For most
readers, understanding horsepower is easy; the bigger the number,
the faster you go. Torque, however, is a figure that underlies
horsepower. Once you understand what it is, and its relationship to
horsepower, you’ll find it an equally useful number to know.

If you
remember your high school physics class, you might remember that
power is the rate at which work is done. In a car engine, the power
produced is called horsepower.

The term was
coined by James Watt, a Scottish engineer who improved the
efficiency of the steam engine so dramatically that he is credited
with inventing it. He did not. But he did invent a number of other
things, including the concept of horsepower.

In 1782, a
sawmill ordered an engine from Watt’s company to replace 12
horses. Watt used data from the sawmill to determine that a London
draft horse could work at a rate of 22,000 foot-pounds per minute in
an eight-hour day. Just to be safe, Watt increased the figure by 50
percent. He defined one horse’s power as the ability to move
33,000 pounds one foot per minute or 550 pounds one foot in one
second.

To determine
how much horsepower a car engine has, automakers use a dynamometer,
which measures the twisting force produced by the engine’s
crankshaft at various speeds, or revolutions per minute (RPM). In
reality, however, the dynamometer is not measuring horsepower. It is
measuring torque.

Torque is the
twisting force brought to an object. You do it all of the time.
Remove a twist-off cap from a soda bottle; you’ve applied torque
to do it.

Once the
engine’s torque is determined, a mathematical formula — torque
multiplied by RPM and divided by 5,252 — is used to determine
horsepower. Torque can be scientifically measured; horsepower
cannot.

So how are the
two related? Remember, torque is the amount of force brought to an
object; horsepower is the rate at which it is applied. How they work
in your car depends upon the gearing of a vehicle’s transmission,
differentials and axles.

Consider both
a pickup truck and a sports car with a 5.0-liter V-8 and the same
horsepower rating. A pickup truck will be geared lower so that more
torque is available at low speeds for towing and hauling. By
contrast, it doesn’t take much torque to move a sports car.
Instead, the torque is used to get the sports car through its gears
as quickly as possible. So a sports car usually has more torque at
high rpms.

Knowing this,
you can tell whether a vehicle’s powerplant has a lot of grunt
down low for hauling or is one that’s highly strung for speed. The
Ford F-150’s 3.5-liter EcoBoost V-6 engine has a torque rating of
420 pound-feet at 2,500 rpm, making it ideal for towing, since the
torque peaks at a mere 2,500 rpm. By contrast, torque in a sports
car typically peaks at higher rpm; rev the engine and you get more
speed, not towing ability.

In the end,
torque makes a big difference not only in how the car feels, but how
it performs. Know the numbers, and you know what you’re in for the
next time you test-drive a car or truck.