Carburetor CFM = (Engine RPM x Engine Cubic Inches)/3,456
Let’s take a 350 ci engine that we will run about 5,000 RPM at most.
Carburetor CFM = (5,000 x 350)/3,456 = 506 CFM –> A 500 CFM carb would work for this engine.
Note: Engine RPM should be the top RPM your engine will reach. Be reasonable with the number. Choosing 8,000 RPM in the formula will change the Carburetor CFM versus 5,000 RPM. When you get odd numbers you just have to find the carburetor that is close to the number you come up with.
The monkey wrench in using this equation is volumetric efficiency. Volumetric Efficiency is the efficiency that an engine can move the air/fuel mixture into the combustion chamber shown as a percentage. The better your engine moves fuel and air, the more power you can make because the engine can burn more air and fuel. Some engines may only have a 75% volumetric efficiency. If that’s the case, your 500 CFM carb in our example only needs to be 375. We got this number by multiplying the CFM number we got from our formula by the volumetric efficiency (.75). If your engine’s volumetric efficiency is 90%, the 500 CFM carb technically only needs to be a 450 CFM carb. 450 CFM is much closer to 500 than our 375 we got with 75% efficiency. The closer your engine is to 100% volumetric efficiency, the more accurate the basic formula is.
Blowers usually increase volumetric efficiency over 100%. You can see how a blown engine with a 125% volumetric efficiency would need a higher CFM carburetor(s) versus the same engine with no blower. The basic formula above may work for a blower motor. These situations are more complex. You should ask your blower manufacturer or speed shop for their recommendations.
There are some things you can do to increase your engines volumetric efficiency: Porting and polishing heads, for example, can increase Volumetric Efficiency. Blowers as mentioned above also help.