"Torquing Points" about Horsepower
Which is Better?
by Jim Bray
Horsepower is everything - or is it?
Horsepower is wonderful, and the more the merrier (unless you're concerned
with things like gas mileage), but buying a car based only on the amount of
horsepower it puts out is only looking at part of the picture.
Why? Because horsepower measures how much "oomph" your vehicle's
engine is capable of cranking out, not how much force actually makes it to
those little patches of rubber attaching your tires to the road.
The term horsepower has been around since, well, the days when horses were
doing the work of today's internal combustion engines. Rather than getting
into the nuts and bolts here, suffice it to say that when James Watt coined
the term "horsepower" it related to the weight of coal a horse could
raise a particular distance up a mine shaft in a given amount of time - for
example lifting 550 pounds of coal 100 feet in one minute, 55 pounds 1000 feet
in one minute, and so on.
Put even more simply, horsepower indicates the amount of work an engine is
capable of doing. All things being equal, since a 200 horsepower engine can
do twice as much work as a 100 horsepower engine, it should follow that a 200
horsepower car should be twice as fast as a car with only 100 horsepower.
Alas, in the real world all things are never equal, so a 200 horsepower car
that weighs twice as much as that 100 horsepower car won't be able to drive
twice as quickly. This is why, rather than just looking at horsepower, you
should figure out a car's power to weight ratio (horsepower divided by weight).
If you're really concerned with figuring out how long a strip of rubber your
car should be able to lay (depending on the condition of your tires, of course),
you should also pay attention to its torque rating.
Expressed in pound feet, torque is actually a measurement of "twisting"
or "rotating" force. A car engine may be rated at a certain amount
of horsepower, but it uses that horsepower to create torque by turning the
crankshaft that turns the next doohickey that, at the end of the mechanical
chain, eventually turns the wheels and tires.
So it's a vehicle's torque rating that actually lets you know how that horsepower
will translate into black strips on the road and that marvelous feeling of
being pushed back into your seat.
Naturally, this is a very superficial explanation, but it should help put
to rest the perception that horsepower's everything. In fact, the two go hand
in hand (along with weight, speed, etc.), which is why a race car and a bulldozer
that may have the same horsepower will perform very differently when it's time
for them to do the jobs for which they've been designed.
The bulldozer will undoubtedly have a big, low revving engine that allows
it to tool along at the breathtaking speed of a couple of miles per hour compared
with the race car's smaller, ultra-high revving engine that tops 200 mph. But
when it comes to that twisting force called torque, the bulldozer will blow
the race car away because its blade needs to push horrendous amounts of heavy
dirt and rock ahead of the vehicle, while the race car just needs to push itself
and a driver through the air.
RPM, revolutions per minute (which has nothing to do with politics) is also
an important factor in an engine's performance, and lower RPM's can actually
be better than higher, depending on the job.
Part of the reason, generally speaking, is longevity. The higher an engine's
RPM's are the harder it's working and the shorter its potential life span can
be. Because the bulldozer's engine is designed to last for decades of dependable
service (otherwise its high cost wouldn't be justifiable to its owner) and
it doesn't need to go fast, its rpm's stay low. The race car, however, is designed
to make it to the last lap of the race (and, with luck, the victory lap!) before
being taken apart and rebuilt.
And remember: torque that comes on at low revs, for example 3000 rpm, gives
a more powerful feel to the car - and you don't have the wind it out as much
to get pushed back into your seat.