We hear the term “horsepower” used all the time, but what is it? Well read on and you’ll find out!

The word horsepower was introduced by James Watt (the inventor of the steam engine among other things) around 1775.  If you read my section on electricity, you’ll know a little about Watt. Mr. Watt came to the realization that “a strong horse could lift 150 pounds to a height of 220 feet in one minute.”  Don’t ask me how he came up with these parameters or why he was having a horse lift 150 pounds to a height of 220 feet – poor horse!

One horsepower is also commonly expressed as lifting 550 pounds one foot in one second or 33,000 pounds one foot in one minute.  These are just different ways of saying the same thing.

You’ll notice that when quantifying horsepower, one always needs to include force (pounds), distance (feet), and time (minute or second).

Back to horses, imagine a horse raising dirt out of a pit. A horse exerting one horsepower would raise 550 pounds of dirt one foot every second. 

Here is an example of another way horsepower can be directly measured. Imagine you have a horse hitched to a plow.  In the hitch is a spring scale (like the kind used to weigh produce at the market).  The horse pulls the plow one foot every second and 550 pounds is measured on the scale. The horse is generating one horsepower.  I find this example a little easier to understand.

As can be seen by the example above, horsepower can be directly measured.  However, there is a problem when directly measuring horsepower of internal combustion engines because they produce rotary, not linear, motion. In addition, unless the engine is geared down, the speed at which they do work is too great for practical direct measurement of horsepower. 

The solution is to directly measure torque (rotational force expressed in pounds at one foot radius) and RPM (time and distance with distance measured in circumference at the one foot radius) and from these calculate horsepower.  Torque and RPM can be measured directly.  Early dynamometers used a brake device to load the engine. A torque arm was attached to the brake’s stator and the brake’s rotor was coupled to the engine’s crankshaft. A spring scale or other measuring device connected the torque arm to the stationary fixture holding the engine and brake.  During a test the brake’s application loaded the engine.  Torque and engine RPM were then observed and recorded.

Modern day dynamometers calculate horsepower directly by measuring the torque and RPM related to water flow. On a dynamometer, it takes more water flow to the water brake to increase the load on the engine being tested.  As the test engine’s torque rises more water flow is needed. As the test engine’s torque drops less water flow is needed.  The dynamometer’s water brake does not respond to horsepower. Major adjustments to water flow are needed as an engine crosses its torque peak but none are needed as it crosses its horsepower peak. In other words, the water flow to the brake during a dynamometer test follows the engine’s torque curve and not its horsepower curve.  Torque is what twists the tire. Horsepower helps us understand an amount or quantity of torque over a period of time (torque + time and distance).

If we are measuring only torque and RPM, how is horsepower calculated?  You may be familiar with the equation HP=TORQUE * RPM / 5252, but where did it come from?  Well, using Watt’s observation of one horsepower as raising 150 pounds to a height of 220 feet in one minute, we first need to express 150 pounds of force as foot pounds of torque. This is done as follows:

• Pretend the force of 150 pounds is applied tangentially to a one foot radius circle.  This is 150 foot pounds of torque.

Next we need to express 220 feet in one minute as RPM. How is this done?  Like this:

• The circumference of a one foot radius circle is 6.283186 feet (Pi * diameter or 3.141593 * 2 feet).
• The distance of 220 feet, divided by 6.283185 feet, gives us a RPM of 35.014.

We are then talking about 150 pounds of force (150 foot pounds of torque), 35 RPM, and one horsepower.

Constant (X) = 150 ft. lbs. * 35.014 RPM / 1hp

35.014 * 150 / 1 = 5252.1

5252 is the constant.

Therefore, HP = torque * RPM / 5252

To look at it a different way, 150 foot pounds at 35.014 RPM = one horsepower.

So let’s take this calculation and apply it something we’re familiar with, a RC nitro engine!  Suppose we measure that at 40,000 RPM (crankshaft) the motor is generating 1/4 ft. lbs. of torque (usually on a small engine it’s measured as oz. inches but to keep things simple I’ll use ft. lbs.), what’s it’s horsepower output?  Simple, if you follow the equation: (0.25 ft. lbs. * 40,000 RPM) / 5252 = 1.9 horsepower.  Easy!

Calculating horsepower for an electric motor is even easier, simply multiply amps * volts to get watts and divide watts by 746 and you get horsepower.

Lastly, here’s something that’s pretty interesting.  Below 5252 RPM, any engine’s torque number will be higher than it’s horsepower number and above 5252 RPM any engine’s horsepower number will always be higher than its torque number. At 5252 RPM, the horsepower and torque numbers (or curves, if you are graphing) will cross and be exactly the same.

Well there you go, that’s a crash course in horsepower.  So, next time your buddy is bragging about his RC truck’s horsepower (or full size truck!), you’ll be able to bring him down to earth if his horsepower numbers have lost touch with reality!

Drop me a line at webmaster@e-maxximumrc.com!