I’m confused about the meaning of this word. Is it how much a horse can pull translated into how much the engine can pull? Also, what is the actual “metric”? Why do we still use this? It seems archaic.
Also, what type of horse was originally used to get the measurement?
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Horsepower is a calculated metric, based on the torque output of an engine at any given RPM
Horsepower = Torque * RPM / 5252
The term has evolved over time, but was originally used to describe the amount of power output a steam engine could provide. The method of calculation on dynamometers has also changed, gross vs net, back in 1971.
1 Horse does not necessarily produce 1 horsepower, either. It was an averaged metric over a day’s time. If a horse really exerts itself, it can produce upwards of 15hp (think sprinting vs long distance running).
How does it relate to engines? Some engines produce considerable torque, but can’t maintain that torque throughout the rpm range (high torque, low horsepower, like diesels). Some engines produce considerable torque, but only at high rpm (high horsepower, low torque, such as a sportsbike or smaller displacement engines). Some engines (my favorites) produce gobs of torque at low and high rpm, having a “flat curve” (high horsepower and high torque, such as a supercharged V8). This is a *greatly simplified* explanation of how it all works and is calculated.
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