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BMEP Calculator (Brake Mean Effective Pressure Calculator)

Calculate engine efficiency with our BMEP calculator using the BMEP formula. Accurate and easy to use for all your brakes mean effective pressure needs.

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Brake mean effective pressure1.2578 MPa12.578 bar, 182.4 psi
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
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
Airflow required190.5 CFMcubic inches × rpm × VE ÷ 3456 — the standard carburettor sizing formula
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.