Why Do We Measure Engines Against The Power Of Horses?

Horsepower.  The word alone makes one want to queue up the classic Tim “The Tool Man” Taylor grunt from the ’90s American sitcom Home Improvement. A word that, when preceded by a number, represents the total power output of a vehicle’s engine. Whether this breaks down into engine horsepower (hp), brake horsepower (bhp), or wheel horsepower (whp) is dependent on where it is measured and what kind of dynamometer is used. However, this article’s focus is not on the differences of how the power is measured, but why the power is measured in such an obscure unit as the Horse. In the related video for this article (above), Jason Fenske of Engineering Explained gives us a full mathematical breakdown of the famous unit.

To learn the history of this oddly-favored unit of measurement we must jump in our DeLorean and head back in time. Our flaming tire tracks land us in the late 18th century, shortly after the birth of the United States of America and a short time before those unfortunate souls had complications with Dysentery along the Oregon Trail. A time where the horse and buggy was the dominant method of transport and the horse itself was commonly used for work-related tasks. Here we find the perfecter of the mighty steam engine: James Watt.

Regardless of what units you prefer to measure in, what is actually measured on a dyno is torque. Power — be it horsepower, PS, CV, or kilowatt — is calculated from the torque readings on the dyno.

The Power of the Horse

As Jason kindly breaks down for us in his video, James Watt created the horsepower to act as a measuring unit for the power exerted by his now-perfected steam engines. The theory is that one horse exerts 180 pounds of force (or lbf) to turn a 12-foot-radius grinding mill at a rate of 2.5 rotations per minute, or RPM if you will. Oddly enough, there is no history or knowledge as to how Watt came to determine the initial output of said horse was 180lbf.

Even weirder still, Watt decided to simplify the final power output calculations by ignoring the true results of the power formula (33,929 foot-pounds of force per minute, or ft-lbf/min) and adapting his own result of 32,400ft-lbf/min based on the horse traveling roughly 60 yards per minute around the mill.  Later down the line Watt rethought his approach and rounded to 33,000ft-lbf/min, which takes us even further into the realm of quantum confusion. This time, at least, the numbers were pretty close to accurate.

Now, with all of that out of the way, we can get to the brass tacks of horsepower. To simplify, you can break 33,000 ft-lbf/min down to 550ft-lbf/s. What this means is that a horse can lift an estimated 550 pounds 1 foot high in 1 second (via a ceiling-mounted rope and pulley, for example). So, to write this in its simplest form, you get: 1hp = 550ft-lbf/s. Based on this final calculation, in a rudimentary way, we can now surmise that you can measure the power of a vehicle’s engine by strapping the car to the same pulley system and seeing how far the weight is lifted in a single second. For example, if the vehicle pulls the weight 15 feet up in one second, that is 15 times stronger than the horse. This results in a final measurement of 15 horsepower (15 x 1hp = 15hp).

A Horse in Germany is Still A Horse

It’s time to hop across the pond and confuse you, European style. It is commonly known that Europe uses the Metric system instead of the American Imperial system. Not only does this mean the calculations are a bit different since they use kgf-m/s (kilogram meters of force per second), but the nomenclature is different depending on what country you are in.

Whether you are using the German PferdeStärke (PS), Italian Cavallo Vapore (CV), or the French Cheval Vapeur (CH), it all translates the same: Horsepower.

European power ratings are generally lower than those of the same vehicle in the USA, and Jason does an excellent job at breaking down the math and telling you exactly why at the 7:00 minute mark of his video.

Some of Fenske’s math explaining how the horsepower unit was (and is) calculated.

Keep it Simple

“Why do we still rely on this ‘back of the napkin math’,” asks Fenske, when there is a simpler unit of measure: the Watt. Named after the man himself, 1 watt = 1 kgm²/s³. A seemingly simpler measurement… Right?

So, why DO we?

Because horsepower is king.

About the author

Cory Barker

Able to identify nearly any car on the road since the age of six, he turned his passion into a lifelong career. Starting from his garage and working his way to Crew Chief for a private racing team, his knowledge encompasses nearly anything on wheels.
Read My Articles

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