You Asked:
What is the difference between horsepower and torque? — Alan, Montreal, Canada

Marshall Brain Answered:

Torque is a measurement of twisting force. When you put a wrench on a nut and apply a force to the end of the wrench, you create torque on the nut. The nut may not budge, but the torque is still being applied.

An engine produces torque when burning gasoline creates pressure in a cylinder. See how car engines work for details on the process. The pressure pushes down on the top of a piston, which conveys the pressure to the crankshaft, which creates a twisting force. The amount of pressure in the cylinder along with a) the diameter of the piston plus b) the number of pistons and c) the geometry of the crankshaft determines the maximum torque the engine can produce. Usually the torque changes with RPM, as shown here:

You can see that the engine has more torque at 2,000 RPM than at 7,000 RPM, but the horsepower peaks at 6,000 RPM.

Once you know the engine’s torque, you can calculate the horsepower using this equation:

(Torque x Engine RPM) / 5,252 = Horsepower

You can see from the equation that if you have an engine capable of creating a lot of torque at high RPM, the engine will have a lot of horsepower. For more info see:
- How Horsepower Works
- How do you convert engine torque to horsepower?

A really nice explanation of torque curves in cars, and the effect on horsepower: