I use a slightly different method of calculating CFM- - - - -since it takes two turns of the engine to get a 4 stroke cycle on all the cylinders, I plug in cubic inch displacement times 1/2 the desired RPM- - - -which gives "cubic inches per minute". Divide by 1728 (cubic inches in a cubic foot) to get "theoretical" CFM. For a more realistic guesstimate, use 90% of that number to account for less-than-complete cylinder scavenging on the exhaust stroke.
Example- - - - -
350 cubic inch Chevy V8
6K RPM
350 X 3000 = 1,050,000 cubic inches per minute, or 608 CFM theoretical volume
608 X 0.9 = 547 CFM- - - - -now- - - -why does somebody need a 750 CFM carburetor (or bigger) on that engine?
Jerry
This is exactly why a spread bore 4 bbl. is so effective on the V8, small primary bores for great idle and mid-range drivability with repeatable crisp performance. Vacuum actuated secondary with large bores will allow the amount of air/fuel that the engine is capable of inhaling. Even though the Rochester/Carter Q-Jet was rated at 700 - 750 CFM for the small block and a stock big block Q-Jet was rated for 800 CFM, they would flow only the cubic feet of what the engine could flow.
