Carburetor CFM Calculator – Find Recommended Carburetor Size
Enter the engine size, max RPM, and volumetric efficiency to calculate the recommended carburetor CFM.
How to Calculate the Carburetor Size for an Engine
The carburetor in an engine is a device that controls the amount of air-fuel mixture that enters the cylinders. Equipping your engine with the right size carburetor is critical for maximizing its performance.
Too small of a carburetor and the engine will be starved of fuel, while too large a carburetor and it will respond sluggishly when you accelerate.
Carburetors are sized by the amount of airflow they can handle in cubic feet per minute, or CFM.
So, what size carburetor do you need? There’s a simple formula to calculate just the right size of carburetor for your engine.
carburetor size = engine size × max rpm × efficiency ÷ 3,456
The engine size is the displacement in cubic inches. If you know the displacement in liters, try converting using our liters to cubic inches calculator. Try our engine displacement calculator to find the size of your engine
The max RPM is the maximum speed of the engine in revolutions per minute.
Another factor to consider is the volumetric efficiency of the engine. It’s common to see efficiency values of 70-80% for stock engines and up to 95-110% for race engines.
For example, let’s find the recommended carburetor size for an engine with a displacement of 302 cubic inches, a max speed of 7,500 RPM, and 80% volumetric efficiency.
carburetor size = 302 × 7,500 × .8 ÷ 3,456
carburetor size = 524.31 CFM
Because carburetors typically come with CFM ratings that are rounded off to the nearest 50 or 100, you would select a carburetor with a CFM rating that is the next step up from the result of your calculation.
In the example above, you would likely select a 550 CFM or 600 CFM carburetor, depending on what a given carburetor manufacturer offers in this range.