for a v8 it is 2hp/cfm max
No, not max, but it is a good rule of thumb for a very hot street/light but serious race car.
Don't get fooled into numbers. I have heads that make 2.4 hp/cfm.
Put a 300 cfm head on a 9:1 engine. Put the same heads on a 15:1 engine. There are many things that affect how well an engine consumes air.
^^This^^ There is no hard and fast rule, too many variables. I've seen 2.2 on a well thought out pump gas stroker.
X's 2 on Justin & yellow roses post.
Another thing about head flow numbers is that they will change not only with compression changes but also bore size changes.
To nut shell it is difficult and leaves way to much on the table as far as information goes. With a head that flows 300 cfm, in theroy there should be enough there to make 600hp. That would not be a max power output. But that cold be pushing it for a given cubic inch size and/or pump gas fuel.
It is most important NOT to get hung up on a lot of things like head cfm flow. While it is a good indicator of the possible performance it has, a well flowing set on top of a bad combo will create a crappy performing engine.
Do remember there are racers out there in very restricted classes turning out magnificent ET's and trap speeds with stock or mildly ported heads. The focus should be more on execution of what should be an excellent chassis and optimized for the task at hand, be it track or street.
For an interesting look at such, look up member Locomotion and check out his car & site. He is not making a ton of mad house power yet he can clean a lot clocks with there higher dollar and modified parts.
