The following was quoted from ebay carb buyersguide. USE AT OWN RISK

This is a question I get asked a lot. What carburetor would be best for my engine? Whether it be a 1, 2, or 4 barrel, here is a little help for you. There is a formula that will get you very close to the proper cfm for your engine.

The formula goes like this:

cubic displacement divided by 2 (ex: 351=175.5)
maximum rpm divided by 1728 (ex: 6000 rpm=3.47)
multiply those two figuires (ex: 175.5 x 3.47=608.98 cfm)

1 cc = 0.0610237441 cubic in

Hold on we are not done. You now have to factor in volumetric efficiency. A Nascar race engine runs at 85%-90% volumetric efficiency. A normal street car wil be 75-80%, depending on what you have done with your heads and exhaust.
Ex: 608.98 x 75%=456.74cfm


With carburetors, bigger is not better. You will not get very good results with an 850 double pumper on your small block Chevy. If you are after good idle quality, fair fuel mileage, and good performance, stick with this equation. The boys at GM and Ford used this equation for many years and it works! You can not change the laws of physics. I would like to thank Jon Enyeart of Pony Carburetors for teaching me how to properly restore and install a carburetor. He is the Grand Wizard of carburetion.
Kurt Praxl
Kp Carbs