Horsepower - what is it?

"You gotta start somewhere, this is a good place."

Before we start talking about turbo this or nitrous that, we need to lay some groundwork. Every gearhead wants more horsepower, but what exactly IS horsepower? What does it measure? Horsepower is just like inches or pounds, an arbitrary unit created from a common reference point that everyone can understand. In today's world of advanced scientific instruments, horsepower hangs on, even though it is a little imprecise. Those high minded keepers of the units and standards that quantify everything with precision would rather toss out this well known measure and substitute kilowatts. That Corvette has 298.28 kilowatts, hmm... 400hp just sounds better.

Where did horsepower come from?

James Watt, who did quite a bit of work on steam engines back in the 1700's, needed a way to measure their output. Watt used a common reference, the horse, as the basis for his calculations (like the inch was based on the width of a man's thumb). The exact process he followed to find out what a horse could do is open to speculation, everyone seems to have their own favorite story, but the end result was: 1 horsepower = 550 foot-pounds per second, which means, in Watt's calculations, a horse can lift 550 pounds one foot in one second.

There are only seven base units of measurement: length, time, mass, temperature, electric current, amount of substance and luminous intensity. Each unit can be determined by scientifically reproducible results (no more horses and thumbs!) and all units and standards used today can be derived from those basic seven. An international system, SI, maintains the agreed upon standards for all of these basic units.

Horsepower conversion formulas

The neat thing about defining a reference point with numbers is how easy it is to convert that reference to some other unit of measure.

1 horsepower = 550 foot-pounds/second
1 horsepower = 33,000 foot-pounds/minute
1 horsepower = 42.44 Btu/minute
1 horsepower = .7456999 * kilowatts

1 kilowatt = 1.34102 * horsepower

Horsepower is one measure of power

All of those formulas and conversions are different ways of saying how much work is being done, which is exactly what power is. Power is work done over time.

P = W / t

where P is power, W is the work done and t is time.

Watts are the more common term for measuring power which is why the conversion to and from horsepower is good to know. One watt is 1 joule/second. And that can be converted to ... well, you'll have to do the rest of that research yourself because we could go on forever.

A few more horsepower details:

There have been a few different horsepower rating systems over the years. The old "SAE horsepower" is no longer used because it gave a misleading number due to the fact that an engine was tested without any auxillary items installed like alternators, air cleaners or fans. Since 1972 the "SAE net horsepower" is used to indicate the engine was tested with everything in place.

More recently, the Society of Automotive Engineers (SAE) released a new testing procedure, J2723, to determine horsepower. The 7.0 liter LS7 engine for the 2006 Z06 Corvette, was the first engine certified under the new procedure. The output of the LS7 is certified at 505 horsepower at 6300 rpm and 470 lbs.-ft. of torque at 4800 rpm.

You will also hear the term "brake horsepower" (bhp) which is the power measured at the crankshaft. In the very early days, a carefully measured braking force was applied to a drum being driven by the engine in a 1:1 ratio. Now the brake is a dynamometer driven by the engine but the old term remains.

"Rear wheel horsepower" or more frequently today, "Horsepower at the wheels " is just what you would think it is, it's the horsepower available at the drive wheels. Since the driveline between the engine and the wheels uses some of the engine's power, there will be less available to move the car, truck or motorcycle. This number is one of the best comparisons since driveline losses vary from vehicle to vehicle.


Now you know where horsepower came from and what it is.

Next, be sure to read our discussion of torque.