Auto Questions: How Horsepower Works

Often times car manufacturers will use terminology we may not completely understand, like horsepower. This article will define what horsepower is and how it works.

The term horsepower is used quite frequently by manufacturers to describe their product lines - whether it is a Chevrolet Corvette with four hundred horsepower or a Ford Mustang with three hundred. Since the muscle car era in the 1960's, big horsepower outputs in cars have been used to draw buyers into dealerships and increase sales. But what is horsepower?

Horsepower is a term that was invented by James Watt back in the 18th century before cars were around. He came up with this term to measure the amount of energy needed for one horse to haul a load of coal out of a mining pit. The simple definition of horsepower is therefore energy required to lift 550 pounds of coal one foot in the air in one second.

Obviously times have changed, and very few people use their cars to haul coal these days. In simple terms, automotive horsepower is an indication of how able a vehicle will be at moving quickly from one point to another. Horsepower will also be directly related to a vehicle's top speed - a three hundred horsepower Mustang will have a higher top speed than a base Mustang with just two hundred.



The horsepower numbers that manufacturers quote in their marketing material is the estimated amount of power produced at the engine's crankshaft. As a result of internal combustion, the pistons inside an engine move in an up and down motion inside the cylinders. The cylinders are connected to the crankshaft by means of connecting rods and this movement causes the crankshaft to turn. The turning power of the crankshaft is therefore the gross horsepower output of an engine.

Gross horsepower is the measurement taken before any load is placed on an engine. When load is added to an engine, power is sapped by the frictional forces needed to turn the belts and pulleys (that drive the power steering, air conditioning and alternator) as well as the transmission, drive shaft, differential and axles. Net horsepower is therefore the measurement of power taken at the wheels.

The most accurate measure of net horsepower is to place a vehicle on a dynamometer. On this machine, the vehicle's wheels are strapped down onto large metal rollers and the twisting force or torque of the engine measured as the car accelerates through different gears. Torque is the measurement taken by the dynamometer, but this figure is converted into horsepower, as the two values are inter-related. Horsepower is the calculated from torque as follows: (torque x rpm)/ 5252 = horsepower. Net horsepower will always be less than the gross horsepower from an engine.

It is important to note that there will be a difference in net horsepower for two vehicles within the same model line. These differences are caused mainly by the variations within manufacturing tolerances used on the assembly line of a mass-produced car. So while two Corvettes may both be rated from the factory at four hundred horsepower, slight differences during the engine building process could cause one car to make three hundred and fifty horsepower at the wheels while the other makes only three hundred and forty.

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