Carburetor CFM Calculator
Airflow the engine can actually swallow at peak RPM. The 3456 divisor covers cubic inches per cubic foot and the fact that a four-stroke inducts once every two revolutions.
| Required airflow | 516.5 | CFM |
| At 100% VE | 607.6 | CFM |
- CFM = displacement × RPM × VE ÷ 3456
- 3456 = 1728 cu in per cu ft × 2 revolutions per intake stroke
When you need it
When sizing a carburetor for a combination, or checking whether the one already on the engine is sensible.
Volumetric efficiency is the input that matters
The arithmetic is trivial; the answer is only as good as the VE you assume. A tired stock engine with small ports might be seventy-five to eighty percent. A well-assembled street engine with decent heads and a matched cam, around eighty-five. A race engine with good heads and a tuned intake can exceed one hundred percent over a narrow band.
If you do not know, eighty-five percent is a reasonable street starting point. Guessing high is how people end up with too much carburetor.
The ratings are not comparable
Four-barrel carburetors are rated at 1.5 inches of mercury; two-barrels at 3.0. The same physical airflow gets a bigger number under the two-barrel standard, so a 500 CFM two-barrel and a 500 CFM four-barrel are not the same carburetor. Compare within a standard or not at all.
Bigger is not better on the street
A carburetor that is too large loses signal at the booster when airflow is low. That shows up as soft throttle response, poor part-throttle metering, a stumble off idle and worse fuel economy — all the things you actually notice driving.
Err small for a street engine; the top-end loss is smaller than the driveability gain. A race engine that lives in a narrow RPM band can afford to size up, because it never operates where the signal is weak.