Definition of torque

The simplest definition of torque is the force around a point. The measurement for torque is the pound-foot, often abbreviated to lb-ft., where the pound is the amount of force applied and the foot is the distance from the central point.

How does torque apply to cars?

When car manufacturers state the torque for a car in lb-ft., they are describing the force applied to the crankshaft, or the part of the drive system that sends power to the wheels, from the central point of that crankshaft.

Torque vs. horsepower

Torque is a measurement, while horsepower is a calculation from a formula based on torque at a given number of revolutions per minute (rpm) of the crankshaft. The formula for horsepower is (torque x rpm)/5252. This formula is always the same, meaning horsepower is always a function of torque and engine speed. Flip the formula and notice that torque = (horsepower x 5252)/rpm.

Torque and acceleration

Remember that torque is measured relative to the engine rpm, so when you see the advertised torque number for a car, this is at the ideal number of rpm. The car will accelerate best when kept closest to the rpm where horsepower and torque are at a peak, usually close to the car's redline, or highest engine rpm possible.

Torque and horsepower disparity

Engines that have to spin very hard to reach their peak horsepower will have much lower torque value than horsepower. Think of a sports car like the Honda S2000, which makes its best horsepower near 9000 rpm. Larger engines generally have more max torque than horsepower, like big truck diesel engines. Here the torque comes much lower, allowing the car to carry heavy loads and haul big things like trailers and boats.

What does it all mean?

Cars with much higher horsepower than torque lead to fun driving because the driver must push the engine hard to reach the peak power. Cars with more torque are easier to drive because the torque, and the pull from the line, comes at a much lower rpm. The horsepower and torque are not independent measurements, but linked together by an equation as shown above.

Source:

MotorTrend: Sibling rivalry: Horsepower vs. Torque

epi-eng.com: Power and Torque