Before diving into the differences, it's essential to understand what horsepower means. Historically, horsepower was defined as the amount of work a horse could do in a minute, which James Watt standardized to 550 foot-pounds per second.
In modern terms, it's a unit of power that measures how much work an engine can perform over time. For an internal combustion engine, this typically means the ability to move the vehicle efficiently and rapidly.
What an engine naturally produces, however, is torque. Think about one piston in a gasoline engine. When the gasoline ignites, it pushes on the piston, and the piston exerts pressure on the crankshaft, causing it to turn. The crankshaft feels some number of foot-pounds of torque in the process. There are three variables that affect torque:
- The size of the piston face
- The amount of pressure that the ignited fuel applies to the face of the piston
- The distance the piston travels on each stroke (therefore the diameter of the crankshaft). The bigger the diameter of the crankshaft, the bigger the lever arm and therefore the more torque.
There is a direct relationship between horsepower and torque. You can convert torque to horsepower with the following equation:
HP = Torque * RPM / 5,252
That 5,252 number, by the way, comes from dividing 33,000 by (2 * pi). Imagine taking 33,000 foot-pounds and walking it around in a circle rather than a straight line. For example, if you took a 10 foot pole and attached it to a vertical axle, the circumference of its circle is:
circumference = 10 2 pi = 62.8 feet
You can see from the horsepower equation that high RPM values favor horsepower. If you take an engine with a certain torque and run it at very high revs, it can generate lots of horsepower even though its torque hasn't changed at all.