Head CFM vs. HP per c.i.?

[Nastytls]

Is there a standard of how much HP you should be able to achieve given the amount of CFM your heads flow? Meaning, if your head flows 300CFM you can achieve (x) hp ci.

[Ohio HD]

It's all dependent on everything else being maximized and optimum as well.


http://www.wallaceracing.com/calcafhp.php

[kd]

   What he said. I think it's better to think in terms of what it will "support" and not what it will "produce" because the other components team up to help the "produce" part.

[Don D]

Assuming what a hurricane head that flows 372 @.7 lift 28" at 124" VS a CVO head that flows 330 similar conditions, all parts being the same and assuming a proper tune will the bigger heads make 12+% more power?

[No Cents]

   too many unknown variables to give a positive answer to the OP's question. Calculators can put you in the ball park...but they are not an exact science.
   Unless you can find the same exact combo you are questioning...the answer would be to dyno the combination you have in question to see what the results will show you. Changing one simple part in a combo can change the outcome you will see on a dyno.
   A good set of flowing heads won't do crap for you if you don't have the other needed accompanying parts to go with it. A good induction system with a good exhaust are critical to make a good set of flowing heads work up to their potential.

[Hossamania]

It also depends on who is making them flow 300 cfm. I bet with a little practice, I could get 300 cfm flow numbers.
But I'm pretty sure they are not going to work very well, no matter what combination of parts you use!

[Nastytls]

  What he said. I think it's better to think in terms of what it will "support" and not what it will "produce" because the other components team up to help the "produce" part.

OK, so I think you have verbalized it better than I did. The above posted calculator kind of does what I was thinking. According to it, 300cfm will support approximately 1.2HP per ci. in a 2 cylinder engine.
What made me think of the question was the amount of CFM the 4valves are flowing vs. the power they seem to be producing. The twin cam heads that are flowing these large numbers are generally the real high HP builds

[K4FXD]

Velocity is important. The 370 head will support more HP but it might only make power starting at 5000 RPM. The 320 CFM head will make less HP but might start making power at 2000 RPM.

That is a very generalized description cause I'm sure a Head Porter will chime in and tell us his 370 CFM heads make good velocity starting at 1500.

[Matt C]

Velocity is a function of the port's cross sectional area (CSA), the displacement of the cylinder
it's feeding and the targeted RPM range.

That formula only tells you how much air you have to move to achieve a certain HP. It doesn't
take into account weather you move it through a 2" opening or a 2' opening. Moving the right
amount of air through the correct CSA is the key.

[Matt C]

Additionally, if the engine for example, demands 250 CFM, putting a head on it that delivers
350 CFM, will likely hurt performance b/c the CSA will be too big to maintain a high enough
velocity through the runner.

The engine will theoretically be able to deliver more HP with the bigger head, but it would likely
be an RPM that would not be attainable or one that would destroy the engine. And it wouldn't have any TQ
until the velocity got up to around 250-350 ft/s.

[1FSTRK]

Is there a standard of how much HP you should be able to achieve given the amount of CFM your heads flow? Meaning, if your head flows 300CFM you can achieve (x) hp ci.

It is a yes and no answer. This is why large data samples are so important, the best number you can find for an engine family is the standard at the time. This is one reason I have posted so much on the M-8 engines not living up to their potential.

The ratio of air to hp gives you an indicator of the over all efficiency of that engine so when the CFM of the heads is to high in relation to the hp it tells you something is wrong in the combination, the hard part for most people is it does not tell you what is wrong.

[Deye76]

"Velocity is important. The 370 head will support more HP but it might only make power starting at 5000 RPM."

Guy I know has Hurricane heads on a 120" @ 12.4:1 cr, says it's a PIA to ride under 3500 rpm. He is rapidly learning that bigger isn't always better.

[rigidthumper]

I had a local put MVA heads on his 103 with 999 cams and wonder why it was a slug!

[Don D]

  What he said. I think it's better to think in terms of what it will "support" and not what it will "produce" because the other components team up to help the "produce" part.

OK, so I think you have verbalized it better than I did. The above posted calculator kind of does what I was thinking. According to it, 300cfm will support approximately 1.2HP per ci. in a 2 cylinder engine.
What made me think of the question was the amount of CFM the 4valves are flowing vs. the power they seem to be producing. The twin cam heads that are flowing these large numbers are generally the real high HP builds

How do you know 300 cfm will support 1.2? That engine, the air pump, may be 88" or 131" for example.  It has so much demand at the VE and peak rpm dictated by the rest of the system forming both individual and a system constraint.
Velocity can be your friend or enemy. Too much and there are pumping losses and a very hard to tune motor. Too slow and the ports are too big to feed the pump, result reversion low VEs, and high brake specific fuel consumption. All total a very inefficient engine. A larger slower port is more easily dealt with especially with an efi fed engine.  Harley just did this clad in emissions legal trim. It's called the m8.
Oh by the way what about the exhaust side? Some of the best of harley offerings have an awful handling of the waste, just saying.

[Nastytls]

  What he said. I think it's better to think in terms of what it will "support" and not what it will "produce" because the other components team up to help the "produce" part.

OK, so I think you have verbalized it better than I did. The above posted calculator kind of does what I was thinking. According to it, 300cfm will support approximately 1.2HP per ci. in a 2 cylinder engine.
What made me think of the question was the amount of CFM the 4valves are flowing vs. the power they seem to be producing. The twin cam heads that are flowing these large numbers are generally the real high HP builds

How do you know 300 cfm will support 1.2? That engine, the air pump, may be 88" or 131" for example.  It has so much demand at the VE and peak rpm dictated by the rest of the system forming both individual and a system constraint.
Velocity can be your friend or enemy. Too much and there are pumping losses and a very hard to tune motor. Too slow and the ports are too big to feed the pump, result reversion low VEs, and high brake specific fuel consumption. All total a very inefficient engine. A larger slower port is more easily dealt with especially with an efi fed engine.  Harley just did this clad in emissions legal trim. It's called the m8.
Oh by the way what about the exhaust side? Some of the best of harley offerings have an awful handling of the waste, just saying.

"I" don't know, I said according to the posted calculator that's what it will do.

[Don D]

OK my point is CFM/hp at 2 cylinders is not enough information to come to any decision. Bore, stroke, VE all are important data needed. This is where the software calculators such as dynomation come in and do a good job of helping calculate the engines demands when the user has some real world experience to temper the results. Also the software included in the flow bench programs helps calculate velocity based on CSA. Or you can calculate that long hand.
If this is of interest I suggest a book
Engine Airflow, A Practical Guide to Airflow Theory, by Harold Bettes ISBN-10: 1557885370

[Thermodyne]

Head flow can be used to calculate volumetric efficiency.  That's one of the numbers you'll need to plug into any good HP calculator. 

VE =  3456 x CFM / CID x RPM

[Don D]

Well not exactly
That calculation would be how much the air pump moves at 100% VE
Problem is VE is a factor and varies according to the efficiency of the whole system. An average Harley is not running that high, 100%. A well healed LS chevy new story, a F1 better yet

[prodrag1320]

Having a build with parts that compliment each other,with the proper compression & quality headflow is way more important than CFM amount

[1FSTRK]

Head flow can be used to calculate volumetric efficiency.  That's one of the numbers you'll need to plug into any good HP calculator. 

VE =  3456 x CFM / CID x RPM

I believe this formula is used to calculate VE at a given rpm when using air turbine data from a dyno run.

[Matt C]

Having a build with parts that compliment each other,with the proper compression & quality headflow is way more important than CFM amount

As long as the air speed in the intake runner is sub-mach, you're good. Velocity is 10X more important than CFM .
It provides a "ram" effect which can cause VE to exceed 100%. "Ram tuning" is far more effective in filling a cylinder
than the piston pulling the charge in.

[pwmorris]

Having a build with parts that compliment each other,with the proper compression & quality headflow is way more important than CFM amount

As long as the air speed in the intake runner is sub-mach, you're good. Velocity is 10X more important than CFM .
It provides a "ram" effect which can cause VE to exceed 100%. "Ram tuning" is far more effective in filling a cylinder
than the piston pulling the charge in.

This^^^^^^^^^^^^^^^^^^^^^^
That's why I shake my head at "best" cam threads, "best" pipe, etc, as the correct combo (with many variables involved) working together as ONE, always produces the best power and achieves the goal of the owner/builder.
This is also why top race teams, and top street builders and porters consistently make more horsepower than the rest-they know (don't guess) what works and what doesn't. 
My street heads flow well over 400, but that ain't why my street motor makes good power.....it's the head and intake velocity, and the correct combo of parts from valve train all the way down and out the pipe-planned in advance based on the goals of the build.

Some great learning on Speedtalk, as many top pro head guys share knowledge and experiences with similar principles that apply here....
https://speedtalk.com/forum/viewforum.php?f=1

[Matt C]

Jessel rocker/S&S pro stock top ends and SA cases aren't really standard fare around here.
Apples and oranges. But I get the point.

Just thinking out loud...

[build it]

I'm'a put my waders on.

[motolocopat]

Interesting conversation.
My youngest son, 17, has an interest in being a Mechanic and the other day he stated something about Air Flow being important, got me to thinking he's actually read/studied some on the subject. He's not the best at the righty tighty lefty loosey stuff but he's pretty good on Math etc and I've thought he'd be better off more on a design/Engineering path  . . . anyhow I've sent him an email directing him to this thread and bought the book referenced on the matter, and another on Engine Formulas.