It doesn't matter whether you stop by your local motorcycle dealer or favorite Friday night hangout; sooner or later, chitchat gravitates to performance. And sometime during the conversation, the word "horsepower" is tossed around with abandon. Typically, someone says they dyno'd their engine and it made huge horsepower. Another guy quips that he is building a monster motor and expects to get 150hp. Another performance guru says he bolted on a new carb and picked up big horsepower. Nevertheless, the common thread in the conversations is often "horsepower."
Clearly, horsepower has panache. Although the term is tossed around frequently, no one really defines it. And to make matters even more confusing, the word "torque" is often tossed into the conversation. As such, questions often left unanswered are what is horsepower, how do you define it and where does torque fall into the mix?
Work
Truth be told, horsepower isn't something discernible by touch. Instead, it is a measure for the amount of work an engine can perform in a given time. Eighteenth century engineer and inventor James Watt spearheaded the development of the high-pressure steam engine. That led to the need for a method of measuring the amount of work the steam engine could perform over a given amount of time. Since the steam engine would perform work commonly done in that epoch by draft horses, Watt related an engine's work to the work a horse could perform. Using a progression of tests along with empirical data, Watt surmised that moving 33,000 pounds 1 foot in one minute was the equivalent of 1hp, which has become the standard for measuring the force or power output of the internal combustion engine. But to explain horsepower first requires a discussion of force and torque, because horsepower is a calculated number derived by first measuring torque at a given rpm.
Torque
Work is measured as a force exerted in a straight line. However, common items that involve force, such as an engine's crankshaft, nuts and bolts when they are tightened or loosened and a bicycle pedal crank, rotate around an axis. The rotational or twisting force of these items is called "torque." Torque is a measure of the ability of a force to cause twisting or rotation, which is measured in "pound-feet" units of force times the distance from the axis of rotation. For example, if you have a wrench 1 foot long and apply a force of 100 lb-ft at the end of it, you are applying a torque of 100 lb-ft. If the same wrench were 3 feet long, 100 lb-ft of force would apply 300 lb-ft of torque. In other words, if your V-twin engine makes 100 lb-ft of torque, it would take 100 pounds of force on a 1-foot lever to stop its rotating motion.
Today, engine torque is typically measured on a dynamometer and can be defined as the potential to do work. Unlike horsepower, however, torque does not take into consideration the element of time, which gauges the rate at which an engine can perform work. An engine's power rating is actually established by first measuring torque at a given rpm and then mathematically calculating horsepower.
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Both of these engines make the same peak torque at the same rpm but have unequal areas und
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This power chart shows that torque peaks at a low 3000 rpm and then drops off quickly beca
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By increasing the engine's ability to breathe (more cam, higher-flowing induction/exhaust
Horsepower
Horsepower is a measurement of how much work (force over distance) an engine can do while including the element of time it took to do the work. Therefore, horsepower is a function of a given amount of force (torque) acting over a given distance from the axis of rotation within a given amount of time (rpm). A simple example of performing work over a specified distance is applying a force of 1 pound over a distance of 1 foot. This is equivalent to 1 lb-ft of work (force over distance). However, the definition of horsepower also includes a time factor. So let's now assume we applied a force of 1 pound over a distance of 1 foot and did it in one minute. That would be equivalent to a small fraction of a horsepower, because James Watt's definition of 1hp is performing 33,000 lb-ft of work in one minute.
Furthermore, the definition for horsepower does not need to remain rigid for producing 1hp. The force, distance and time factors can be varied as long as the result is equivalent to performing 33,000 lb-ft of work in one minute. For instance, applying a force of 66,000 pounds over a distance of 1 foot in two minutes or applying a force of 300 pounds over a distance of 100 feet in one minute or applying a force of 33,000 pounds over 3 feet in three minutes are all equivalent to James Watt's definition of 1hp. Following are the equations for calculating horsepower and torque.
horsepower = torque x rpm /5252
torque = horsepower x 5252 /rpm
The constant 5252 is derived from James Watt's 18th century definition of horsepower. The constant enters the "time factor" into the equation.
What It All Means
We can relate horsepower to the internal combustion engine by associating the three factors involved--force, distance and time--in the following manner: (1) Force is equivalent to the amount of combustion pressure applied to a given square area of the piston dome. (2) Distance is equivalent to the engine's stroke length. (3) Time is defined by the rpm or speed at which the engine is turning.
The following are four important principles to remember when designing, building and tuning an engine: (1) At any given rpm, horsepower is directly proportional to torque. (2) By increasing torque at a specified rpm, horsepower increases at a corresponding amount. (3) If torque remains constant but rpm increases, then horsepower increases in direct proportion to rpm. (4) Even when torque starts to drop off (beyond the engine's torque peak), as long as rpm increases faster than torque drops, horsepower will still increase. What these axioms tell us is that every engine is a torque engine. And the only difference between a torque engine and horsepower engine is where the engine makes its torque: Torque engines make lots of torque low in the rpm band while horsepower engines make gobs of torque on the top-end.
Horsepower can also be described as how much and how often a cylinder fills and how often it fires in a specified time. However, keep in mind that horsepower is a calculated number, so the only practical method for determining horsepower is by first measuring engine torque and rpm with a dynamometer.
Since horsepower is equal to torque multiplied by rpm, any increase in torque increases horsepower at a given rpm. That is why it is better to concentrate on improving torque than horsepower for achieving the best engine performance. When designing or building an engine, always concentrate on improving torque within the most important rpm range the engine operates.