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 |
| Required airflow | 877.5 | m³/h |
| At 100% VE | 607.6 | CFM |
Demand rises in direct proportion to RPM, so the only things that move the line are displacement and VE. The axis stays in CFM in both unit systems because that is how carburetors are rated everywhere.
- CFM = displacement (cu in) × RPM × VE ÷ 3456
- 3456 = 1728 cu in per cu ft × 2 revolutions per intake stroke
- from litres: m³/h = displacement (L) × RPM × VE% × 0.0003
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.