# What does “horsepower” mean for an Engine?

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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?

In: 36

Horsepower is the average output of a horse throughout a day. Horses can still outpace horsepower fairly routinely, just not in a sustained way. When an engine is said to have “100 horsepower”, it means it has as much output as 100 horses on average

It is indeed an archaic unit of Power (approx. 740 watts), but like many archaic units it survives because it sounds cool and only serves as a subjective comparison of “which is better”, so it’s actual objective value isn’t really important.

I don’t know if it’s even originally related to horses, but there’s probably a source on the internet that would explain.

Before steam engines, horses were used to pull plows and cranes and mills.

When steam engines started displacing horses in these applications in the 1800s it was necessary to measure the working capacity of these machines in a way that people understood.

The obvious point of comparison is the horse they’re replacing, so the unit “horsepower” was developed to approximate the working capacity of a draft horse.

This isn’t a horse at full sprint or max load, it’s what a working horse can calmly do all day long.

One horsepower is the power to lift 550 pounds by 1 foot per second in a horse-drawn crane.

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.

Here’s an answer. Torque is kinda like how strong an engine is a certain instant. Like if you pedal your bike really fast the max torque is like the hardest you pedaled out of all your pedals. But horsepower is more like adding up all your pedals over time and dividing them to get a number we can compare across vehicles or machines. Horsepower very specifically is a measurement that involves TIME and POWER.