasket ... patiently waiting all of the lesson ...

... step away from the grinder ...

I think World is leaving the spark plug out when they test!
I am pretty sure my numbers are accurate and the equipment is calibrated regularly. I noticed that Brezezinski reports similar numbers to mine in stock form though they advertise 225-230 cfm for their max ported version ($2,295 assembled!)

I figure another 10 cfm is not out of the question but that means opening up the chamber and thinning the push rod wall. I'm not sure I want to go that far. World may be reporting potential flow if blue printed, not "as recieved". Variations in testing techniques and other factors may influence the results, but I feel confident in my numbers.

I wouldn't be too dissappointed in the numbers. 214 CFM can support up to 440HP.
I don't think the engine will make that much but these heads are more than capable of around 400 hp. I'll copy the ports to the rest and maybe do some fine tuning. I can retest later. I might be able to get another 3-5 cfm. I'll need to cc the chambers and see how much I can unshroud the valves without putting the SCR in the toilet.

I didn't have any time left to test the camels. That will have to be another day also, but my measurements show most camel humps have the same cross section dimensions as the S/R's. While there may be some castings that have slightly better cross sections and certainly casting variations, but unless the push rod is moved you are constrained on width.

Brezezinski reports stock unported:
'492 heads flow 206 cfm Intake and 141 cfm exhaust.
Bowtie #14011034 heads flow 212 CFM Intake and 140 CFM exhaust.
Vortec '062 218 cfm intake and 150 cfm exhaust.

So Kev's ported heads are somewhere between stock Bowtie's and Vortecs. 214 cfm Intake @.525" & 149 cfm Exh. @ .600"

Here's the cross section difference between the Sportsman and the S/R.
The sizes are "as measured" though I'm guessing the offset is about .050" towards the pushrod side. By moving the pushrod slightly the dog leg straightens out a bit.
The port is higher as well. Notice the Sportsman cross section area* is increased significantly over "stock". (*note: Cross section was calculated with Solidworks Cad program which took into account the .125" radius)

This picture is not the port opening! It's the minimum cross section area inside the port at the pushrod bend. Flow velocity hinges on the minimum area. Peak torque rpm hinges on flow velocity. According to my calculations, Peak torque will be no higher than 3,200 rpm, depending on intake manifold, exhaust system and cam timing. This will help give us an idea on what stall speed you'll want for best acceleration.

Flow data was recorded at 28":

Pretty good numbers from some truck heads. How do they stack up against other published vortec numbers? What kind of numbers are you expecting?

Kev

My exhaust to intake relationship @ .400" lift is 72% with the pipe. My average exhaust to intake relationship from .400" - .600" lift is 76% with the pipe. Here's what CHP came up with:

http://chevyhiperformance.com/howto/p177903_image_large.jpg

After comparing my real flow numbers from my Vortecs to other published flow numbers, I'm wondering if the piece used for the intake inlet could cause the differences in flow numbers?

http://highperformancepontiac.com/features/0211hpp_part2flow05_zoom.jpg

Yes, the flow guide makes a difference, however a lot of flow testers just use clay and mold it into a radius. This isn't consistant so the machined guide (IMO) is better.
Discrepancies between flow test can be attributed to many factors, including incompetance (leaving the spark plug out or not sealing the head to the stand).

There are no controls or calibration audit's on flow bench equipment like on gas pumps. You have to rely on your tester to be accurate. Most have no idea the scientific basis of flow testing and just report the figures. There is a certain amount of judgement involved. If the meter is bouncing between 240 and 250 because of port turbulence and the operator writes down the 250 figure (because it's bigger and the customer wants bigger) and doesn't report the turbulence what is that called? Incompetance? Malicious compliance? Ignorance?

Comparing absolutes is like comparing dyno tests.
You can get day to day testing variations so it's not always useful to focus just on peak numbers. It's very important to record and control test parameters so you can determine if you really have an improvement or a testing error. For example my S/R test numbers were less than the results in the CHP database. I'm not sure why their #'s were so high but I'm certain my results were accurate.
I've had it drilled into my head that you do a baseline (avg of 3), make a change (test avg of 3) and then repeat the baseline (avg of 3).
That's 9 tests for one change. It's very tedious but it's more accurate than rapid checks and multiple changes at once)

The seats and valves were re-done. Most factory grinds are "ok" but I've never seen one that passed my criteria for flow and sealing. I spend a lot of time with machinist dye to define the sealing band. As Dave is finding out from his machinist there's power in the valve job. I prefer using a Serdi machine but at home I have to make do with conventional grinding stones. It takes care, precise pilot centering and attention to detail to get it right. Every pass with a stone needs to be thought out before hand. You can always remove more material, but can't put the material back on. There is a risk of grinding too much and "sinking" the seat. This is why many old swap meet heads are such a "good deal". The seats have been ground into oblivion. SBC seats are cast integral to the head. That means you have to machine them out and replace them with new, machined seats. Not a simple or cheap operation.

When grinding the bowl near the seat it takes a very steady hand and the right tools.
I have many types of cutters, stones and sanding rolls. I use a Makita electric grinder with an AC speed controller. The air grinders turn way to fast and can jump around, which can ruin a port or badly nick a seat in an instant. I always dye the seat so I can see it and tell immediately if it got nicked.
The trick when blending the chambers is to use an old valve to protect the seats.

Here's an older picture of some of the tools I use, though I use the "dirty bench" instead of the clean bench to do the actual porting. A strong adjustable light illuminates the port. I also have a wall mounted vacuum that keeps the port clear of chips. I should weigh the chips that came off the S/R's. I'm betting it's significant.

Here's a picture of the seat from the class winning DD1.0 "Wright Stuff" heads.
The sealing band is even all the way around, square and concentric to the centerline.

Here's one before rework. Notice the chatter marks from the "factory fresh" valve grind. This was a brand new Sportsman head. These minor defects are not visible without the machinist dye which helps reveal these kind of low spots.
An interesting note on this technique. If you've ever heard of the term, "hand scraping the ways" on a milling machine, it's an old process that gives very flat and true surfaces with only hand tools and machinist dye. I adapted the technique to valve grinding in the seventies and then brought it to Roush 20 years ago. They were skeptical until I showed their best valve grinds had leaky chatter marks. The process is still being used today but nobody remembers who first introduced it.

More combustion pressure goes hand in hand with more HP. Performance engines can develop over 1,000 psi. This kind of combustion pressure will eventually find any potential leak path prematurely and usually at the least opportune time.
Exhaust seats are the worst because once the hot gases start flowing past a seat, it's like a blow torch.

Sealed Power is not just a brand name. It's a fundamental engine concept. You want combustion pressure to have no other option than pushing the piston down.

My head guy called me today with his peak flow results. His work took into consideration the limited lift & spring pressure I'm going to run with these press-in studs. Keep in mind that these flow numbers are the results of only a valve job and some bowl work. We also kept the stock 1.94/1.50 valves. The chambers and runners were not touched, thus keeping max port velocity. The actual valve and bowl work performed will remain super-secret squirrel information...his trade secrets need to remain his.

I will have the complete flow data next week, but here are the peaks. We're now looking at 237.5 cfm @ .450" on the intake and a whopping 182 cfm @ .500" on the exhaust. These things should scream on the 383 ;) :D

Here's the final flow bench results after the valve job and some bowl work:

100 - 61.86 - 50.65
150 - 93.87 - 87.01
200 - 126.05 - 111.8
250 - 158.47 - 133.98
300 - 185.38 - 150.92
350 - 207.8 - 164.85
400 - 226.04 - 171.13
450 - 237.4 - 175.84
500 - 223.05 - 178.98
550 - 221.26 - 182.1
600 - 221.26 - 182.75

heres a chart FROM THE BOOK,HOW TO BUILD BIG-INCH CHEVY SMALL BLOCKS with some common cross sectional port sizes
(measured at the smallest part of the ports)
...........................sq inches........port cc
edelbrock performer rpm ....1.43.............170
vortec......................1.66.............170
tfs195......................1.93.............195
afr 180.....................1.93.............180
afr 195.....................1.98.............195
afr 210.....................2.05.............210
dart pro 200................2.06.............200
dart pro 215................2.14.............215
brodix track 1 .............2.30.............221
dart pro 1 230..............2.40.............230
edelbrock 23 high port .....2.53.............238
edelbrock 18 deg............2.71.............266
tfs 18 deg..................2.80.............250

I found it interesting how some of the smaller cc heads would still share the same cross sectional area as a larger cc head. I assume this is mainly what will affect torque/hp at any given rpm range. Can anyone elaborate on this and maybe point out where the restrictions would be different on each head? Most seem to agree that the pushrod pinch or throat beyond will be the most restrictive once the bowl is opened to the port size and the short turn radius has been smoothed. I assume the restriction in the intake tract could be tuned to play with the rpm to get a desired power range by cross section and runner length.

Hi,

I guess I don't understand where the cross section measurement was taken.

Just my opinions here but this gets kinda complicated to explain, especially for me. To add to the confusion, sometimes just "raising" the intake will greatly effect performance.


Jeff

Can anyone elaborate on this and maybe point out where the restrictions would be different on each head?

Those measurements are all taken at the pushrod area on those heads.

found it interesting how some of the smaller cc heads would still share the same cross sectional area as a larger cc head.

this is because without using an offset rocker and/or moving the rockerstud you only have so much room to open the pushrod area up. With a head that has a larger bowl area or raised runner, you can increase the volume of the head quite significantly without opening up that pushrod cross section.

Most seem to agree that the pushrod pinch or throat beyond will be the most restrictive once the bowl is opened to the port size and the short turn radius has been smoothed.

Not always. Without going into alot of detail, any part of a port that has localized velocities of more than about 350 f.p.s. in a measured state will have difficulties with separation. This separation is more detrimental to power loss than having a "restriction" in the pushrod pinch area. The short turn area/shape is critical. In large, high rpm race engines we will have valve bowl areas that are upwards of 108% of the valve. In a live running motor anything over about 615 f.p.s. or .55 mach is going to introduce a "choke" that will limit power past that point. This will be a moving target based on cubic inch and rpm.

I assume the restriction in the intake tract could be tuned to play with the rpm to get a desired power range by cross section and runner length.

Sort of. Until head speeds reach a certain point, those areas don't pose a restriction, and making them larger/smaller isn't always an option. Just getting a cylinder head with the correct cross section measurements to meet the area needed to attain the rpm range desired for a given cubic inch is not all that needs to be looked at. That's a mouthful. It is something that needs to be considered when determining an entire combonation. Combonation is the key. Total intake runner length does play a role also. The problem is packaging. Most cars with regular hoods don't have the needed area to take advantage of the strongest wave tuning pulse. Most people also don't have the desire to have a sheet metal intake built to get the correct taper/cross section/length to fit their particular application. That leaves you with mass produced intakes that you pretty much don't have any control over.

With all of that being said, the small block chevy is a beautiful piece because of the tons of options available. You can buy something off the shelf that is going to get you great power without having to do many modifications to it. If you are class racing or looking for that last little bit, then you'll start to understand why winning race engines are so expensive. You are paying for the expertise it takes to make that last little bit to win.
shawn

I found it interesting how some of the smaller cc heads would still share the same cross sectional area as a larger cc head. I assume this is mainly what will affect torque/hp at any given rpm range. Can anyone elaborate on this and maybe point out where the restrictions would be different on each head? Most seem to agree that the pushrod pinch or throat beyond will be the most restrictive once the bowl is opened to the port size and the short turn radius has been smoothed. I assume the restriction in the intake tract could be tuned to play with the rpm to get a desired power range by cross section and runner length.


Your observation shows how port volume isn't a reliable parameter for comparison. If you had two tubes of equal length, the one with more volume would have a larger cross section. Ports are not linear tubes. They have an irregular shape with drastic cross section variations.

A fundamental problem is the need to put the pushrod somewhere that can line up with the valve centerline and not interfere with the port. GenI 23 degree heads wrap the port around the pushrod creating a dog leg. This becomes the minimum cross section. The bowl area and the port area might be substantially larger which affects the total port volume but the restriction is the air speed limiter.

Air velocity is measured by the Mach Index (Z) which is a number relative to the speed of sound. Text books list Z= .6 as the maximum (60% of the speed of sound) for efficiency. Air has mass and therfore inertia when at this speed. It's this what causes the inertial ram effect which overfills the cyulinder beyond 100% (inertial supercharging)

It's easy to make generalizations and leap to conclusions, because this is where science and common sense are often at odds.

Torque is a product of cylinder filling but peak HP is dependant on total airflow capacity.

A recent example in another thread we had two heads with the same minimum cross section but one has better airflow, we saw the torque is similar but the HP was much greater for the head with more airflow.

Hot rodders love to choose parts and specs based on choosing the one with bigger numbers.
I call this the additive approach. If I add up all the big numbers I get a bigger output.
The reality is the the parts interact in subtle ways that this often doesn't work.
By understanding how the variables interact it's possible to build a better performing engine.

I'll tell you what I see alot but don't seem to be able to make some people understand and that's over opening the valve. They buy a cam that opens the valve much more than the head can flow. I call it the bigger must be better mentality. The dirt guys seem to be better at understanding this than the drag racers but not by much. Same with picking a carb. My 468 BBC is faster with a 850 carb than with the 1050 Dominator and I run the little carb because of this. Go to the track and everyone is running a Dominator. When they see my little carb they all say man that thing would really run with a Dominator on it, but that's just not true. Bigger must be better......LOL. RM