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BMEP calculator

Enter torque, or power and the rpm it was made at, and the displacement. You get brake mean effective pressure in psi, bar and kPa, a way to compare engines of any size on the same scale.

Engine output

Start from

Engine type

Torque

lb-ft

Enter a number greater than zero.

Displacement

Enter a displacement greater than zero.

Displacement unit

Use crank (flywheel) torque or power. Wheel figures include drivetrain losses and give a lower BMEP.

BMEP is not a pressure you can measure in the cylinder. It is the steady pressure that would do the same work per cycle as the engine does at the crank.

What the inputs mean

Torque: crank torque at the rpm you care about, usually the torque peak.
Power and rpm: if you only have power, the calculator turns it into torque first: torque = hp × 5,252.1 ÷ rpm.
Displacement: total swept volume, all cylinders.
Engine type: a four-stroke makes one power stroke per cylinder every two revolutions, a two-stroke every revolution.

The formula and where it comes from

The work one full cycle does at the crank equals BMEP × displacement. A four-stroke cycle takes two revolutions, 4π radians, and work from torque is torque × angle, so:

torque × 4π = BMEP × displacement
BMEP = 4π × torque ÷ displacement

In US units, torque in lb-ft × 12 gives lb-in, and cubic inches give psi:

BMEP (psi) = 4π × 12 × lb-ft ÷ CID = 150.8 × lb-ft ÷ CID

In metric, 1 L is 0.001 m³, so 4π × Nm ÷ (L × 0.001) gives pascals, and dividing by 1,000 gives:

BMEP (kPa) = 4π × Nm ÷ L 1 bar = 100 kPa

For a two-stroke, use 2π instead of 4π: half the four-stroke figure for the same torque.

Worked example

400 lb-ft from 350 ci.

US: 150.8 × 400 ÷ 350 = 172.3 psi.
Metric: 400 lb-ft is 542.3 Nm and 350 ci is 5.735 L. 4π × 542.3 ÷ 5.735 = 1,188.2 kPa = 11.88 bar.
Check: 1,188.2 kPa ÷ 6.895 = 172.3 psi, the same answer both ways.

From power instead: 400 hp at 5,500 rpm is 382.0 lb-ft, which gives 164.6 psi (11.35 bar) from the same 350 ci.

What BMEP is for

Torque grows with engine size, BMEP does not. That makes it a fair way to compare how hard a small engine and a big one work each cylinder, or to compare one engine before and after a change. Boost raises it, because more air is in the cylinder each cycle; see the boost calculator.

Related calculators

Work out the static ratio first with the compression ratio calculator, and the pressure ratio of a boosted engine with the boost calculator. The engine set also covers dynamic compression, rod ratio, volumetric efficiency and turbo airflow and compressor map points.

Questions

How do you calculate BMEP?

For a four-stroke, BMEP = 4π × torque ÷ displacement. In US units that is 150.8 × lb-ft ÷ cubic inches: 400 lb-ft from 350 ci gives 172.3 psi.

How do I convert torque to BMEP in bar?

Multiply Nm by 4π, divide by liters, and you have kPa; divide by 100 for bar. 200 Nm from 2.0 L is 1,256.6 kPa, 12.57 bar (182.3 psi).

Where does the 150.8 constant come from?

It is 4π × 12. The 4π is the angle of a four-stroke cycle (two revolutions) in radians, and the 12 turns lb-ft into lb-in so that dividing by cubic inches gives psi.

Is BMEP different for a two-stroke engine?

Yes. A two-stroke fires every revolution, so BMEP = 2π × torque ÷ displacement, half the four-stroke figure. 400 lb-ft from 350 ci would be 86.2 psi.