What is Horsepower?

How do you define horsepower? Ask a car enthusiast and most of the time you'll get a blank look, a shrug of the shoulders and maybe a guess along the lines of "What a horse can do!". That answer begs the question: What horse? A thoroghbred race horse that can carry the small weight of a jockey with a lot of speed, or a working horse that can pull heavy loads albeit slowly? Obviously there is a more precise answer. Car manufacturers, despite their reputation for being creative regarding the horsepower ratings of their products for marketing reasons, require a more stable definition.

Horsepower is defined as work done over time. The exact definition of one horsepower is 33,000 lb.ft./minute. Put another way, if you were to lift 33,000 pounds one foot over a period of one minute, you would have expended one horsepower. Even more interesting is how the definition came to be. It was originated by James Watt, (1736-1819) the inventor of the steam engine and the man whose name has been immortalized by the definition of Watt as a unit of power. The next time you complain about the landlord using only 20 watt light bulbs in the hall, you are honoring the same man. To help sell his steam engines, Watt needed a way of rating their capabilities. The engines were replacing horses, the usual source of industrial power of the day. The typical horse, attached to a mill that grinded corn or cut wood, walked a 24 foot diameter (about 75.4 feet circumference) circle. Watt calculated that the horse pulled with a force of 180 pounds, although how he came up with the figure is not known. Watt observed that a horse typically made 144 trips around the circle in an hour, or about 2.4 per minute. This meant that the horse traveled at a speed of 180.96 feet per minute. Watt rounded off the speed to 181 feet per minute and multiplied that by the 180 pounds of force the horse pulled (181 x 180) and came up with 32,580 ft.-lbs./minute. That was rounded off to 33,000 ft.-lbs./minute, the figure we use today. Put into perspective, a healthy human can sustain about 0.1 horsepower. Most observers familiar with horses and their capabilities estimate that Watt was a bit optimistic; few horses could maintain that effort for long. Although the standard for rating horsepower has been available for over 200 years, clever car manufacturers have found ways to change the ratings of their engines to suit their needs. During the famous horsepower wars of the 1960s, manufacturers could get higher figures by testing without auxiliary items such as alternators or even water pumps. High ratings backfired when insurance companies noticed them and started to charge more for what they saw as a higher risk. Manufacturers sometimes responded by listing lower horsepower figures, forcing enthusiasts to look at the magazine test reports to determine what was going on. In the early seventies the SAE (Society of Automotive Engineers) stepped in with standardized test procedures and the fiqures were more consistent. Between 1922 and 1947, the Royal Automobile Club used a horsepower rating that was the basis for an automobile tax. The horsepower of an engine was determined by multiplying the square of the cylinder diameter in inches by the number of cylinders and then dividing that figure by 2.5. Using this dubious method, What we know of as a 385 horsepower motor found in the Z06 Corvette would be rated at only 48.67 hp! There is a metric horsepower rating, although it is rarely used. The two methods are close, with one SAE horsepower equal to 1.0138697 metric horsepower. One horsepower also equals 745.699 watts. This means that if you really want to confuse people, you could complain about the .02682 horsepower light bulb your landlord has in the hallway as opposed to the mundane 20 watt measurement.

             

Simple Horsepower Calculator

horsepower

The horsepower (hp) is a unit in the foot-pound-second (fps) or English system, sometimes used to express the rate at which mechanical energy is expended. It was originally defined as 550 foot-pounds per second (ft-lb/s).
A power level of 1 hp is approximately equivalent to 746 watts (W) or 0.746 kilowatts (kW). To convert from horsepower to watts, multiply by 746. To convert from watts to horsepower, multiply by 0.00134. To convert from horsepower to kilowatts, multiply by 0.746. To convert from kilowatts to horsepower, multiply by 1.34.
While the horsepower, the watt, and the kilowatt are all reducible to the same dimensional units (they each represent a certain rate of energy expenditure, or power), the horsepower is rarely used to express power in any form other than mechanical. You will likely get raised eyebrows if you talk about a 1-hp microwave oven, just as you would feel uncomfortable talking about a 37-kW outboard motor.