What is horsepower and why do the ratings differ by country?

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Kyle Smith

There are numerous comparison points between various makes and models of cars, but there is one that both enthusiasts and the common driver latch onto with unapologetic fervor—horsepower. It’s a number manufacturers have always put front and center in advertisements, and it’s also a tidy number for enthusiasts to discuss and determine a hierarchy.

But what is horsepower? Many enthusiasts know a dynamometer can tell you how many horses you have, but what is that dyno doing in calculating that number? Does it actually relate to the power of a horse? Jason Fenske of Engineering Explained returned to his old form and dug into exactly what horsepower is and how it differs based on where you are in the world.

The core of measuring power is the mathematical formula force multiplied by distance. To move a car a determined distance requires a certain amount of force. Add time to that equation and now you have power. James Watt is the one who brought horses into the conversation. Watt was a steam-engine builder more than a century ago, and thus was selling his machines as an alternative to horses. Therefore, he had to devise a way to compare the two and show that his engine was more productive than an animal.

One of the interesting aspects is that the initial calculation done by Watt cites the pulling force of a horse as 180 pounds. Watt noted that he was not sure if this was correct, and there are no other notes from the period that determine why this number is used. Of course, this all means we have been using napkin math done by a salesman to compare our cars for over a century. Odd.

The second part of the video is a discussion regarding horsepower versus metric horsepower. Honestly, the most interesting part is watching as Fenske goes through the math longhand while admitting it is all basically meaningless. The math checks out though, or at least I think it does. I have to be honest, I didn’t score real high in algebra. I was too busy doodling pro-street Corvairs and pickups. My bad.

The conclusion lies in the units though. As Fenske outlines, converting SAE units of pounds and feet into kilograms and meters makes for a 1.4 percent difference. This is why European cars have higher ratings in their home countries compared to what is on the Monroney sticker in the American dealer lot.

So horsepower is a bit of a baseless number. The fact is, however, if we weren’t using this measurement we’d have some other way of measuring output—maybe one based in real-world science. I doubt it would take over at this point though; humans are pretty engrained in our ways and are reluctant to change.

So, horsepower it is. We’ll just continue to ignore how absurd the math is.

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