Big Horsepower Or Gobs Of Torque | Horsepower VS. Torque | Tech

In Depth Stop by a newsstand and check out the cover headlines on almost any motorcycle magazine and the first things you'll see are topics such as "Gain 10 easy horsepower," or "Increase horsepower and torque with quick bolt-ons," or "Add pound-feet of torque the easy way." Terms like horsepower and torque are tossed around frequently, but seldom are they defined. Consequently, you are often left with many unanswered questions. Regardless of whether you ride a heavyweight bagger, medium-size cruiser or lightweight Sporty, understanding the differences between horsepower and torque can help you make educated decisions when choosing performance parts for your model of bike and riding style.

Interestingly, horsepower and torque are closely related. The point in the engine's rpm band at which torque is made and the amount of torque made will determine whether the engine is considered a torque- or horsepower-type of motor. Astute engine builders select engine components that improve torque and/or horsepower to optimize a bike's application and the rider's style of riding. In order to understand horsepower, we must first understand something called "work" and then define torque.

Defining Work
In simple terms, horsepower is a measure for the amount of work an engine can perform in a given time. Eighteenth century engineer and inventor James Watt headed the development of the high-pressure steam engine. That led to the need for a method of measuring the amount of work a steam engine could perform over a given amount of time. Because the steam engine could perform work commonly done in that epoch by draft horses, Watt related an engine's work to the work a horse could perform. Using a progression of tests along with empirical data, Watt surmised that moving 33,000 pounds 1 foot in one minute was the equivalent of 1 horsepower, which has become the standard for measuring the force or power output of the internal combustion engine. However, to explain horsepower first requires a discussion of force and torque because horsepower is a calculated number derived by first measuring torque at a given rpm.

Torque
Work is the measurement of a force exerted in a straight line. Common items that involve force include an engine's crankshaft, nuts and bolts as they are tightened or loosened, and a bicycle pedal crank. All three examples rotate around an axis. The rotational or twisting force of these items is called "torque." Torque, then, is defined as a measure of the ability of a force to cause twisting or rotation, which is measured in "pound-foot" units of force times the distance from the axis of rotation. As an example, let's assume you have a wrench 1-foot long and apply a force of 100 pound-feet at the end of it. In essence, you are applying a torque of 100 pound-feet. If the same wrench were 3-feet long, 100 pound-feet of force would apply 300 pound-feet of torque. In other words, if your V-twin engine makes 100 pound-feet of torque, it would take 100 pounds of force on a 1-foot lever to stop its rotating motion.

Engine torque is normally measured on a dynamometer and can be defined as the potential to do work. However, unlike horsepower, torque does not take into consideration the element of time, which gauges the rate at which an engine can perform work. An engine's power rating is actually established by first measuring torque at a given rpm and then mathematically calculating horsepower.