Horseless Carriage Education
Technical Articles

What is the difference
between Torque and Horsepower

Author: Harold Sharon

So, What is the difference between torque and horsepower? Don't shy away from a couple of very simple formulas. A formula is just shorthand for ordinary words.

So, here we go: Torque is a measure of how powerfully we can turn a shaft. A common unit of measurement is foot-pounds. Any other unit of dimension and force will do. Grab a "torque wrench" with a foot long handle and pull sixty pounds on it and the torque exerted is sixty foot-pouds. Nobody mentioned motion or time here, just force and lever arm.

If you pull the nose of a car up to a tree, and put two huge torque wrenches on the rear wheel nuts, it's very easy to spin the wheels of the car. (Or strip the threads on the axle nuts).
But you cannot do this rapidly unless you have some machinery that will turn the big wrenches at a fairly high speed. That's horsepower!

Horsepower measurement includes the element of time. The very simple measurement of horsepower is usually denoted in pound-feet per minute. If you write the formula out, as an 8th grader would do, you'll note that both pounds and feet are in the numerator, and time is in the denominator. You can restate the formula as feet per minute times pounds. So it's now speed times force! Sit in the back of a pick-up truck and hold a rope which is attached to the old car behind, Have someone drive the truck, and you pay attention to the tension in the rope. As the truck speeds up, the force gets greater ( wind drag, tire drag, etc). You are exerting more horsepower through the rope.
Got it? Harold


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