How does one normally make a direct comparison of two head flow tables when one is done using a 4.060" bore and 1 3/4" exhaust pipe and the other a 4.125" bore and 1 7/8" exhaust pipe, assuming that both heads would be used on a 4.060" bore & 1 3/4" exhaust pipe?

I have an M.E. degree and it gives me tired-head just thinking of doing this one on my own. I don't want to re-do perfectly good work if it is already done.

(P.S. - This is AFR LT4 website data I am talking about.)

 

Rectal extraction conversion comes to mind.

"tired-head" is a good term. I haven't heard it before. (Old time ME here)

Seriously, if Lloyd Elliott (Nighttrain 66) or Larry Meaux (maxracesoftware) sees this they might have some good input. Especially about AFR head numbers.

 

All depends on the head too... Some heads don't change flow numbers from a change in bore that small other heads like the AFR BBC heads have a drastic change in flow from 4.250-4.500" bores.

Bret

 

Not only can I not help ya, but it brings up a simpler question that I have. (Just shows ya what kind of guy I am)

When computing intake to exhaust ratio's..... Do you use exhaust flow numbers w/o using a pipe or not? It changes the ratio by about 5% and my assumption is that you would NOT use flow numbers using a pipe andor compensate in some way.

 

Well I have an opinion on that.

There is not much of a pressure differential between atmospheric air and the other side of the intake valve. There is a large pressure difference between the two sides of an exhaust valve (post combustion).

Having never once been paid for my engineering knowledge post-graduation, I am having to draw upon my IC engines class of 6 years ago. Without opening my Heywood book, that is my answer.

 

Youngster.

We used C.F. Taylor's stuff 6^2.06 years ago when I took IC engines.

 

It effects the high lift numbers the most to start, and the average is changed less. The highest pressure going out of the exhaust valve is right when it opens too, so the low lift on a exhaust side is pretty important.

The valve curtain area isin't changed when you add the pipe on the exhaust so that part of the total E/I% is not effected.

Should you flow with a pipe? Yeah you should because the motor sees that. In fact you should flow with the header on the head but that's not easy to fit on the bench. Not many places flow with a pipe let alone the whole dam headers.

Bret

 

if you have Flow developed a Intake port for a certain Intake valve size, valve location relative to Bore and Location , and have maxed-out that combination,...
then its very possible to see less Mid to High Lift Flow by putting that same Cyl Head on a larger Bore diameter.

Flowing the same Cyl Head on larger Bore sizes doesn't always automatically increase the Flow Numbers/Flow Curve .

the general "trend" is increasing Bore size = increasing Flow Curve,
but not an always 100 % Rule to go by.

the best you could expect is to assume the same Cyl Head would now flow
a little Less CFM on the 4.060 Bore w/ 1 3/4" Pipe

its very hard to compare 2 different FlowBenches ,
much less 2 different FlowBenches with also differing Bore and Pipe sizes.
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Exhaust Port side =>

its almost 28" inches of water = 1 PSI

if you only have 100 psi left in the Cylinder pressure at the time the Exhaust Valve cracks open...then for a brief time an Exh Port that Flows 200 CFM at 28" will Flow approx. = 1058.3 CFM at EVO

FlowBench
Intake Port Flows = 267 CFM @ 28"
Exhaust Flows = 200 CFM @ 28" = 75 % PerCent Exh/Int Flow Ratio
but exh flows 1058 CFM at EVO ..about 4 times more than what the Intake Port flowtested.

 

Wow never realized it was that high. Makes sense to go to a stronger bench then, add on top of that what tuning pressures could do to the intake PSI and now I really understand the high vacuum bench idea.

Bret

 

I have never seen anything more than 2-3 cfm at peak from using a 4.060 to a 4.155 bore. I have seen over 20 cfm on a BB Chevy head from using a 4.310 to a 4.500 like someone earlier mentioned. I think it is more the fact that the BBC chamber is shaped in a way that they over hang the bore and the larger bore helps this out. The SBC heads that I have tested fit fine on the 4.060 bore and so the 4.155 doesn't correct any big OVER HANG like on a BBC, it just allows a lil less shrouding.

The exhaust pipe also eliminates that 90 degree turn from the edge of the exhaust port to the header flange. This is a pretty drastic change right after the port makes the turn from the shortside and all of the air has not "collected" with itself after making that turn. A 6" strait pipe will ALWAYS help flow #'s on a bench since it allows all of the air time to collect and straiten out for a while before exiting the pipe.

A truer test would be to use the header that you are gonna run on the car that has a 90 degree turn a few inches after the header flange. The strait pipe is porobably more of a true test than comparing flow #'s with NO pipe and from cylinder head to cylinder head but unless you are running a chassis car or ZOOMIES (LOL), you are not gonna have a header that has 6" of strait pipe after the header flange.

It is just tough to compare head from different benches and different test methods. There again, if you could compare them, what are you looking for ???? How does the port react at a MUCH higher depression? what does it sound like? A person like Larry Meaux or a shop like Air Flow Develoment can have a MUCH better WORKING port that might flow the same as someone elses head BUT be designed like it should be. The right cross section in the right places, the right velocity in the right places, etc. His stuff could be 20 HP better than a seemingly better flowing head because he knows what to look for and what he wants the port to do.

That is why he charges what he does and is SWAMPED with work. He DELIVERS the technology that alot of other places talk can only about, LOL. I am still learning and my stuff is steadily getting better but a guy like Larry Meaux is light years ahead of me. The 28" flow #'s might now show it but track is the true testing place. At the rate at wich I am learning, I would be FOOLISH to think that I know it all and have nothing else to learn.

Not everyone needs (or can afford) a set of head from Larry or AFD and that is why I stay so busy but the product that they offer is a step or 2 above most of us and 3 or 4 steps ahead of some.

Lloyd Elliott
972-617-5671
Eportworks.com

 

That's why there is the money difference that there is. It's hard to beat Lloyd for bang for the buck, and on the same end it's hard to beat Larry for Knowledge at the price he charges.

Bret

 

Thanks Bret and Lloyd !!
but i'm still learning everday something new about Head Porting..looks like its a neverending subject ?..always something new pops up or discovered.

like from a Quote from a Movie i saw ,
"Enough of me, ... how about you ? "

Bret , you and your Dad (OldSStroker) are 2 very sharp people here on this Forum taking your valuable time off from Work and freely helping others here

and Lloyd, everything i've heard about your Port Work and work ethics were excellent !!
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FlowBench
Intake Port Flows = 267 CFM @ 28"
Exhaust Flows = 200 CFM @ 28" = 75 % PerCent Exh/Int Flow Ratio
but exh flows 1058 CFM at EVO ..about 4 times more than what the Intake Port flowtested.

Forgot to mention or didn't want anyone to misunderstand my previous Posts
that if there's 100 psi at Exh Valve Open point, then it would be similiar to a Test Pressure of approx. 2800 inches of water
and at 2800" ..so if you had an exh port that flowed 20 cfm @ .050" Lift its possible for a very brief time it might flow 200 cfm @ .050" ?? depending of temperature and speed of sound in that region.

the 1058 CFM was estimated from .70 % of max lift to near max lift starting from the 200 cfm Exh Flow baseline, from some observed Cylinder Pressure psi readings i've seen