Otto Explains — Horsepower and Torque
Horsepower vs Torque: What Do They Really Mean?
Short Answer
Torque is the engine's twisting force: the turning effort it puts on the crankshaft and, through the drivetrain, on the wheels. Horsepower (or kilowatts) is power: how quickly the engine can do work, which equals torque multiplied by engine speed. Torque is what you feel when a car pulls away, carries a load or climbs a hill; power tells you how much work the engine can do each second. The two are linked, so neither figure tells the whole story on its own.
What is torque?
Torque is a twisting force: the force applied multiplied by its distance from the centre of rotation. That is why a longer spanner loosens a tight nut more easily with the same push. In an engine, the crankshaft turns the pistons' up-and-down movement into torque, which the drivetrain sends to the wheels. Plenty of torque helps a car pull away and do hard work, such as carrying a heavy load or climbing a steep hill.
What is horsepower?
Horsepower is a unit of power, and power is the rate of doing work: how much work is done each second. For anything that rotates, power equals torque multiplied by rotational speed. So the same torque delivered at a higher engine speed produces more power, and an engine's power figure is calculated from its torque and the engine speed. Torque describes the turning effort; power describes how quickly the engine can keep delivering it.
What do kW, PS, hp and Nm mean on a specification sheet?
Torque is given in newton metres (Nm) or pound-feet (lb·ft); 1 lb·ft is about 1.356 Nm. Power is given in kilowatts (kW) or in horsepower: one mechanical horsepower is about 745.7 watts and one metric horsepower (PS) about 735.5 watts, so the two kinds of horsepower are close but not the same. Manufacturers usually state both power and torque. Toyota's specifications for the GR Yaris, for example, list 200 kW (272 PS) and 370 Nm for its 1.6-litre turbocharged engine, and 88 kW (120 PS) and 145 Nm for the 1.5-litre engine.
Which matters more when you drive?
Both, in different ways. Torque is what you feel when the car pulls away, overtakes at low engine speed or climbs with a full load; diesel engines tend to produce more torque than petrol engines, which is why they suit heavy work. Power reflects how much work the engine can do each second. Electric cars are a clear example of torque at work: an electric motor reaches its peak torque almost immediately, and the car's reduction gear lowers the speed and raises the torque that reaches the wheels. Forced induction can also be used to raise low-speed torque: Volvo, for example, uses a supercharger in one of its engines to fill in torque at low revs.
Can an engine's power and torque be changed?
The manufacturer's figures come from the engine's design and its software. ECU tuning changes settings such as fuelling, ignition timing and boost pressure to alter how the engine delivers its power, but results vary with the car, its condition and its fuel. At AutoTronix, tuning is handled by AutoTronix PERFORMANCE, subject to the vehicle's compatibility and a technical assessment, and no power figures are promised before that assessment. If a car simply feels weaker than it used to, that is a different question, covered in why a car loses power.
Common Questions
- Are hp, PS and kW the same thing? They all measure power, in different units. A kilowatt is 1,000 watts; one mechanical horsepower is about 745.7 watts, and one metric horsepower (PS) about 735.5 watts. That is why the same engine shows slightly different numbers in hp and in PS.
- Why do diesel engines feel strong at low speed? Diesel engines tend to produce more torque than petrol engines, and torque is the twisting force you feel when pulling away, carrying a load or climbing a hill.
- Why do electric cars accelerate so quickly from a standstill? An electric motor reaches its peak torque almost immediately, instead of having to build engine speed first, and the car's reduction gear multiplies that torque on its way to the wheels.
- Is a higher torque figure always better? Not on its own. Power is torque multiplied by engine speed, so an engine with less torque that keeps working at higher speeds can still make more power. How the figures suit you depends on how you drive and what the car carries.