Generally it would be true to say that with a well tuned combo and compression that will support pumpgas you can typically see 1.9 to 2 hp per cfm. With higher racing compressions I have seen as much as 2.2 per cfm on raised runner 23 degree heads. That seems to be a pretty good number for race engines.
It does depend on where you see that cfm increase. If you dont see it until the higher lift numbers it probably wont make as much difference as if you see an across the board 5 cfm increase. And you probably wont see any difference if the increase comes at a lift that is above what you cam has for lift.
2 hp per cfm is a good pump gas mark but it's out of the reach of most people who are just picking very complimentary parts from a catalog and bolting the stuff together. it takes a bit of development work to get that from a streetable every day driver. most will be closer to 1.8 hp per cid and i believe that to be a much more realistic goal for the do it yourself guy with sound building skills and good machine work. and yes you can make 2.2 hp from raised runner 23º heads but for those considering such a build, unless rules restrict them, you probably should have went with a more race oriented head in the first place cause then you could make 2.5-2.6 hp per cid or make the same power with a less aggressive camshaft profile.
Originally posted by Dave69Z Could you explain what is the port area rule?
The short answer is that if velocities anywhere in the intake port exceed more than 0.55 to 0.60 the speed of sound (about 690 ft/sec or so), the air has a tough time accelerating past this velocity every cycle and you get an "inertial block" on the mass flow, as Vizard calls it, when airflow into the engine and power take a dump. The research on this was done by C. F. Taylor, a while ago. Taylor and Taylor wrote The Internal-Combustion Engine textbook.
The minimum port area, when the piston is at it's max velocity (75-80 degrees ATDC) and valve lift is past 25% of the valve diameter (or about .400 in.) isn't the valve curtain area, but in a typical SBC head, the area where the port passes the pushrod.
With some empirically developed constants based on type of camshaft (race roller, flat hydraulic, etc), Vizard published a few formulae for determining limiting port velocity (LPV), or min port area or max power rpm based on min. port area. Bore^2 and stroke (displacement) are part of each.
A larger displacement engine should therefore reach the limiting port velocity at a lower rpm than a smaller displacement area if there is a limiting port area in the heads used. It's a mostly linear relationship with displacement.
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383 LT-4 and a few other goodies
You will gain 10.28Hp with a 5CFM increase,however you get it.
Maybe and maybe not. If you do something to disrupt the wet flow chacteristics of the port you'll hurt power... don't care if you gain 10-20 cfm. I can name some examples I've been involved with that did just that.