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"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.


Jim Bray is a member of the Automobile Journalists Association of Canada. His columns are available through the TechnoFile Syndicate.

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