I used 4500 RPM as a redline for a street engine with a .8 volumetric efficiency; and 5000 RPM for a race engine with a .9 volumetric efficiency.
For the above:
Street - 298 CFM
Race - 388 CFM
Once upon a time here in Missouri, there was a hillbilly with no shoes. At age 17, his uncle volunteered to buy him a pair of boots. The kid walked barefooted to town and came back wearing a pair of shiny new boots. His uncle asked what size. The kid answered. "Well, the salesman said my size was nine, but 10's felt so good, I bought 11's".
The same can be said for carburetion.
As I mentioned above, I placed a 400 CFM on a 300 CID 6 with a 4 speed. The truck would not run fast enough to engage the secondary in fourth gear. The 400 was plenty in the other 3 gears.
Fuel economy over the original 1 barrel Holley almost doubled! But that may not be the norm. The 1 barrel was really restricting the engine. The four barrel, coupled with some better exhaust allowed the engine to breathe.
Jon.
