Introduction 

Chances
are you've heard about horsepower. Just about every car ad
on TV mentions it, people talking about their cars bandy the
word about and even most lawn mowers have a big sticker on
them to tell you the horsepower rating.
But what
is horsepower, and what does the horsepower rating mean in
terms of performance? In this article, you'll learn exactly
what horsepower is and how you can apply it to your everyday
life.
The term
horsepower was invented by the engineer James Watt. Watt lived
from 1736 to 1819 and is most famous for his work on improving
the performance of steam engines. We are also reminded of
him every day when we talk about 60watt light bulbs.
The story
goes that Watt was working with ponies lifting coal at a coal
mine, and he wanted a way to talk about the power available
from one of these animals. He found that, on average, a mine
pony could do 22,000 footpounds of work in a minute. He then
increased that number by 50 percent and pegged the measurement
of horsepower at 33,000 footpounds of work in one minute.
It is that arbitrary unit of measure that has made its way
down through the centuries and now appears on your car, your
lawn mower, your chain saw and even in some cases your vacuum
cleaner.
What horsepower
means is this: In Watt's judgement, one horse can do 33,000
footpounds of work every minute. So, imagine a horse raising
coal out of a coal mine as shown above. A horse exerting 1
horsepower can raise 330 pounds of coal 100 feet in a minute,
or 33 pounds of coal 1,000 feet in one minute, or 1,000 pounds
33 feet in one minute. You can make up whatever combination
of feet and pounds you like. As long as the product is 33,000
footpounds in one minute, you have a horsepower.
You can
probably imagine that you would not want to load 33,000 pounds
of coal in the bucket and ask the horse to move it 1 foot
in a minute because the horse couldn't budge that big a load.
You can probably also imagine that you would not want to put
1 pound of coal in the bucket and ask the horse to run 33,000
feet in one minute, since that translates into 375 miles per
hour and horses can't run that fast. However, if you have
read How a Block and Tackle Works, you know that with a block
and tackle you can easily trade perceived weight for distance
using an arrangement of pulleys. So you could create a block
and tackle system that puts a comfortable amount of weight
on the horse at a comfortable speed no matter how much weight
is actually in the bucket.
Horsepower
can be converted into other units as well. For example:
 1 horsepower
is equivalent to 746 watts. So if you took a 1horsepower
horse and put it on a treadmill, it could operate a generator
producing a continuous 746 watts.
 1 horsepower
(over the course of an hour) is equivalent to 2,545 BTU
(British thermal units). If you took that 746 watts and
ran it through an electric heater for an hour, it would
produce 2,545 BTU (where a BTU is the amount of energy needed
to raise the temperature of 1 pound of water 1 degree F).
 One
BTU is equal to 1,055 joules, or 252 gramcalories or 0.252
food Calories. Presumably, a horse producing 1 horsepower
would burn 641 Calories in one hour if it were 100percent
efficient.


Measuring
Horsepower 

If you
want to know the horsepower of an engine, you hook the engine
up to a dynamometer. A dynamometer places a load on the engine
and measures the amount of power that the engine can produce
against the load.
You can
get an idea of how a dynamometer works in the following way:
Imagine that you turn on a car engine, put it in neutral and
floor it. The engine would run so fast it would explode. That's
no good, so on a dynamometer you apply a load to the floored
engine and measure the load the engine can handle at different
engine speeds. You might hook an engine to a dynamometer,
floor it and use the dynamometer to apply enough of a load
to the engine to keep it at, say, 7,000 rpm. You record how
much load the engine can handle. Then you apply additional
load to knock the engine speed down to 6,500 rpm and record
the load there. Then you apply additional load to get it down
to 6,000 rpm, and so on. You can do the same thing starting
down at 500 or 1,000 rpm and working your way up. What dynamometers
actually measure is torque (in poundfeet), and to convert
torque to horsepower you simply multiply torque by rpm/5,252.
Torque
: Imagine that you have a big socket wrench with a 2footlong
handle on it, and you apply 50 pounds of force to that 2foot
handle. What you are doing is applying a torque, or turning
force, of 100 poundfeet (50 pounds to a 2footlong handle)
to the bolt. You could get the same 100 poundfeet of torque
by applying 1 pound of force to the end of a 100foot handle
or 100 pounds of force to a 1foot handle.
Similarly,
if you attach a shaft to an engine, the engine can apply torque
to the shaft. A dynamometer measures this torque. You can
easily convert torque to horsepower by multiplying torque
by rpm/5,252.
Graphing
Horsepower : If you plot the horsepower versus the rpm
values for the engine, what you end up with is a horsepower
curve for the engine. A typical horsepower curve for a highperformance
engine might look like this (this happens to be the curve
for the 300horsepower engine in the Mitsubishi 3000 twinturbo):
What a graph like this points out is that any engine has a
peak horsepower  an rpm value at which the power available
from the engine is at its maximum. An engine also has a peak
torque at a specific rpm. You will often see this expressed
in a brochure or a review in a magazine as "320 HP @
6500 rpm, 290 lbft torque @ 5000 rpm" (the figures for
the 1999 Shelby Series 1). When people say an engine has "lots
of lowend torque," what they mean is that the peak torque
occurs at a fairly low rpm value, like 2,000 or 3,000 rpm.
Another
thing you can see from a car's horsepower curve is the place
where the engine has maximum power. When you are trying to
accelerate quickly, you want to try to keep the engine close
to its maximum horsepower point on the curve. That is why
you often downshift to accelerate  by downshifting, you
increase engine rpm, which typically moves you closer to the
peak horsepower point on the curve. If you want to "launch"
your car from a traffic light, you would typically rev the
engine to get the engine right at its peak horsepower rpm
and then release the clutch to dump maximum power to the tires.


Horsepower
in Highperformance Cars 

A car
is considered to be "high performance" if it has
a lot of power relative to the weight of the car. This makes
sense  the more weight you have, the more power it takes
to accelerate it. For a given amount of power you want to
minimize the weight in order to maximize the acceleration.
The following
table shows you the horsepower and weight for several highperformance
cars (and one lowperformance car for comparison). In the
chart you can see the peak horsepower, the weight of the car,
the powertoweight ratio (horsepower divided by the weight),
the number of seconds the car takes to accelerate from zero
to 60 mph, and the price.

Horsepower

Weight
(lbs)

Power:Weight

060
mph (seconds)

Price

Dodge
Viper

450

3,320

0.136

4.1

$66,000

Ferrari
355 F1

375

2,975

0.126

4.6

$134,000

Shelby
Series 1

320

2,650

0.121

4.4

$108,000

Lotus
Esprit V8

350

3,045

0.115

4.4

$83,000

Chevrolet
Corvette

345

3,245

0.106

4.8

$42,000

Porsche
Carrera

300

2,900

0.103

5.0

$70,000

Mitsubishi
3000GT twinturbo

320

3,740

0.086

5.8

$45,000

Ford
Escort

110

2,470

0.045

10.9

$12,000

You can
see a very definite correlation between the powertoweight
ratio and the 0to60 time  in most cases, a higher ratio
indicates a quicker car. Interestingly, there is less of a
correlation between speed and price. The Viper actually looks
like a pretty good value on this particular table!
If you
want a fast car, you want a good powertoweight ratio. You
want lots of power and minimal weight. So the first place
to start is by cleaning out your trunk. 



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