Join Date: Mar 2002
Location: Austin, TX, USA
I use this formula and a fudge factor:
CFM = (RPMpk * CID / 3456 ) * VE
Where RPM pk is the peak RPM, CID is cubic inch displacement, and VE is the volumetric efficiency.
VE is almost always going to be less than 100%. Although, with the right heads and cam you can get well into the 90% range at certain RPMs. That's the fudge factor. Make a value judgement of your motor's VE, peak RPM, and you'll have the size.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI