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Question: Please give me some information about the terms 'horsepower' and 'brake horsepower'. I believe the two are related, but would like to know more. 


Answer:  We're not motor engineers, but we'll try to give you some help.

As you rightly say, brake horse power is related to horse power.

Now for a little history:

One horsepower was originally defined as the amount of power required to lift a mass of 33 000 pounds through a vertical distance of 1 foot in 1 minute. This is also stated as 550 foot pounds per second (fps). James Watt worked out this figure for the horsepower when he did some experiments on horses hauling coal. He found that horses could haul coal at the average rate of 22 000 foot-pounds per minute. He then applied a correction factor which gave the current value of 33 000 or 550 fps.

We're talking about the power of an engine here, not its weight, nor the weight of the car. You could have an extremely light weight powerful engine in a heavy car or various permutations based around this. Three different horsepower values are used in talking about engine performance: (1) Indicated horsepower is the theoretical efficiency of a reciprocating engine, which is determined from the pressure developed by the cylinders of the engine; (2) Brake or shaft horsepower is more commonly used to indicate the practical ability of the engine, or the maximum performance, which is the indicated horsepower minus the power lost through heat, friction, and compression; (3) Rated horsepower is the power that an engine or motor can produce efficiently for sustained periods of time. Obviously the second of these is the one that is of interest to you. In Britain, engines are classified in rated horsepower, but their brake horsepower may be four to six times the rated horsepower.

To measure the brake horsepower is a sophisticated business involving computers nowadays, which are able to monitor how much power is produced at different parts of the power cycle and how engine speed affects power output. If you pick up a car brochure you will see in the engine specification, data referring to power outputs. Engine capacity and engine type are further factors that can affect power output. Small engines can be manufactured to produce as much power, or even more than larger engines.

The bhp can be given as a gross figure, or can be given based on bhp per ton which is related to weight. Just because the manufacturer boasts a car has a high bhp it doesn't guarantee a high performance. Cars that are better engineered tend to drive better than those that aren't. Try a BMW M Sport!!




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