bhp

Brake horsepower measures the real mechanical power output from an engine, as opposed to the theoretical power generated inside the cylinders. It is determined by applying a load (called a brake) to the engine output shaft while the engine runs at a fixed speed, then measuring the force and torque required to hold the shaft steady. The name comes from the brake itself, traditionally a friction or eddy-current device that absorbs the engine's power and converts it to heat.

The difference between bhp and raw cylinder power matters because engines lose energy to internal friction, pumping losses in the intake and exhaust systems, and the mechanical work of turning crankshaft bearings and auxiliary equipment like the water pump and alternator. A gasoline engine might produce 200 indicated horsepower inside the cylinders but deliver only 160 bhp at the crankshaft. This gap grows larger in engines with heavy accessory loads or poor breathing characteristics.

Measurement and comparison

Bhp is measured on a dynamometer, a machine with an absorbing unit coupled to the engine output. Modern dynamometers use electric motors or hydraulic units to apply the braking force, while older test setups used mechanical friction brakes (hence the term). The engine is warmed to operating temperature, run at a specified speed such as 5,500 rpm, and the dynamometer records the torque value. Horsepower is then calculated from torque using the formula: horsepower equals torque in foot-pounds multiplied by rpm, divided by 5,252.

Bhp figures depend heavily on test conditions: fuel octane rating, air temperature, barometric pressure, and whether the engine is running with or without installed accessories. Manufacturers often test engines with the alternator and power steering pump disconnected, so published bhp figures can exceed what reaches the wheels on a road vehicle. Some engines tested at sea level on a cold day will produce more bhp than the same engine tested at high altitude on a hot day.

In automotive specifications, bhp has largely been superseded in Europe by kilowatts (kW), where one kilowatt equals roughly 1.34 bhp. However, bhp remains the standard unit in Britain, North America, and among enthusiasts worldwide. It remains distinct from SAE net horsepower, which accounts for engine-driven losses, and should not be confused with RAC horsepower, an older theoretical calculation based on engine displacement alone.