CFM vs HP How does it work?

[Jesse/DVK]

Been searching online to get my head around this.

When looking for heads most of the time you see the CFM figures advertised. How do they measure it? Most of time the time it states @ 25 or @ 28. What's the difference?

With the superflo formula it is possible to estimate the max HP a set of heads can deliver. For example 180 CFM = 185 hp. But that depends on more parts than only the heads..

Can someone explain to me what to look for when buying a set of heads? Is it CFM/size of ports/ shape of ports??

[dragvw2180]

 It is all that and more.  Decide first whether this is going to be a street car, purely a drag car, or a mix of both, N/A or turbo ?  Drag racing only heads generally do not have as many cooling fins ( Super Flow , Comp Eliminators ) and are not really easy to use with sheet metal. Generally the bigger the engine the more Cubic Feet Minute of air is required ( CFM) along with maintaining air speed , also the more RPM's the better the heads needed for your application. Huge heads on an engine not capable of using the airflow will be a waste of money and very disapointing performance wise . When you see the numbers you were asking about the heads have been run on a" flow bench" to measure how much air is passing through the intake port. There is also a ratio of intake to exhaust flow that you may want to compare also, and it is expressed  like 250 @ 25. Do not buy the heads on flow numbers alone, but get with a knowledgeable engine builder to come up with a combination of parts to achieve your goals, CID, compression, cam , heads, ect... that work together. Here are a few links to look at, Mike

http://www.superflow.com/Flowbenches/sf110_120.html
http://www.flowperformance.com/
http://en.wikipedia.org/wiki/Air_flow_bench

[Speed-Randy]

What he said

[jamiep_jamiep]

Is 'cfm' just used as a measure of how easily the air/fuel mix can enter the combustion chamber? I would imagine that whether or not the heads would ever pull a certain cfm would be down to a lot of other factors such as intake arrangement, cam timing, valve lift etc.

A certain size combustion chamber would I imagine only hold x amount of air/fuel at any given time (at atmospheric pressure at least), so is it the ability of the head to allow the charge in at a given flow rate that means the pistons have to expend less of their energy in the downward stroke 'pulling' that charge through the 'bottle neck' of the port/valve?

So, how does port velocity come into play? Does this just help in keeping the fuel air mix as a suspension or? As port diameter increases and volumetric flow rates in the port increase, is it inevitable that port velocity drops off, for a given cylinder volume and rpm?

It all seems like a black art to me and its a subject I'd love to know more about!

[dirk zeyen]

hi jesse,

search for apfelbeck......