More on dynamometers: Horsepower vs. torque

Publish Date: 
Jul 17, 2014
By Tracy Martin

In part one of dynamometers explained, we covered the history of the dynamometer and the various types used in the industry. Dealers may hear customers ask, “Which is better, more horsepower, or more torque?” or say, “I don’t care about horsepower. I just want lots of torque.” We’ll provide you with definitions of both terms, as well as the answers to the above question.

It is a misconception that an engine makes horsepower. In fact, engines only produce torque at specific rotational speeds, or engine revolutions per minute (RPM). Horsepower is merely a calculation using the results of a formula that factors in engine RPM -- and torque.

Torque is the twisting force or energy that an engine produces. Torque as a force can also be used to describe a pushing or pulling motion. In pure engineering terms, torque is also called moment or moment of force and is defined as energy required to rotate an object (an engine’s flywheel, for example) about its axis.

We’ll define torque as the twisting force measured at an engine’s crankshaft or a motorcycle’s rear wheel. Torque is measured in foot-pounds (ft. lbs.). In addition to measuring the rotational output of an engine, service manuals provide specifications for tightening a bolt in foot-pounds or inch-pounds.

A torque wrench is used to measure how much twisting force is applied, in ft. lbs. or inch lbs. When tightening a bolt, a torque wrench doesn’t read the final torque value until the bolt stops turning. This type of torque is called static torque because there is no rotational acceleration involved when tightening a bolt.

Dynamic torque is different; it involves acceleration as the speed of the rotational force increases. An engine can produce static and dynamic torque. For example, if a motorcycle is being ridden at a steady throttle opening on a flat surface, the engine produces static torque, because there is no acceleration of the engine. When the throttle is opened, and the bike accelerates, dynamic torque is produced. Torque can be measured from the engine’s crankshaft, flywheel, transmission output shaft, or most commonly at the rear wheel of a motorcycle.

In engineering terms, power output is expressed as torque multiplied by rotational speed around an axis. The formula for horsepower is: Torque x Engine Speed, divided by 5,252. The number 5,252 is the result of lumping several different conversion factors together, while taking into account the 33,000 ft. lbs. of work that a single horse can perform in James Watt’s formula for horsepower (as discussed previously). For the details regarding how the number 5,252 is derived, search online for “horsepower formula.” 

One activity that many motorcyclists participate in is bench racing, using horsepower and torque numbers to spar verbally with one another. Owners of cruisers, whose bikes might have lower horsepower than racing bikes, might say “Torque causes acceleration, not horsepower.”

This thinking, which may make some kind of intuitive sense, is incorrect. Horsepower and torque are linked by the fact that horsepower is calculated from torque in ft. lbs. and engine rpm (hp = T x rpm / 5,252). Because of this formula, horsepower and torque are not independent from each other in relation to engine power.

Continued