There is a lot of confusion about the relationship between horsepower and torque, I was guilty until I decided to do some investigating. Here is my summary.

Lets cut to the chase, in the context of this article it is torque and torque alone that accelerates a car. That's great you think, my big block sticks out 750lbf-ft of torque and that small high revving engine only puts out 250lbf-ft, I'll blow him away. Sadly its not quite as straight forward as that, there is a small caveat I did not mention, what I should of said was it's torque at the wheel and torque at the wheel alone that accelerates a car. All things being equal the car that has the most torque at the wheel will be accelerating harder. Well your first reaction may be 750lbf-ft is 3 times 250 so there is no contest, wrong. It's all about gearing, the best way to explain is an example, see diagram below.

OK, we have two engines both putting out 714hp, one makes 750lbf-ft at the engine and the other 250lbf-ft. The important factor is the speed at which the torque is made. The small engine is revving 3 times faster, so we can gear it down with a 3:1 ratio to achieve the same rpm at the output shaft (the wheels in real life). The consequences of this is the torque is multiplied 3 times, hey presto we have the same torque at the output shaft.

So after having said that torque alone accelerates a car, the torque figure is meaning less if we don't know at what rpm the torque is being made, this is where horsepower come in. Think of horsepower as a way of factoring in rpm to the torque equation.

You may be surprised, I know I was, but horsepower is simply a calculation based on rpm and torque. Dynos and rolling roads only ever measure torque. Horsepower is calculated after. Here's the formula:


Horsepower = Torque * RPM / 5252


So how is this formula arrived at, and where did 'horsepower' come from?