Carburetor CFM calculator
How much air your engine can use at peak rpm, and which common carburetor sizes fit it. If you've used the displacement calculator, your cubic inches are already filled in.
Airflow needed
- Nearest common size
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- Next size up
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- At 90% of max rpm
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Vacuum-secondary carbs are generally more forgiving of a little extra size on a street car. Mechanical secondaries reward precise sizing and an engine that can use them.
The carburetor formula
3,456 converts cubic inches per minute to cubic feet per minute (1,728) and accounts for a four-stroke engine filling each cylinder every other revolution (× 2). Volumetric efficiency is how well the engine fills its cylinders compared with their full volume. Stock engines usually sit around 80%, and well-developed race engines can exceed 100%.
Bigger isn't better
An oversized carburetor lowers air speed through the venturis, which weakens the fuel signal at low rpm and hurts throttle response. Pick the size that matches the rpm the engine actually reaches, not the rpm you wish it did.
Formula as published by Speedway Motors ("What Size Carburetor Do I Need?") and used throughout the industry.
Frequently asked questions
Is bigger CFM always better for a carburetor?
No — an oversized carburetor can actually hurt throttle response and low-speed drivability because it never operates in its efficient range at everyday rpm, so matching CFM to your actual engine and rpm range beats simply buying the largest carburetor available.
What volumetric efficiency number should I use?
A stock engine typically runs somewhere around 80-85% volumetric efficiency, while a well-built performance engine with good heads and cam can reach 100% or slightly beyond at its power peak — pick a number that honestly reflects your specific engine's state of tune rather than assuming the best case.
Why does this need my displacement in cubic inches specifically?
The formula is built around cubic inches because that's the traditional unit carburetor sizing charts and most American engine specs use — if you only know your displacement in liters or cc, the displacement calculator on this site converts it for you first.
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