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Home Garage Here's why horsepower is a dumb metric

Here’s why horsepower is a dumb metric

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Horsepower is often the first figure people throw out in bench-racing sessions, but is it really the best way to measure a car’s power output? Jason Fenske at Engineering Explained doesn’t think so.

As a quick refresher, power is defined here as work over time which, for cars, the amount of force needed to propel a car a given distance, divided by the time it takes to cover that distance.

Horsepower was devised by James Watt, inventor of the practical steam engine. Knowing his engine would primarily compete against horses, Watt came up with a way to compare the power output of machine and animal. This would show that the steam engine could do the work of multiple horses.

How Watt defined horsepower is disputed, but it’s thought Watt based it on a horse turning a mill. The measurement is based on the horse walking a 12-foot diameter circle to turn a shaft, and exerting 180 pounds of force on that shaft. But these figures are arbitrary. Horses don’t all have identical strength, after all, and it seems Watt rounded up the amount of force in his writings, Fenske noted. Bottom line, one horsepower equals one horse lifting 550 pounds one foot in one second.

Horsepower is also measured differently on the United States and metric systems. Metric horsepower (often abbreviated as PS or CV), uses metric units of measurement, meaning kilograms instead of pounds, and meters instead of feet. That means 1 metric horsepower is only equal to 98.6% of 1 U.S. horsepower.

It’s also why European supercars sometimes have confusing names. The McLaren 765LT, for example, is named for its output in metric horsepower. In the U.S. system, it’s rated at 755 horsepower.

Is there an alternative to horsepower? Fenske suggests the Watt (named after James Watt, after his death) because it’s divisible by one and thus much easier to work with. Some automakers already list power output in kilowatts (1,000 watts) alongside horsepower, and it’s becoming more relevant for electric cars.

What do you think? Is it time for a change, or should the auto industry stick with horsepower?

Click on the video above for a deeper dive into the definition of horsepower.

This article was originally published by Motor Authority, an editorial partner of ClassicCars.com.

7 COMMENTS

  1. The question should be “Should the AMERICAN auto industry stick.to horsepower?”
    The rest of the world has been using the much more easily understood metric system for years. It’s time USA caught up.

    • Yea that’s the general rule however measuring torque is even more vague than measuing horsepower. And once you correctly arrive at horsepower and torque numbers as we all know their isnt a direct correlation to vehicle quickness or top speed.

  2. Not that I have the math skills to calculate the formulas anyway but I prefer to stay with horsepower. It’s what I know and I’m familiar with.

  3. Here’s the thing, you can claim it is a dumb metric yet it is a widely accepted metric. While horsepower will never completely measure a cars speed, output or efficiency it can give a general idea of what to or what you should expect. Knowing that a Honda Civic puts out 200 hp and a Camaro ZL1 puts out 650 hp can give you a pretty good idea of maybe just how much faster the Camaro is going to be. Only metric better is the far more fun in the seat of the pants feel.

  4. Horsepower IS dumb as it is only obtained at maximum revs, and how often does the average car run like that? It’s torque that get you there i.e. acceleration, which we all use all the time. Horsepower is irrelevant if it takes an age to get there.

    A long time ago I read (from memory), that oversteer is when the rear of the car hits the wall, understeer is when the front hits the wall, horsepower is how hard you hit the wall, and torque is how far you move the wall.

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