# The difference between horsepower, and torque in a car.

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The difference between horsepower, and torque in a car.

In: Physics

Torque is the twisting force of the engine. Power is how much energy the engine can produce in a measure of time.

Mathematically, power is torque x rotational speed.

Specifically, HP is torque (in lb-ft) x rpm / 5252

You will feel torque in terms of being able to break the tires loose or slam you back in the seat, but you need horsepower to win a 1/4 mile drag.

Torque is a measure of the number of grocery bags you can carry inside at one time in one trip.

Horsepower is a measure of how quickly you can get yourself and all of the grocery bags inside.

In the context of vehicles, torque is important for towing heavy loads or climbing steep hills, while horsepower is important for acceleration and speed.

The relationship between horsepower and torque is speed. High torque at low speed have the same horsepower as low torque at high speed. A gearbox changes the speed but not the horsepower going through it. You can therefore get high torque at low speed or low torque at high speed out of the same engine just by changing the gears.

You often see just a single number being quoted for an engine. This is usually the peak horsepower and torque respectively. So how many horsepower can it produce at the optimal speed, typically a very high speed, and how much torque can it produce at a different optimal speed for torque. These numbers can be misleading as the engine rarely stay at optimal speed for either of these numbers. Peak horsepower might be related to maximum speed but when you are accelerating the engine have to go through a lot of different speeds for each gear. The torque curve for an engine tends to be more flat so peak torque tends to be something you can get at any speed. So high peak torque engines tends to accelerate better as they often have a descent amount of horsepower outside of the optimal speeds as well.

Think of a tractor. They require a large amount of torque to be able to pull things, but they don’t need to move the wheels quickly. In this case, a tractor has high torque but relatively lower horsepower, as horsepower is determined by torque combined with RPM. A tractor doesn’t need to move quickly so it’s RPM is low.