What is horsepower, by definition?
I’m not talking about the speed index of race horses here, I’m talking about cars, and trucks and well any vehicle really. When it comes to talking about vehicles, and in particular, the performance or power of a vehicle, the term horsepower (HP) is used. What exactly does it mean? Should you look at the horsepower rating when buying a vehicle?
When you ask Google to define: horsepower, the answer you’ll get is this;
- a unit of power equal to 550 foot-pounds per second (745.7 watts).
- the power of an engine measured in terms of this.“a strong 140-horsepower engine”¹
There is mechanical and metric horsepower. When referring to vehicles, mechanical horsepower is what we are talking about here.
Is horsepower related to horses?
Yes, it IS related to horses, and it was a Scottish engineer who came up with the term. James Watt² born in 1736 is known for the work he did on steam engine development. He wanted a way of comparing the power of his steam engines to the power of draft horses.³
“One horsepower was deemed to be the equivalent of one horse lifting 33,000 pounds over one foot in one minute on the surface of the Earth.”¹¹ (550 foot pounds per second).
How does horsepower relate to vehicles?
Horsepower is a measurement of power. Manufacturers advertise horsepower figures to show how much power the vehicle has. ie. in relation to how fast the vehicle is. Without getting too complicated, as this is not always the case, but the higher the horsepower number, the more powerful the vehicle is. As an example, the 2017 Fiat 500 Pop has 101hp, the 2017 Dodge Demon has 840hp, so the Demon has more power. A LOT more power.
Should you look at horsepower figures when comparing vehicles?
Yes, but as a rough guide. If there are two vehicles within 20hp of each other, drive them both and see if you even notice a difference.
There are ways that HP figures can be skewed. Larry Webster wrote in a Car and Driver article that, “Since minute differences in the dimensions of engine components that naturally occur during series production—the compression ratio could be a smidge higher in one engine versus another—can result in two identical-looking engines making slightly different horsepower…”¹²
There are also different ways of quoting figures, such as BHP (brake horsepower), IHP (indicated horsepower), SHP (shaft horsepower), WHP (wheel horsepower). On top of that, other countries in the world have different systems. Germany uses Pferdestärke, which is quoted as PS. For example, a German vehicle might be rated at 200PS. 1 PS is not quite equivalent to 1 HP (PS x 0.9863 = 1HP).
There doesn’t seem to be any regulations as far which of these numbers should be quoted by manufacturers, although from my understanding, they do have the option to have the number SAE certified. If you were a manufacturer, you’re probably going to choose the highest number, which would be the BHP measurement at the crank. “Brake horsepower (bhp) is the power measured at the crankshaft just outside the engine, before the losses of power caused by the gearbox and drive train”³.
Other factors to consider
An HP figure alone will not tell how powerful a vehicle is, as there are other variables that affect power, performance, and speed of a vehicle. The weight of the vehicle is a huge factor. A car that weighs 3 ton is going to need more power to move it than a car that weighs 1 ton. Torque is another factor, along with vehicle design, aerodynamics, loss of power through other components used, and altitude etc.
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¹”Define:horsepower.” Google Search. Google, n.d. Web. 12 July 2017.
²Kingsford, Peter W. “James Watt.” Encyclopædia Britannica. Encyclopædia Britannica, Inc., n.d. Web. 12 July 2017.
³”Horsepower.” Wikipedia. Wikimedia Foundation, 08 July 2017. Web. 12 July 2017.
¹¹Vousden, Mike. “What Is Horsepower? HP, BHP & PS Explained.” Carwow.co.uk. Carwow, 1 Feb. 2016. Web. 12 July 2017.
¹² LARRY WEBSTER. “Horsepower Confusion and Resolution – Column.” Larry Webster: Horsepower Confusion and Resolution – Column – Car and Driver. Caranddriver, 01 Apr. 2006. Web. 12 July 2017.
Lauren has been working in the automotive industry both in the U.K and in the U.S. for over 10 years. She has driven hundreds of vehicles, not only new cars but beaters without heaters, fast cars on fire, slow cars in snow, off-road trucks in the mucks, and old pickups with pups. She’s driven heaps of Jeeps, miles in muscle and once took her gran in a car from Japan.