How do you define a horsepower? Ask a car enthusiast
and most of the time you'll get a blank look, a shrug of
the shoulders and maybe a guess along the lines of "What a
horse can do!".
That answer begs the question: What horse? A
thoroghbred race horse that can carry the small weight of
a jockey with a lot of speed, or a working horse that can
pull heavy loads albeit slowly? Obviously there is a more
precise answer. Car manufacturers, despite their
reputation for being creative regarding the horsepower
ratings of their products for marketing reasons, require a
more stable definition.
Horsepower is defined as work done over time. The exact
definition of one horsepower is 33,000 lb.ft./minute. Put
another way, if you were to lift 33,000 pounds one foot
over a period of one minute, you would have been working
at the rate of one horsepower. In this case, you'd have
expended one horsepower-minute of energy.
Even more interesting is how the definition came to be.
It was originated by James Watt, (1736-1819) the inventor
of the steam engine and the man whose name has been
immortalized by the definition of Watt as a unit of power.
The next time you complain about the landlord using only
20 watt light bulbs in the hall, you are honoring the same
To help sell his steam engines, Watt needed a way of
rating their capabilities. The engines were replacing
horses, the usual source of industrial power of the day.
The typical horse, attached to a mill that grinded corn or
cut wood, walked a 24 foot diameter (about 75.4 feet
circumference) circle. Watt calculated that the horse
pulled with a force of 180 pounds, although how he came up
with the figure is not known. Watt observed that a horse
typically made 144 trips around the circle in an hour, or
about 2.4 per minute. This meant that the horse traveled
at a speed of 180.96 feet per minute. Watt rounded off the
speed to 181 feet per minute and multiplied that by the
180 pounds of force the horse pulled (181 x 180) and came
up with 32,580 ft.-lbs./minute. That was rounded off to
33,000 ft.-lbs./minute, the figure we use today.
Put into perspective, a healthy human can sustain about
0.1 horsepower. Most observers familiar with horses and
their capabilities estimate that Watt was a bit
optimistic; few horses could maintain that effort for
Although the standard for rating horsepower has been
available for over 200 years, clever car manufacturers
have found ways to change the ratings of their engines to
suit their needs. During the famous horsepower wars of the
1960s, manufacturers could get higher figures by testing
without auxiliary items such as alternators or even water
pumps. High ratings backfired when insurance companies
noticed them and started to charge more for what they saw
as a higher risk. Manufacturers sometimes responded by
listing lower horsepower figures, forcing enthusiasts to
look at the magazine test reports to determine what was
going on. In the early seventies the SAE (Society of
Automotive Engineers) stepped in with standardized test
procedures and the fiqures were more consistent.
Between 1922 and 1947, the Royal Automobile Club used a
horsepower rating that was the basis for an automobile
tax. The horsepower of an engine was determined by
multiplying the square of the cylinder diameter in inches
by the number of cylinders and then dividing that figure
by 2.5. Using this dubious method, What we know of as a
385 horsepower motor found in the 2001 Z06 Corvette would
be rated at only 48.67 hp!
There is a metric horsepower rating, although it is
rarely used. The two methods are close, with one SAE
horsepower equal to 1.0138697 metric horsepower.
One mechanical horsepower also equals 745.699 watts or
.746 kW (kilowatts) of electrical horsepower. This means
that if you really want to confuse people, you could
complain about the 0.0268 horsepower light bulb your
landlord has in the hallway as opposed to the mundane 20