Carburetor CFM Calculator

This carburetor cfm calculator will help you to estimate the correct carburetor size for your engine.

Airflow required190.5 CFMcubic inches × rpm × VE ÷ 3456 — the standard carburettor sizing formula

Total displacement1.9982 litres121.94 cubic inches, 1,998.2 cc

Per cylinder499.56 cc

Bore area58.088 cm²

Bore to stroke ratio1square

Mean piston speed17.2 m/stwice the stroke per revolution — around 20 m/s is the usual limit for a production engine, and above 25 m/s is racing territory

Piston speed in feet per minute3,386 fpm

Brake mean effective pressure1.2578 MPa12.578 bar, 182.4 psi

What that BMEP indicatesa well-developed naturally aspirated enginenaturally aspirated petrol engines reach about 10–13 bar; turbocharged ones go well beyond

Torque per litre100.09 N·m/l

Power at that torque and speed125.6637 kW168.5 hp

Specific output84.3 hp per litre

In cubic metres a minute5.3952 m³/min

The formula

displacement = π/4 × bore² × stroke × cylinders

Displacement, and the two divisors

Swept volume is the area of the bore times the stroke, times the number of cylinders. Bore is a diameter, so it enters squared — which is why a small increase in bore changes capacity far more than the same increase in stroke.

The bore-to-stroke ratio shapes an engine's character. Oversquare engines, with a bore wider than the stroke, have lower piston speeds at a given rpm and so can rev higher; undersquare engines have a longer lever on the crank and make torque lower down. Mean piston speed is the real constraint on revs, and around 20 m/s is where production engines sit.

Two formulas here carry a factor that depends on the cycle. Brake mean effective pressure uses 4π for a four-stroke and 2π for a two-stroke, because a four-stroke fires only every other revolution. The carburettor CFM formula divides by 3456 for a four-stroke and 1728 for a two-stroke for the same reason. Using the four-stroke constant on a two-stroke halves the answer.