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Discussion Starter · #1 ·
You often hear people say their heads flow 500hp.
What I dont understand is how you can derive a number from air flow alone.
I mean you can justify saying these heads flow 290cfm at .6" inlet lift andthat they have a 75% exhaust/inlet ratio etc etc.
But thats where its stops without further qualifiers surely.

The power is from BTU or the thermal energy of the fuel mix.

A set of heads flowing 250cfm may make 300 hp or 500 hp dependant on the fuel and comp ratios etc or do I have this AAF.

Why dont head builders only quote flow numbers???
 

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The HP numbers the manufacturers quote are just hype to sell the product. Edelbrock claims the Vic Jr small block heads "are good for 500HP" and AFRs web site has some real inflated dyno numbers for their heads.CFM listings are also suspect as they are not always checked using the same equipment or criteria.Magazines also have pie in the sky HP numbers which depend on how much advertising money the manufacturers spend on their ads. You just have to decide who to believe I guess.
 

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I think what they are trying to get at with the HP ratings is, if the heads are rated to support up to 500hp, and you put them on a combo that should be making around 600-700hp naturally aspirated, then the heads might be a little too small. And also, if you're building a 300hp engine, the heads rated up to 500hp might be a little big for your application. Basically it's just a guildeline for the average idiot who is shopping for heads, but knows very little about them.
 

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Discussion Starter · #4 ·
There is a direct connection from flow to power. If you cant get the mix in you cant make the bang.

Now a stock set of heads will flow all sorts of numbers dependent on what cam you run until you reach the flow limit of the smallest part. ie if the valve hole flows 200cfm but your inlet or carb only flow 150, then 150 is all you get (theoretically).

I was talking to a guy last night who does up heads. He says the rule of thumb is...
cfm x .43 x #cyls = HP
He showed me some engine dyno numbers and his head flowbench charts and that equation is pretty much spot on.
So a set of D3VE heads flows 160cfm approx at .6" lift inlet, so all things being in tune with that ie exhaust around 75% plus of inlet, no intake restrictions etc, then you will make 550hp from the heads.

Has anyone got any data to compare this to??

ie heads flowed x cfm and engine made xxHP??
 

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What I have studied in the last months from the Internet and magazines is that for a hot street engine the formula for a V8 is :HP= 2 x intake flow at 28 inches of water at max cam lift.Naturally this is only a rule of a thumb but it really looks like it is close enough for a street guy.For example Ford 392 crate puts out appr 460 horses with GT 40 heads with the intake flow of appr 230 cfm.When those heads were changed to AFR 185 which flow appr 275 the power jumped to 537 hp.Close enough in my books.
 

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Discussion Starter · #6 ·
What valve lift is that with???
Also is there an exhaust % available?
Sounds like good math there but also a quite a "big" motor specs wise.

Anyone else got similar number comparisons.

Another interesting thing about the .43 thing is that he has a 6 cyl hemi head there flowing 133cfm at about .5" that makes 330hp

Your 392 crate motor should only flow 133cfm according to the .43 rule which kinda throws a spanner at it.
 

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i got some flow figures on my out of the box windsor snr's and manifold as an assembly, they flowed 215.7cfm @ .600 lift with a hp potential of 440hp(the heads alone flowed 255cfm @ .600 lift),it made 295rwhp which equates to around 400hp flywheel

the engine is a 347 with 10.1comp and 248 @.050 560lift solid flat cam 1 3/4 headers quad webers,the heads are curently being ported to flow 300cfm with a potential hp of 600hp(so they say)and the manifold is being ported as well,once it is ported they tell me the manifold will have a 5% loss/restriction so my(head/manifold) potential hp will be about 580hp should be interesting to see how much hp it actually makes
 

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Discussion Starter · #8 ·
440 hp for 215cfm is around the .26 constant I got from the network54 385 series BB. It was from Scott J of http://www.reincarnation-automotive.com
Seemed to be more valid when looking at stock D3VE heads and HP of std 460.
I'd be interested to know what your results are after porting. What inlet/exhaust ratio are you going to get. Thats a big limiter too.
580hp would be interesting in a street car
 
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