You might have seen the engine power output terms horsepower and torque thrown around in vehicle literature. These terms are often used to describe a motor vehicle’s power. But what do these terms mean, and what is the difference between horsepower and torque?
Engine torque is a unit of rotational force and may be expressed in a number of units including Newton meters (Nm) or pound feet (lb-ft or ft-lbs). As a unit of measurement, it might be best to think of this as a wrench turning a bolt or axle. If that wrench is one foot long and you apply one pound of force to the end of it, you have one foot-pound of torque.
Horsepower is an expression of that force at engine speed (RPM) as determined by the equation HP = Torque x RPM / 5252. As such, a higher horsepower number can be seen through increased torque at a given RPM, or the same amount of torque at a higher RPM. The goal of most passenger cars is to deliver similar horsepower and torque figures for a flatter torque curve (torque over a wider RPM range), as this would typically offer the best driving experience.
At the other end of the spectrum, an engine with high torque and low horsepower (such a would be typical of a diesel engine) would have a more limited operating range and an engine with high horsepower and low torque would be much more peaky and need to operate at high speeds in order to be most effective. Most high-revving engines like this are known as 'over square,' which is to say they use a larger piston size in relation to the stroke of the crankshaft to reduce piston speed at high RPM. They also utilize gear reduction as a means of multiplying torque and quickly getting the engine up to a more optimal operating range.
With a given engine displacement, accentuating either torque or horsepower often means a compromise to the other. A variety of technologies, including everything from variable valve timing and variable intake geometry to superchargers, are designed to widen the power range as much as possible. Horsepower is an important figure, but torque often plays a bigger role in how a vehicle drives. The difference between the two may not always be that obvious, but that is an easy way to think of the difference.