Hi Tracy with all due respects to all the knowledgeable people here, I was taught a different way. This aint a magig bullet but it works real good for my street cars. Say a 350 cubic will turn a max of 5000 rpms 350x5000=1750000. If u have 85% engine efficiency then 1750000x85=14875000 divide that by 3456 which is unknown to me that =4304.1087-or rounding off to 430 cfm for an engine with 350 cubes @max 5000 rpms 85 % efficiency would be a nearly new rebuilt engine. Now if u have variables-rpm-hi lift cam- hedders- hi compression-NOs-or engine wear then u have to start race tuning. But this is a good place to start from, more is sometimes less. Hope this helps