Well according to my info, take the engine size say 460XMax rpm say5000XVolumetric efficiency divided by 3456= CFM. So460X5000=2300000X.85[most rebuilt engines have about 85% efficiency]=1955000 dividedby 3456=565.68287, or less than 600 CFM. This is only a place to start for some, but it has worked real well for me like it is.