I use the old adage, CID x 2 = CFM. Seems to work, my stroker LA is 426" x 2 = 850 CFM, and I run a Thunder AVS @ 800 CFM. I'm probably a little low on that one. Then again, on the Kit Car E58, I have the Street Demon @ 750 CFM, and I think it's over-carbed. Oh well, maybe someday I'll make...