[David Hodgson]

The formula Dwight used is probably this; CFM = (Cubic Inches x RPM) divided by 3456) x Volumetric Efficiency

example 1; standard 302 with factory exhaust/intake etc revving to 5000rpm would have a VE of about 80%. In the formula (302 x 5000)/3456 x 80% = 349.5 cfm, hence the 2 barrel Carter carby used for non performance cars.

Example 2; a strong 302 with aftermarket headers/intake, better camshaft, some head porting, etc revving at 6500 would have a VE up around 95%. Thus, (302 x 6500)/3456 x 95% = 540 cfm. Therefore a 600cfm would work fine, in fact I am using a 600 DP Holley, quite happily.

A bigger cam and major port work, tunnel ram and 7500 could see about 110% VE and would obviously flow more CFM.

The CFM figure is how much the engine is drawing in, so for best results not to cause a restriction, go at least one carby size up. Checked on a dyno, you would also find that several sizes up could help top end power but you would see a drop in bottom end torque, how noticable these gains/loses are under foot, would depend on factors like gearing and weight of vehicle.