The Difference Between Torque And Horsepower

The Difference Between Torque And Horsepower Explained

Ask anyone to explain the difference between torque and hp and you’ll get one of two reactions; glazed eyes and some waffle about the twist needed to exit a corner or a maths lesson and talk of rotational forces - ain’t nobody got time for ‘dat!

In an effort to keep things simple, here are torque and hp explained in simple terms:

1. Torque

Where horsepower determines how fast a vehicle can travel, torque at the wheels determines how quickly that speed can be reached. The greater the torque figure, the faster the acceleration.

The easiest way to picture what torque is, is by loosening a wheel nut. Try to loosen the nut by pulling on the spanner at the same end as the nut (very little torque) and you’ll struggle to loosen it. Pull on the spanner at the end of its shaft (maximum torque) and you’ll loosen it quickly.

As car scribe Ralph Hosier explains: if you weighed 180lbs and the spanner was a foot long and you had all your weight bearing down on the end of the spanner, then the torque at the bolt is 180lb/ft.

2. Horse power (hp)

We touched on this last week - The Difference Between BHP And WHP Explained - but bhp is torque (the twisting force) multiplied by speed (how fast it’s spinning).

Mathematically, 1 HP is the equivalent of 33,000 ft/lbs per minute. This is because three things need to be taken into account: 1) the amount of weight involved 2) the distance the object is being moved and 3) how long this takes.

Which is better: Power or torque?

The answer to this question (for cars) is dependent on your driving style. If you’re running a torquey, but thirsty V6 engine, you’ll find that you don’t need to down-shift quite so often when accelerating from lower speeds - this is because more torque gives you more available access to power at lower revs. This makes driving (generally) more relaxed.

If your engine has little torque, you’ll have to change down a gear more often to explore the rev range to unleash the engine’s power. Cars with lower torque figures are generally cheaper to run, but the engines have to be worked harder to get up to speed.

Let’s take these two cars as an example:

	Alfa Romeo 145	Honda Civic VTEC
Engine size	2.0-litre	1.6-litre
Max power	150 bhp at 6000 rpm	160 bhp at 8000 rpm
Max torque	140 lbft at 3500 rpm	113 lbft at 6000 rpm

Imagine a drag race with the cars above. Having more torque, the Alfa will put down more power at the low RPMs and will initially pull away from the Civic. But because the Honda’s little 1.6 has more power than the Alfa once it hits 8000rpm, the Honda will catch the Alfa given a long enough runway.

Advanced Info

1. Horsepower = Torque x RPM/5252.

2. One horsepower is the power needed to move 550 pounds (249kg) one foot in one second.

3. A car’s gearing can make a high-revving engine put down a lot of effective torque to the ground. If the gearbox and differential reduce crank RPM to wheel RPM by 9 times, you also get 9 times the torque at the wheels. Gearing can essentially convert low engine torque to high effective torque, but at the cost of speed (RPM).