What is…BHP?

When considering what to discuss in this series of blog posts one of the first things that came to mind was BHP: brake horsepower. It is a very common term, but I realised I don’t really know what it is beyond the fact that it’s generally seen as a good thing to have a higher BHP.

A little bit of history
Don’t worry, this won’t be a lengthy history lesson.
The question is: where does the term brake horsepower come from? The term horsepower (without the ‘brake’) comes from James Watt (1736 – 1819) – yes, the one of the light bulbs. While working at a coal mine he calculated that one mine pony could do 22,000 foot-pounds in a minute. He then decided to add 50% which means that 1 horsepower is 33,000 foot-pounds per minute. The additional 50% comes from the idea that a horse is stronger than a pony. I won’t go into detail regarding the differences and similarities of horses and ponies (every pony is a horse, but not every horse is a pony).
He came up with this term to compare the work done by a horse with the work done by a steam engine, which is what he was trying to sell to people who were used to working with horses for generations.

Image courtesy of How Stuff Works

Let’s see how he calculated this exactly.
If you look at the image above, you could use the example that one horse could raise 330 pounds of coal 100 feet in one minute. So 330 pounds x 100 feet is 33,000 foot-pounds. As the How Stuff Works website points out there are various combinations possible; this is one of the more likely scenarios.

How do you calculate a car’s horsepower?

Image courtesy of wikiHow

Looks scary, but it really isn’t.
The website wikiHow (the link is under the image above) explains it in quite simple terms. First of all you have to find out the torque of your car. You should be able to find this in your car’s manual. I will use my own car as an example. I drive a Mazda 2 with a 1.3 litre engine. According to the Mazda website my car has a maximum torque of 119 / 3500 Nm / rpm (I may have to explain Nm in a later post…).

Step two is following the formula above. Let’s look at my car again. The formula would be: HP = (3500 x 119) / 5252.
Let’s break that up a little and start with 3500 x 119 = 416500. Then we divide 416500 by 5252 which is 79.3. There you have it, my car has 79.3 HP. Cute, isn’t it?
Oh, for the record, I used the maximum torque mentioned by Mazda. I wouldn’t be surprised if my car has a little less.

So what is brake horsepower?
I hope you haven’t confused the terms horsepower and brake horsepower. They are indeed something different!
The difference lies in where these units are measured. Horsepower is measured at the crank shaft. Or, in simpler terms, horsepower is the energy generated by the engine.
Brake horsepower is horsepower after the loss of energy caused by the other components. Let’s make that a little simpler: the energy generated by the car’s engine goes through a lot of other components before the car starts moving. Think of, for example, the gearbox, differential; all that kind of stuff. There is a loss of power because of this and therefore BHP is something different than HP.

I have to admit the jury seems to be out a little bit regarding where BHP is actually measured, but they seem to agree on the fact that there is energy loss between the engine and the wheels. And I don’t really want to make it any more complicated that this. However, if you have anything to add or to ask, feel free to post a comment underneath this post. I will happy to investigate further.


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