H O R S E P O W E R.††††† Just what is HP ??
Torque and Horsepower are the most used and least understood words in any engine userís vocabulary. Just know right now that HP can only be a 'calculated number' from Torque, which is a 'measured scale number' off of a Dyno.
These two terms are not as hard to understand as some people, (who believe in the BLACK ART of tuning), would like you to believe. They describe different aspects of the engines performance. The words are only two out of a large selection of terms that tell exactly what goes on inside a working engine. To explain torque and hp properly without understanding some of the other terms is not possible. However, it is a lot easier than you think to learn them, so here goes.
The magic starts when the spark plug fires the fresh intake charge in the just compressed combustion chamber. A tremendous amount of heat is caused be the rapidly burning gases. That heat causes the gases to expand and push the piston down in a straight line with great pressure. This causes the connecting rod to convert this straight line movement into rotary motion by spinning the crankshaft, if this succeeds this is called TORQUE.
To measure torque, you must know 2 things, (1) the amount of force to the rotating object; and (2) the lever distance, which is the distance from the center of the rotation to the point where the force is being applied.
On a crankshaft this lever distance is from the center of the crank, out to the center of the bottom end of the rod where it joins the crankshaft. If the force is 50 pounds and the distance is one foot then the torque is 50 pounds-feet.
Torque figures could be confusing if they had to be actual force and actual lever distance. To make life easier they are converted to a lever distance of one foot. This is easy to do: just multiply the force by the lever length in feet,; i.e. 75 pounds at two feet becomes 150 pounds-feet; 75 pounds at three feet becomes 225 pounds-feet; 75 pounds at three inches (one quarter foot) is 18.75 pounds-feet.
Torque can exist without rotation. If you pull on a torque wrench with 50 pounds of pull, but the bolt isn't turning, the 50 ft.lbs. of torque is still there.
If the force succeeds in moving something then Work is done. Work is a unit of measure that is calculated by multiplying that force applied by the distance is moved. If 100 pounds was used to slid an object 5 feet then the work done was 500 pounds-feet.
To figure the Work done on an engine when a crankshaft turns one full turn, you need to know how far it is around that circle. Calculate this distance by knowing that the circumference of a 2 foot circle is 6.28 feet ( or 2 * 3.14 ). If you use 50 pounds-feet as example then 50 * 6.28 and you get 314 pounds-feet of Work done.
Knowing that, you then have to check on how long it took. One second? One minute? One hour? This answer is calling for another term Power (not horsepower). Power is the amount of work done in a given length of time. If an engine is running at 3000 rpm, you multiply 314 pounds-feet times 3000, and answer is 942,000 pounds-feet of work per minute, or Power generated at 3000 rpm .
The above example is for a TWO-STROKE engine, the force is applied every revolution. A four stroke fires every other stroke, think in pairs of rpm. A four stroke with same lever-force specs as a two stroke, the crank pin would have to do twice as much work to per firing due to distance being double. The power would be the same.
HORSEPOWER is the estimate that a horse can lift 33,000 pounds of weight one foot in one minute (or 550 pounds one foot in one second). This was conceived by James Watt about 200 years ago. So measure of work-per-minute was designated one-horsepower and engines are now rated this way.
If an engine produces ten horsepower, that means it can lift 330,000 pounds one foot one minute, or 33,000 pounds ten feet in one minute, or 33,000 one foot in six seconds (one-tenth of a minute). Measured by weight, distance or time the engine can do ten times the work of a horse.
To calculate above example the engine did 942,000 pounds-feet of work at 3000 rpm. So 942,000 divided by 33,000 pounds-feet per minute, and answer is 28.5 horsepower at 3000 rpm.
To simplify all this we now just multiply the torque by the rpm and divide by 5252 (a constant number used to compute horsepower using torque), the horsepower output is directly related to its torque and rpm.
This means to alter its horsepower an engine has to change either the amount of torque made, or the rpm at which it makes this torque.
At any specific amount of torque at one specific rpm any engine will make the same horsepower, regardless of number of cylinders, two or four strokes, or anything else.
-Tom E. Turner -- TSR Software