Q&A

# Mailbag: The Math Formula for Determining Carburetor Size

(Image/Street Muscle)

Q: I am in the process of rebuilding my 1967 Chevelle. I am looking for the formula to figure out the proper size carburetor. Do you know it? If so, would you please share that?

A: We do, and we will.

Carburetor CFM = (plannned rpm x cubic inches) / 3,456

For clarity, your carb CFM should equal your planned rpm multiplied by your engine’s cubic-inch displacement, and then divided by 3,456.

Important note: This formula assumes 100% volumetric efficiency and doesn’t take vehicle weight nor rear axle gear ratio into account. If you want to adjust for street or race use, multiply the formula result by .85 for a street car (85% volumetric efficiency), or 1.1 for racing (110% volumetric efficiency).

We’ve included this formula, along with 12 other key automotive formulas, in our 13 Key Automotive Formulas You Should Know post. Check it our for valuable information on compression ratio, rear gear ratio, and more. You can also find interactive calculators to help with your math.