Introduction to How Horsepower Works
Chances are you've heard about horsepower. Just about every car ad on TV mentions it, people talking about their cars bandy the word about and even most lawn mowers have a big sticker on them to tell you the horsepower rating.
But what is horsepower, and what does the horsepower rating mean in terms of performance? In this article, you'll learn exactly what horsepower is and how you can apply it to your everyday life.
The term horsepower was invented by the engineer James Watt. Watt lived from 1736 to 1819 and is most famous for his work on improving the performance of steam engines. We are also reminded of him every day when we talk about 60watt light bulbs.
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The story goes that Watt was working with ponies lifting coal at a coal mine, and he wanted a way to talk about the power available from one of these animals. He found that, on average, a mine pony could do 22,000 footpounds of work in a minute. He then increased that number by 50 percent and pegged the measurement of horsepower at 33,000 footpounds of work in one minute. It is that arbitrary unit of measure that has made its way down through the centuries and now appears on your
car, your lawn mower, your chain saw and even in some cases your vacuum cleaner.

What horsepower means is this: In Watt's judgement, one horse can do 33,000 footpounds of work every minute. So, imagine a horse raising coal out of a coal mine as shown above. A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1,000 feet in one minute, or 1,000 pounds 33 feet in one minute. You can make up whatever combination of feet and pounds you like. As long as the product is 33,000 footpounds in one minute, you have a horsepower.
You can probably imagine that you would not want to load 33,000 pounds of coal in the bucket and ask the horse to move it 1 foot in a minute because the horse couldn't budge that big a load. You can probably also imagine that you would not want to put 1 pound of coal in the bucket and ask the horse to run 33,000 feet in one minute, since that translates into 375 miles per hour and horses can't run that fast. However, if you have read How a Block and Tackle Works, you know that with a block and tackle you can easily trade perceived weight for distance using an arrangement of pulleys. So you could create a block and tackle system that puts a comfortable amount of weight on the horse at a comfortable speed no matter how much weight is actually in the bucket.
Horsepower can be converted into other units as well. For example:
 1 horsepower is equivalent to 746 watts. So if you took a 1horsepower horse and put it on a treadmill, it could operate a generator producing a continuous 746 watts.
 1 horsepower (over the course of an hour) is equivalent to 2,545 BTU (British thermal units). If you took that 746 watts and ran it through an electric heater for an hour, it would produce 2,545 BTU (where a BTU is the amount of energy needed to raise the temperature of 1 pound of water 1 degree F).
 One BTU is equal to 1,055 joules, or 252 gramcalories or 0.252 food Calories. Presumably, a horse producing 1 horsepower would burn 641 Calories in one hour if it were 100percent efficient.
In this article, you'll learn all about horsepower and what it means in reference to machines.
Measuring Horsepower
If you want to know the horsepower of an engine, you hook the engine up to a dynamometer. A dynamometer places a load on the engine and measures the amount of power that the engine can produce against the load.
Torque Imagine that you have a big socket wrench with a
2footlong handle on it, and you apply 50 pounds of force to that
2foot handle. What you are doing is applying a torque, or turning force, of 100 poundfeet (50 pounds to a 2footlong handle) to the bolt. You could get the same 100 poundfeet of torque by applying 1 pound of force to the end of a 100foot handle or 100 pounds of force to a 1foot handle.
Similarly, if you attach a shaft to an engine, the engine can apply torque to the shaft. A dynamometer measures this torque. You can easily convert torque to horsepower by multiplying torque by rpm/5,252. 
You can get an idea of how a dynamometer works in the following way: Imagine that you turn on a car engine, put it in neutral and floor it. The engine would run so fast it would explode. That's no good, so on a dynamometer you apply a load to the floored engine and measure the load the engine can handle at different engine speeds. You might hook an engine to a dynamometer, floor it and use the dynamometer to apply enough of a load to the engine to keep it at, say, 7,000 rpm. You record how much load the engine can handle. Then you apply additional load to knock the engine speed down to 6,500 rpm and record the load there. Then you apply additional load to get it down to 6,000 rpm, and so on. You can do the same thing starting down at 500 or 1,000 rpm and working your way up. What dynamometers actually measure is torque (in poundfeet), and to convert torque to horsepower you simply multiply torque by rpm/5,252.
Graphing Horsepower
If you plot the horsepower versus the rpm values for the engine, what you end up with is a horsepower curve for the engine. A typical horsepower curve for a highperformance engine might look like this (this happens to be the curve for the 300horsepower engine in the Mitsubishi 3000 twinturbo):
What a graph like this points out is that any engine has a peak horsepower  an rpm value at which the power available from the engine is at its maximum. An engine also has a peak torque at a specific rpm. You will often see this expressed in a brochure or a review in a magazine as "320 HP @ 6500 rpm, 290 lbft torque @ 5000 rpm" (the figures for the 1999 Shelby Series 1). When people say an engine has "lots of lowend torque," what they mean is that the peak torque occurs at a fairly low rpm value, like 2,000 or 3,000 rpm.
Another thing you can see from a car's horsepower curve is the place where the engine has maximum power. When you are trying to accelerate quickly, you want to try to keep the engine close to its maximum horsepower point on the curve. That is why you often downshift to accelerate  by downshifting, you increase engine rpm, which typically moves you closer to the peak horsepower point on the curve. If you want to "launch" your car from a traffic light, you would typically rev the engine to get the engine right at its peak horsepower rpm and then release the clutch to dump maximum power to the tires.
One of the areas where people talk most about horsepower is in the area of highperformance cars. In the next section, we'll talk about the connection there.
Horsepower in Highperformance Cars
A car is considered to be "high performance" if it has a lot of power relative to the weight of the car. This makes sense  the more weight you have, the more power it takes to accelerate it. For a given amount of power you want to minimize the weight in order to maximize the acceleration.
The following table shows you the horsepower and weight for several highperformance cars (and one lowperformance car for comparison). In the chart you can see the peak horsepower, the weight of the car, the powertoweight ratio (horsepower divided by the weight), the number of seconds the car takes to accelerate from zero to 60 mph, and the price.






















































You can see a very definite correlation between the powertoweight ratio and the 0to60 time  in most cases, a higher ratio indicates a quicker car. Interestingly, there is less of a correlation between speed and price. The Viper actually looks like a pretty good value on this particular table!
If you want a fast car, you want a good powertoweight ratio. You want lots of power and minimal weight. So the first place to start is by cleaning out your trunk.
For more information on horsepower and related topics, check out the links on the next page.
Lots More Information
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More Great Links
 OffRoad.com: Torque and Horsepower
 RevSearch: Engine Dynamometer Information Links
 Erbman's Engine Emporium  article describing factors that affect engine performance
 Engine Science