The Myths of Power and Torque, (What About Weight?)

- by Atul H. Patel, Editor-in-Chief

You see horsepower figures advertised all the time, and the race keeps escalating.  You'll scarcely find a vehicle that has less than 150 hp, and now 300 hp is not that rare in cars, as well as trucks.  So why shouldn't you pay so much attention to these numbers?  First of all, the advertised horsepower is the peak horsepower that the engine can put out, most likely at 5,000 rpm or higher.  Chances are, you don't drive at 5,000 rpm very often, unless you're a leadfoot.  That means you won't see the fully advertised horsepower much at all.  It's power that accelerates you and your vehicle, but then what is torque?

Torque is just spinning force and the faster it spins, the more power you get.  Therefore 175 lb-ft of torque at 1000 rpm constitutes half the power of 175 lb-ft of torque at 2000 rpm.  The formula is as follows...

Power  =  (Torque  x engine rpm )  /  5252
.. where power is in horsepower, and torque is in lb-ft

Regardless of how much power you have at a given engine rpm, the more torque you get at any rpm, the better "pull" you feel.  A flat torque curve implies that there is a high amount of torque available from low rpms up to high rpms.  (Diesel engines provide lots of torque from low rpms but don't spin as fast so their peak horsepower ratings tend to be comparatively low).  Turbos, (once spooled up), compress air coming into the cylinder and maximize oxygen combusted and thereby maximize torque across most of the engine rpm range. In day-to-day driving, torque availability at low and mid rpm's  makes a vehicle more driveable.  Since the engine has enough torque at lower rpms, you don't have to rev it as much and so it is quieter and seems more responsive because you don't have to wait for then engine to rev up to get the torque you want.

Don't get me wrong.  A free-spinning high revving engine can be fun, but if you're driving the vehicle with a manual transmission, you'll be downshifting often to maintain speed in higher gears going up grades, and you'll need to downshift a lot more often to make a spirited pass.

But all this is moot if you drive a heavy vehicle relative to the power and torque available.  300 hp in a 6,000 lb vehicle gives you the same power-to-weight ratio as 150 hp in a 3,000 lb vehicle, and most likely similar acceleration (not considering the weight of the people inside).  That's why many of these larger heavy SUV's have plenty of power, but have a difficult time keeping up with average family sedans.  Transmission gearing and final drive ratio are also factors which affect both performance and fuel economy, but I don't want to complicate matters for the purposes of this article.

To sum up, my advice is this... The next time you look to buy a vehicle and want to consider acceleration and driveability, check out the engine's torque curve to see how much you have to rev to get to substantial torque, and then check out the weight of the vehicle and divide it by the peak horsepower to determine the power-to-weight ratio.  You might be surprised that 300 hp may be hard to reach by stepping into the gas pedal, and even when you do, it might not be enough to accelerate the vehicle as well as you want.

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