What is horsepower? What type of horse? Thoroughbred? Clydesdale? Tennessee Walking Horse?
The short answer is that Horsepower is a measurement of amount of work done over time. The exact definition of one horsepower is 33,000 lb.ft./minute. Put another way, if you were to lift 33,000 pounds one foot over a period of one minute, you would have been working at the rate of one horsepower. In this case, you’d have expended one horsepower-minute of energy.
How the name came about and some history: The term “Horsepower” was originated by James Watt, the inventor of the steam engine and whose name, “Watt” is now a unit of power itself. To help sell his steam engines, Watt needed a way of rating their capabilities. The engines were replacing horses, the usual source of industrial power of the day. The typical horse, attached to a mill that ground corn or cut wood or to pull up coal from a mine, walked a 24 foot diameter (about 75.4 feet circumference) circle. Watt calculated that the horse pulled with a force of 180 pounds, although how he came up with the figure is not known. Watt observed that a horse typically made 144 trips around the circle in an hour, or about 2.4 per minute. This meant that the horse traveled at a speed of 180.96 feet per minute. Watt rounded off the speed to 181 feet per minute and multiplied that by the 180 pounds of force the horse pulled (181 x 180) and came up with 32,580 ft.-lbs./minute. That was rounded off to 33,000 ft.-lbs./minute, the figure we use today. Most observers familiar with horses and their capabilities estimate that Watt was a bit optimistic; few horses could maintain that effort for long.
Put into perspective, a healthy human can sustain about 0.1 horsepower.
If you thought that was interesting and you’re still reading, here’s how torque is different: When driving a car, it is torque that you feel, not horsepower. Torque is the turning force, which is usually measured in foot-pounds. Torque is a measure of the ability of an engine to do work. It’s a component of, but not the same as, the horsepower of the engine, which is the rate at which work can be done. In an automotive engine, power and torque are related by a simple equation that considers torque, engine speed (in revolutions per minute), and a conversion factor:
HP = Torque x RPM ÷ 5252.
Take a moment and really think about this equation. Torque and horsepower are the same at 5252 RPM. Thus, HP is a function of how much force an engine can apply at various RPMs.
Really the way to think about it is that if you have two engines that have the same torque but one has more horsepower then that means that the one with more HP can produce its torque at a higher RPM which, of course, results in add’l speed or acceleration.