Horsepower is one of those terms which has become so commonplace that our fascination with the origin has all but vanished. In the context of evolution, the term horsepower became popularized about five minutes ago. As you may have guessed, the obvious correlation to the horse is indeed the nexus of the equation to which horsepower relates to.
We begin our story 268 years ago, far, far away in Glasgow, Scotland. It was a cold and damp morning with a fog that felt like sheets of wet paper. Our character, Mr. James Watt finds himself working with a team of ponies lifting coal in a coal mine. Like so many great thinkers Mr. Watt not only performed a task but he analyzed the layers of even the most mundane of tasks. Watt, being an engineer, began to calculate the amount of coal one horse could move a specified amount of feet every minute.
He determined that an average pony can move 22,000 foot-pounds of work in one minute. Watt then increases that figure by 50% and 1 horsepower is born, weighing in at 33,000 foot-pounds of work per minute. Simply stated; a horse can raise 330 pounds, 100 feet in the air, in one minute.
Race forward 268 years in five minutes and now let’s apply this thinking to my 66′ Shelby Cobra in the garage. This car has an engine that produces 425hp and the car weighs in at 2200 lbs. The power to weight ratio for my Cobra is an unbelievable .193. Compare this to a $135,000 Ferrari 355, which has 375hp, and weighs in at 2975lbs., yielding a power to weight ratio of .126, and you tell me who sees who in the rearview.
What you really want to know is, how do I know the Cobra actually has 425hp. The way horsepower is measured today is by measuring torque on a dynamometer, or “dyno”, then multiplying by rpm divided by the constant 5252, (torque X rpm/5252). Torque is used as the guide to horsepower because it is easily quantified. Torque is most simply explained as the measure of force in foot-pounds applied to an object.
For example, a one-foot wrench that applies 50-foot pounds of force can be made to apply 100 foot-pounds of force, with the same amount of energy, by adding a one-foot long cheater bar. In the case of a piston-driven motor, the pistons and the crankshaft are the equivalents to the wrench in the example above. They are responsible for providing the torque to the drive shaft, which we can now convert to our beloved horsepower.
The dynamometer measures torque by putting a load on or trying to slow, the engine. The best way to measure this in an automobile is at the drive wheels. This is accomplished by putting the vehicle's drive wheels atop a cylinder that has a brake.
Imagine a man standing on a floating log and trying to spin it with his feet while you try to stop it, (of course you are laughing sinisterly). The amount of brake pressure on the cylinder is compared against the engine's ability to produce rpm’s and then you can quantify torque.
From torque it is just a stone’s throw to horsepower: Remember, (torque X rpm/5252). Of course this all adds up to 425hp in my case. Wanna race? (This is the part that I laugh sinisterly.) P.S. Take a look at any light-bulb. You see the word watt anywhere? That is our guy, James Watt: the father of sacred horsepower, and wattage.