Calculate friction horsepower from indicated and brake horsepower, or from mechanical efficiency, and see how much engine power is lost to internal friction.
Friction Horsepower Formula
Friction horsepower is the power an engine develops in its cylinders but never delivers to the crankshaft, because it is consumed by internal friction and pumping. It is the difference between indicated power and brake power:
FP = IP - BP
The same three quantities define mechanical efficiency, which lets the calculator work from any two known values:
ME = (BP / IP) * 100 IP = BP / (ME / 100)
Variables:
- FP is the friction power, in hp or kW
- IP is the indicated power — the power developed by combustion pressure inside the cylinders
- BP is the brake power — the usable power measured at the crankshaft (what a dyno reads)
- ME is the mechanical efficiency, in percent
Pick what you know at the top of the calculator. If you have both indicated and brake power (from an engine simulation, an indicator diagram, or a motoring test), the tool returns friction power and mechanical efficiency directly. If you only have one power figure plus a mechanical efficiency estimate, the tool reconstructs the missing value first and then reports the friction power.
Friction power lumps together piston ring and skirt friction, bearing and valvetrain drag, oil and water pump drive, and the pumping work of drawing in and expelling gases. It rises roughly with the square of engine speed, which is why mechanical efficiency is highest at low-to-mid RPM and falls off near redline.
Friction Power at Typical Mechanical Efficiencies
This table shows the friction power hiding behind common brake horsepower figures at 85% and 90% mechanical efficiency — typical bounds for automotive engines at full load.
| Brake HP | FHP at 85% ME | FHP at 90% ME |
|---|---|---|
| 100 hp | 17.6 hp | 11.1 hp |
| 200 hp | 35.3 hp | 22.2 hp |
| 300 hp | 52.9 hp | 33.3 hp |
| 400 hp | 70.6 hp | 44.4 hp |
| 500 hp | 88.2 hp | 55.6 hp |
| 600 hp | 105.9 hp | 66.7 hp |
| 700 hp | 123.5 hp | 77.8 hp |
Example Problems
Example 1: Friction power from indicated and brake power.
An engine develops 460 hp indicated power in its cylinders but delivers 400 hp at the crankshaft. The friction power is:
FP = 460 – 400 = 60 hp, and the mechanical efficiency is (400 / 460) * 100 = 87%.
Example 2: Friction power from brake power and mechanical efficiency.
A 350 hp engine is assumed to be 88% mechanically efficient. The indicated power is 350 / 0.88 = 397.7 hp, so:
FP = 397.7 – 350 = 47.7 hp lost to friction and pumping.
Frequently Asked Questions
What makes up friction horsepower?
The largest share comes from the piston rings and skirts rubbing on the cylinder walls, followed by the crankshaft and camshaft bearings, the valvetrain, and the oil and coolant pumps the engine drives. Pumping losses — the work of pulling air past a partly closed throttle and pushing exhaust out — are also included when friction power is measured by the difference between indicated and brake power.
What is a typical mechanical efficiency for a car engine?
Most automotive engines run 80-92% mechanical efficiency at wide-open throttle in their power band. Efficiency is worst at very high RPM, where friction grows roughly with speed squared, and at light throttle, where pumping losses dominate. Diesel engines tend toward the lower end of the range at rated speed because of their higher cylinder pressures.
How can friction losses be reduced?
Builders reduce friction with lower-tension piston rings, roller lifters and rocker bearings, tighter-tolerance machining, lighter valve springs where the cam allows it, and lower-viscosity synthetic oils. Gains of a few percent of total power are realistic on a race build, which is why serious engine programs treat friction reduction as free horsepower that costs no fuel or octane.
