a 2 bbl carb is flowed at 3 inches of vacuum while a 4 bbl is flowed at 1 1/2 inches of vacuum a 500 cfm 2 bbl flows 350 cfm in 4bbl ratings one of the rules told to me by the folks at Barry Grant/Demon Carburetion is the correct carb cfm for max power will ca... Login to read more.