It seems as if every advertisement for a pickup truck possesses one mutual claim. Each one boasts something about its horsepower and towing capabilities. And while the word horsepower is thrown around constantly when describing the specifications of an engine or motor, it is less common for the casual observer to actually know what the term truly represents. One would likely assume that has something to do with the power output of a vehicle compared to the strength of horses. But in reality, it’s a much more complex and specific detail that defines one of the most important elements of performance of a given vehicle. So the first task would be providing a proper definition of horsepower, and then defining how it is measured.

Horsepower Defined

The correct definition of horsepower is “work done over time”, but the more precise classification has changed over time and varies between different parts of the world. According to the British Imperial system it represents “the power necessary to lift a total mass of 33,000 pounds one foot in one minute”. The term was coined in the late 18th century in Scotland by James Watt {also the namesake of the Watt unit of measurement} to market his new and improved version of the steam engine, although the concept was described as early as 1702 in a less precise/more ambiguous manner.

How Horsepower Works

Watt observed that a horse could pull a 12-foot wide mill wheel 2.4 times per minute, which adds up to 144 rotations per hour. This means that the horse would move 2.4·2π·12 feet per minute. A typical human can produce around 0.1 hp over an extended amount of time, and maxes out at approx. 1.2 hp during a more brief time span. A well-trained athlete can maintain up to 2.5 hp momentarily, and somewhere in the range of 0.3 hp for a few hours. When Watt introduced his steam engine, he needed some way to explain how and why it was a superior alternative to the preexisting methods. During this era, horse-pulled carts were the usual way of doing things. Watt uses the term horsepower to specify the increased performance of his steam engine as compared to the abilities of a horse. It should be noted that others define horsepower as “the amount of energy required to lift 550 pounds, one foot, in one second”

There are several different types of horsepower determined by the specific application of the term. One of the most common uses defines the power of a certain engine as displayed by a dynamometer. It indicates the maximum rate of acceleration as well as the top speed of a vehicle, specifically the engine that drives it. Closely related is brake horsepower. While regular horsepower is more defined and specific, brake horsepower lies more on the theoretical side of things, as it is determined in closely-monitored lab tests without any other components connected to the engine. On the other hand, horsepower is calculated with different parts attached to the engine, something closer to the finished product rather than a single piece.


The Difference Between Torque and Horsepower

Torque and horsepower are two concepts that are often mistaken for one another by those that aren’t particular familiar with the inner workings of an engine. These are the two main elements of an engine that form its overall quality of performance. However, there are significant differences between them. Horsepower is centered around force, distance, and time. Torque is defined by the amount of twisting force necessary to drive the engine. It includes a much more specific classification of force, and does not take time or distance into the equation. For all intents and purposes, torque can really be considered a component of an engine’s overall horsepower; but the two words do refer to different things.
As the years have passed, the definition of horsepower is generally the same regardless of one’s location, although the units by which it is measured may be different depending on the region of the world. Establishing an engine’s horsepower {as well as its torque} is not always as simple as one might imagine. There are many different details that have to be taken into consideration to measure horsepower, but it is essentially the output of power of the engine/motor in question.