Carburetor CFM Calculator

Background.

This calculator estimates the airflow a four-stroke engine can consume at a chosen maximum operating speed. It uses the established relationship CFM = displacement × RPM × volumetric efficiency / 3456. The divisor combines 1,728 cubic inches per cubic foot with one intake event every two crankshaft revolutions.

Enter the engine's total displacement in cubic inches, the highest RPM at which you want to estimate demand, and an evidence-based volumetric-efficiency percentage. For a 355 cubic-inch engine at 4,500 RPM and 105% volumetric efficiency, the result is about 485.3 CFM. At 100% efficiency, the same geometric demand is about 462.2 CFM.

Treat the result as an airflow estimate, not an automatic carburetor part-number recommendation. Edelbrock describes the equation as a guideline and notes that carburetor ratings depend on a specified pressure drop. Intake design, booster signal, fuel metering, drivability, altitude and the engine's real efficiency curve all affect selection. The calculation is specifically for four-cycle engines and does not substitute for an engine builder's measurements or the carburetor manufacturer's application guidance.

What is carburetor cfm calculator?

Carburetor CFM is an airflow rating in cubic feet per minute. This calculation estimates engine demand from swept volume, speed and volumetric efficiency.

How to use this calculator.

1. 01Enter total four-stroke engine displacement in cubic inches.
2. 02Use the maximum RPM relevant to the intended operating range.
3. 03Enter a measured or defensible volumetric-efficiency value, then compare the estimate with manufacturer application guidance.

The formula.

A four-stroke cylinder fills once every two crankshaft revolutions. Dividing cubic inches per minute by 1,728 converts volume to cubic feet; the additional factor of two produces the combined divisor 3,456.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Edelbrock, Performance Tech Discussion: gives the four-cycle CID × RPM × VE / 3456 airflow equation, worked examples and selection limitations.
2. [2]Holley, How to Choose Your Next Holley Carburetor: presents the same engine-airflow calculation and explains application factors.