The Twist on Torque

Edmunds.com

The Twist on Torque

Specifically, what are the differences between horsepower and torque? If you flip through the pages of any automotive publication, you'll notice that these two measurements are commonly listed under vehicle specifications. And while the average car enthusiast knows that both horsepower and torque play a role in performance, most of them don't understand exactly how or why.

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The problem with horsepower is that it does not indicate acceleration potential. Acceleration varies directly with force (Acceleration = Force / Mass). Constant force gives constant acceleration. Power varies with speed. If you want constant acceleration you have to increase horsepower linearly with speed. At constant force, mass and acceleration, power doubles when speed doubles. Here's a challenge: You have a car weighing 3,232 pounds. You measure the horsepower at the rear wheel and find it is 100 HP. How fast it is the vehicle accelerating? There is no answer because power and acceleration are not related. You have to know the speed, convert to force, and then calculate acceleration. Dynamometers measure torque, the resulting torque curve is mathematically converted to power.

Force = Mass X Acceleration. Power = Work / Time. Work = Force X Distance, so Power = Force X Distance / Time. Distance / Time = Speed. So Power = Force X Speed. Power is the time rate of doing work.

Power is a term James Watt concocted to compare water lifting capacities of different pumping systems. It has little use in vehicle performance except for top speed potential (remember, velocity related).