Now that you know about work and torque, horsepower can be defined. Horsepower is a measurement of how much work (force over distance) an engine can perform 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 pound-foot of work (force over distance). Still, the definition of horsepower also includes a time factor. So let's now assume we applied a force of one pound over a distance of one foot and did it in one minute. That would be equivalent to a small fraction of a horsepower because James Watt's definition of one horsepower is performing 33,000 pound-feet of work in one minute.
Moreover, the definition for horsepower does not need to remain firm for producing 1 horsepower. As an example, the force, distance and time factors can be varied as long as the result is equivalent to performing 33,000 pound-feet of work in one minute. For instance, applying a force of 66,000 pounds over a distance of one 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 one horsepower.
The constant 5252 is derived from James Watt's 18th century definition of horsepower. The constant includes the "time factor" into the equation.
Horsepower can be related to the internal combustion engine by associating 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 rotating.
When designing, building and tuning an engine, it is important to remember the following four principles:
(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) 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. These adages inform us 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 gobs of torque low in the rpm band while horsepower engines make big torque at the top end.
Another description for horsepower is how much and how often a cylinder fills, and how often the cylinder fires during a given time frame. Keep in mind that since horsepower is a calculated number, the only practical method for determining horsepower is by first measuring engine torque and rpm with a dynamometer.
Inasmuch horsepower is equal to torque multiplied by rpm, an increase in torque at any given rpm increases horsepower at that same rpm. As such, when striving for best performance, wise engine builders concentrate on improving torque rather than horsepower. They also concentrate on improving torque within the most important rpm range the engine operates.
4. These two engines make the same amount of peak torque at the same rpm but the areas under the curve are unequal. Since engine #2's torque curve is broader, it has more area under the curve. Generally, the engine with the broader torque curve accelerates faster and is more suitable for street riding. For optimized performance, maximize torque within the engine's most critical rpm range and don't worry about horsepower.
4. These two engines make the same amount of peak torque at the same rpm but the areas un
5. Here we see that torque peaks at a low 3,000 rpm, then quickly drops off since the engine runs out of air because the induction and exhaust systems are restricted. Horsepower also drops quickly because volumetric efficiency (cylinder fill) is reducing faster than rpm is increasing.
5. Here we see that torque peaks at a low 3,000 rpm, then quickly drops off since the eng
6. Increasing the engine's ability to breathe with more cam timing/lift and higher-flowing induction/exhaust systems will move the torque peak horizontally on the chart to a higher rpm. Horsepower will also increase because the volumetric efficiency is dropping slower than rpm is increasing.
6. Increasing the engine's ability to breathe with more cam timing/lift and higher-flowin
7. Since leverage is torque, a long-stroke engine will make about an identical amount of peak torque as a shorter-stroke engine of equal size. However, the rpm at which the peak torque occurs will be at a lower rpm. Stroker flywheels not only increase displacement but also provide greater leverage for more low-end torque. - Short Block Charlie's
7. Since leverage is torque, a long-stroke engine will make about an identical amount of