EQUAL VS UNEQUAL LENGTH HEADERS
 

I was going thru some old files and found this testing (performed by the University of Northwestern Ohio some years ago).
They did a Test of EQUAL VS UNEQUAL LENGTH HEADERS on a Engine Dyno.
They ran at Street RPM test points vs Race RPM points.

The engine was a 525 CID Big Block Chevy but the comparison is valid.
This would cover the most common Pontiac Displacements (minus the 540/565 sized ones).

Engine rpm: The test points were in 100 rpm increments, from 3000 to 6000 rpm.
I am not going to post the dyno sheet as it was copied to an excel file and can't easily be posted.

They showed at 4500 rpm the torque difference was 715 vs 723 or 8 lbs/ft difference
The HP was 612.6 vs 619.5 or 6.9 horsepower difference. Unequal being actually BETTER.

They showed at 6000 rpm the torque difference was 610 vs 636 or 26 lbs/ft difference
The HP was 696.9 vs 726.6 or 29.7 horsepower difference. Unequal being actually BETTER, again.

So Stan, what is going on here? (Course it WAS a Big Block Chebby Engine).

Tom V.

Tom, i posted near the same findings/ results a few months ago from a David Visard BB Chevy comparison and I was kicked in the head by folks here saying that it can't be true.
The results make perfect sence to me since only 4 sets and 4 sets of Intake ports due to there differing length will at the same rpm given a signal plane Intake , no less a duel plane!

Tom,
I was not at PRI this year but from what I was told someone from Burns gave a talk on this subject. From what I was told the equal length is less important than getting the tube / bends correctly. Maybe someone from PY was there and can give as more details.

Stan

Anyone do a header test like this with a 4 pattern camshaft, or simply changing the firing order around? 4-7 swap?? LS firing order??

Far as I know, the 4 pattern cams and LS fire cams are only available for SBC and BBC and SBF.

4-7 lobe swaps are available on 55mm cam for Pontiac. Not with std journal. 50mm, not sure.

A little off topic but if you have a custom set of headers that are clocked properly and then you change a firing order can it effect the performance?Tom

In some of Jim McFarland's writing he mentioned that unequal can be better because each cylinder has a different rpm peak based on length and it helps average the total out better.

Before I built my headers I did a heap of research on this and spoke to some local header builders on the subject too, they all said length is irrelevant when you're using a collector (moreso if a choke collector) but what is important is to promote "Swirl" in the collector by tuning the firing order into the collector. Clockwise or counter clockwise didn't matter, provided that it swirled. Anyhow, I followed that logic and my 1.875" primaries on my 400 seem to work really well. Car runs a reasonable mph for a 400 and is still pretty good on the street.

In a round about way that's what I was asking.

They are available standard journal. My father is running a Lunati 4-7 swap in his.

But my comment was more related to BBC, SBC that the test was done on because those engines also have the LS firing order cam core available as well as the 4 pattern camshaft available, both of which aren't available for Pontiac that I'm aware of, and since the test was done on a BBC anyway, it had me curious. Just seems to me that would make a significant difference in header choices.

Tom, Rob McCabe is right up the street from me. Getting older now but still building headers as far as I know.
http://www.mccaberacing.com/contact-us.html

I've worked beside him while he was building headers for some of the Pro Mod cars I was painting. Guy is a wealth of knowledge. Matter of fact before he settled here in AZ permanently he worked full time for the Nascar teams. His headers were part of a package that was winning so many races at the time that other teams tried to hire him, find trade secrets etc... and became a real cut throat deal. In the end he got tired of all the traveling, retired from Nascar and now builds headers for anything. Some of the best work I've ever seen. Next time I'm up there I'll pick his brain about this subject. I'd bet he could explain it to me in a few sentences what will work and what won't.

First Guess: "Equal length" means nothing if it's the wrong length.

The exhaust gas pulse in the header tube has to resonate in sync with the exhaust valve closing, so that the resonation pulls a deep vacuum in the combustion chamber just as the exhaust valve shuts--trapping the vacuum which begins the pull of the intake charge into the cylinder. It's the miracle of Valve Overlap.

So the whole mess depends on about two dozen variables, ONE of which is the tube length.

Not surprising to me that unless a LOT of processor cycles went into the header design AND cam timing AND combustion chamber volume AND RPM AND... AND... AND... AND...that focusing on one of the variables results in unexpected performance.






I read--many years ago--that when Honda was building Formula 1 V-12s, four cylinders were tuned to one RPM band, another four were tuned to a higher RPM band, and the last four were tuned to the highest RPM band. This supposedly broadened the torque curve.

If you look at the Ford stuff, the intake and exhaust systems are tuned based on what each cylinder wants which is why we spent so much time doing dyno testing. Course I never told you that. The Boosted stuff is a whole different game with a Turbo. Belt S/C stuff, still tune for each cylinder. We did a boosted Harley Truck one time, it had a special exhaust system that sounded like a Harley Chopper at idle, lol. "potato, potato, potato", LOL! Post up what Mr McCabe's thoughts are on the subject.

Tom V.

Anyone built headers from the Pipemax dimensions?

Length don't matter with 4:1 because 4:1 a wrongk architecture. Maybe a focused/ "merged" 4:1 Collector will show length preferences.

Tri-Y or SD-421 style and the lengths will matter and yield better acceleration (wideband RPM). I'd like to see TRI-Y with merged Collectors...cast?

Then there is those open Zoomie where they play at 6000-9500 RPM.

GOAL: all 8 CYLs showing same HP within 1HP of each CYL, proving uniform inhale.

The cam I just received from Bullet cams is a 4/7 swap. The blanks are OTS even if the grind is custom.