What is the true relation of head flow to the max HP an engine will make? I know the rule of thumb is two times the intake flow is your max HP the head will support (ex. 350int = 700hp). But, can a cylinder head flowing 350cfm make more then 700hp? What other factors could greatly effect the end number? Just wondering...

I have had several different BB stroker motors make more than 2.0 HP per CFM, one on pump gas and one on race fuel
That adage may have applied many years ago to N/A motors, but not today with all the improvements in other parts, especially with power adders

I made 2.25hp per CFM...with pump e85

If you use that formula with your size motors C.I. then it gets harder to make 2.0 HP per C.I. My pump gas 518 C,I. street stroker motor made a best of 775 HP with a set of CNC ported 440-1 heads that flowed 370 CFM, my 526 C.I. race gas bracket motor with the same heads and carb. made 845 HP missing due to the header flanges being to small for those heads It might have made another 5 to 20 HP , not near enough to get 2.0 HP per C.I.
I've built and dyno tested several roots blown 426 Street Hemi type motors that both made over 2.0 HP per C.I. at or above 10 lbs. of boost

Sure, you can make more that 2 hp per cfm if the heads are really good. You can also make a lot less than that if there are other problems.

My 514 dyno mule made over 900 hp with EZ heads that flow in the 360 range so that was about 2.5 hp/cfm. But that was getting close to a max effort combo with 15:1 compression, Q16 gas, dry sump oiling, 0.900 lift cam, etc.

I think the Pro Stock boys get close to 3 hp/cfm with 1500 hp and 500 cfm heads. Most bracket engines are lucky to hit 2 hp/cfm but it can be done if the parts are properly selected.

My 470 dyno mule is making 720 hp with OOTB Trick Flow heads that flow around 330 cfm. That is on pump gas with 10.6 compression. If I was running 13:1 compression and race gas the engine would pick up even more power. So that engine is 2.2 hp/cfm but once again, most bracket guys aren't going to duplicate that engine. It has a belt drive setup and a ported intake and an aggressive solid roller cam. Most bracket guys will dial it down a notch or two to make it more reliable.

Mine flow around 380, motor makes around 900.
Doug

I think modern prostock heads flow well over 600 cfm today.

I have 8 Second Time Slips at 3100 lb..in 4600ft air....I bet math is pretty close

When it comes to math on race cars and performance formula's I don't take any thing for granted, only what two or more time slips say. Same thing on dyno pulls, two or more pulls within 1.0% with no changes
I have slowed more than one race car down to making bad decisions on parts and tuning

I thought the 2hp/cfm was based on 10:1 compression engines with a cam sized to flow the air and rev the required RPM per engine size to use all that air?

I don't know where the 2 HP per CFM came from, probably some automotive writer I remember years ago when Chevy got their 1957 F.I. 283 C.I. Corvette motor to make 1 hp per C.I.,that was a earth shaker for a production motor Mopar was making around 365 HP with the Chrysler 392 C.I. Hemi dual quad solid lifter motors in the 1957 luxury 300 letter series cars

Thanks guys. I should be getting my new motor in the next week or two. Little worried it's going to be a big dummy. Big cube small head combo.

Big engine with small heads causes a different problem so the 2 hp/cfm rule might not apply. There is also a velocity rule that has to be obeyed. If the velocity gets too high or is too low then the engine won't make 2 hp/cfm. Big engine with small heads has high velocity while a small engine with big heads has slow velocity. Either too fast or too slow will kill off the performance.

there is no issue at all with having big heads on a small motor... as a matter fact you can't get a big enough.. Bob books small block mopar Motors at 400 cubic inch... have heads flowing close to 500 CFM....it is all about rpm ...and at 3hp per cubic inch.. I would say very impressive

Have any of you used the on line calc to see the air speed difference from 350 cfm to 400 cfm?I don't think you would lose 10/15 hp even on a bigger flow.A lot of that flow is above .700 but the lower numbers will still help a smaller cam/engine,rpm.

I took a look at the Aussie article WhiteDart. Good God 10,000 rpm makes me cringe.....

Bad for me... the time line looks like I'm going to get this motor just in time for the tracks to close and cold weather move in. I'll get to lean on the fenders and wonder all winter

WRT "there is no issue at all with having big heads on a small motor"

Ford made that mistake with the Boss 302, which won't pull the skin off a chocolate pudding. The cure: 351" by stroke increase.
Harley-Davidson did the same thing with the K model in 1952: a 750cc twin with 1.56" intake ports, 50% larger than needed. The cure: 900cc by stroke increase.

Missing from the "cfm" rule: "as long as the minimum air speed in the required RPM range is about 200 f/s". Adjust the minimum depending on the application (LSR can be slower than oval).
A 3" sewer pipe flows a lot of cfm, but useless on any automotive engine.

I love your calculated responses Polysheric. You don't get enough credit here. Would love to talk with you about my stuff some time.

what did you gap your points at on your two examples.haha.. I don't think they had ignition valve spring or cam technology to take advantage of the heads