Torque - How it relates to horsepower

If you want to start a lively discussion at your next bench racing session, ask your buddies about horsepower vs torque. Then after things calm down, ask everyone to explain what they mean. You'll probably get a lot of vague answers because everyone reads horsepower and torque ratings but not many actually understand the concepts and how they relate to one another. So, let's take a closer look.

Torque - what is it?

If you haven't read our page on horsepower, go there now and get that straight, then come back. We'll wait. ... Back already? Good. Now, remember that figure of 550 foot-pounds? We said that 1 horsepower was equal to 550 foot-pounds "per second." It's important to see that "per second" because horsepower is a calculation not a measurement. Think about that. It means you don't actually measure horsepower, you measure that force exerted over a period of time and make a calculation that results in a number, the number is horsepower. That force being measured is torque.

Cars, motorcycles and most everything we are interested in here have engines that turn wheels. The twisting force necessary to turn them is torque. Torque can be measured in several different units but we'll stick to foot-pounds to keep it simple. If you were to attach a one foot long wrench to a bolt and apply one pound of pressure to the end of the wrench, you would be applying one foot-pound of torque to the bolt. So,... torque is a twisting force measured (in our examples) in foot-pounds.

Torque to horsepower conversion

Now we need just a little math, it's easy but you will have to pay attention. Suppose we attach that one foot wrench to the end of a crankshaft and the engine rotates one revolution against that one pound of resistance. The end of the wrench will move 6.2832 feet (Pi * a two foot diameter circle) against a one pound weight. The end result is 6.2832 foot-pounds of work done at one foot-pound of torque.

Remember Pi? That's the ratio of the circumference of a circle to its diameter. Pi is a constant equal to 3.14159 carried for as many decimal points as you wish.

OK, here we go:

1 horsepower = 550 foot-pounds/second = 33,000 foot-pounds/minute

33,000 foot-pounds / 6.2832 foot-pounds = 5252

So, if the engine rotates against the one pound resistance at 5252 rpm:

6.2832 X 5252 = 33,000 foot-pounds/minute = 1 horsepower

because the one pound of resistance was moved 33,000 feet in one minute

(1 foot-pound X 5252) / 5252 = 1

Therefore, to convert torque to horsepower:

(Torque X RPM) / 5252 = Horsepower

 

To measure torque and horsepower, you'll need a dynamometer.