How this calculator works
One intake event occurs every two crank revolutions in a four-stroke engine. Multiply swept volume by intake events per second and by volumetric efficiency to estimate actual volume flow at the chosen reference condition. This is a starting airflow estimate, not a complete carburetor selection: booster signal, pressure-drop rating and the operating range also matter. Boosted engines can have volumetric-efficiency figures above 100% depending on the reference convention.
Formula
Q = V RPM VE / 120 in m³/s. In customary units: CFM = displacement(in³) RPM VE / 3456.
Worked example
A 6 L engine at 6000 rpm and 80% volumetric efficiency demands 0.24 m³/s, about 508.5 CFM. At 3000 rpm and 100% efficiency, 0.1 m³/s corresponds to 4 L.
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Frequently asked questions
Is this for two-stroke engines?
No. The intake-event factor here is specific to a four-stroke cycle.
Should I round up to the largest carburetor?
No. Use the manufacturer's application guidance and airflow rating conditions as well as this estimate.
What efficiency should I enter?
Use a measured or defensible estimate at the RPM of interest; it is not the engine thermal efficiency.