A dynamometer, or dyno, measures the twisting force of an engine or motor and how fast it turns, then works out the power. Knowing that makes every horsepower figure easier to read.
Brake horsepower is measured with a load dynamometer at a set point. Power at the wheels is lower, because the gearbox, differential, bearings and tyres take their share.
Pick the kind of figure you have been quoted. The bar shows how high it tends to read for the same engine, from the most generous to the most cautious.
Based on the reference definitions of brake, shaft and wheel horsepower and engine test standards. Figures from different methods should not be compared directly.
TWO NUMBERS WORTH KNOWING
Not every horse is the same size.
The imperial horsepower and the metric one differ by a little over one percent.
745.7 W
One horsepower (550 ft·lbf/s) in watts
NIST lists one horsepower, defined as 550 foot-pounds-force per second, as 745.6999 watts. The UK horsepower is listed as 745.70 W.
IMPERIAL HP · NIST
735.5 W
One metric horsepower (PS) in watts
NIST lists the metric horsepower as 735.4988 watts. It is the power to lift 75 kilograms by one metre in one second, about 98.6% of an imperial horsepower.
METRIC HP · NIST
The watt, one joule per second, is the SI unit of power. The SI unit of torque, or moment of force, is the newton metre (N·m). The PS was replaced by the kilowatt as the official unit in EEC directives in 1972.
FOUR CHECKS BEFORE COMPARING
Compare like with like.
Two power figures only mean something side by side if they were measured the same way and quoted in the same unit.
Check the unit
kW, hp and PS are different sizes. One PS is about 735.5 W and one hp about 745.7 W.
Check the place
A rolling-road figure at the wheels will read lower than a crankshaft figure for the same engine.
Check the method
Inertia dynamometers work out power from how quickly a known heavy roller speeds up; load dynamometers hold the engine against a braking force.
Look at torque too
Power is torque multiplied by rotational speed, so the same power can come from high torque at low revs or less torque at high revs.
AI-generated image, not a photograph of a real person or place.
DYNOS IN THE LAB
The road, brought indoors.
AI-generated image, not a photograph of a real person or place.
Dynamometers do more than rate engines. In standard emissions test cycles they simulate road loading on an engine, or on the whole powertrain using a chassis dynamometer, so cars can be tested the same way every time.
In the UK, the Vehicle Certification Agency says official fuel consumption and CO2 figures for new cars have come from the WLTP laboratory test since September 2017, replacing the older NEDC test, with tests still run in a laboratory so cars can be compared.
Questions
Questions people ask
Does a dyno measure horsepower directly?
No. A dynamometer measures torque and rotational speed, and power is calculated from them: power equals torque multiplied by angular velocity.
What is the difference between hp and PS?
One imperial horsepower is about 745.7 watts and one metric horsepower (PS) is about 735.5 watts, according to NIST conversion factors. A PS is about 98.6% of a horsepower.
Why is wheel horsepower lower than a crankshaft figure?
Crankshaft or brake horsepower is the engine's own output. Wheel horsepower subtracts friction in the transmission, universal joints, differential, wheel bearings and tyres.
Where does the word horsepower come from?
James Watt adopted the term in the late 18th century to compare the output of steam engines with the power of draught horses.
What unit should torque be measured in?
The SI unit of torque, or moment of force, is the newton metre (N·m). NIST notes it is written as N·m rather than the joule, even though the base units are the same.
