Understanding Torque And Horsepower - Power Theories

In our two previous Motor Series installments, Vol. 38, No. 4, and Vol. 38, No. 5, we looked at some critical engine dimensions and key terminology that will help you avoid making costly mistakes when buying or building an engine. We also touched upon several relationships influential for achieving component synergy. Having these essentials will help you build an engine with maximized power and performance for your application. As a refresher, you might want to dig out those installments from your performance library. In the following text, we'll deal with two interrelated topics: torque and horsepower. The relationship between torque and horsepower is one of the most important concepts to understand when designing and building an engine that will satisfy predefined performance objectives.

Much is written about dyno testing and engine performance. Peak torque and horsepower numbers are usually pointed out, as is other information. Yet there is more to performance than just peak power numbers, because peak numbers may not indicate an engine's true temperament. In fact, many have asked what exactly the difference is between torque and horsepower, and whether an engine can be built favoring one or the other. By understanding the relationship between torque and horsepower, you'll be well on your way toward maximizing power and performance.

Simply put, torque is a twisting or turning force that engineers measure in "pounds" and "feet," or lb-ft, in the English system. One lb-ft of torque is equal to a 1-lb force applied at the end of a 1-ft lever. Engine torque is typically measured on a dynamometer and can also be defined as the potential to do work. 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 is actually established by first measuring torque at a given rpm and then calculating horsepower.

Basically, horsepower is equal to torque multiplied by rpm. Therefore, any increase in torque increases horsepower at a given rpm. That is why it's better to concentrate on improving torque than horsepower to achieve the best engine performance. Competition engine builders always concentrate on improving torque within the rpm range their engine needs to operate.

Engine torque is determined by the percentage a cylinder is filled at a given rpm. The greater the cylinder fill, the greater the torque will be. If power is to be increased, it is crucial to improve the engine's ability to breathe. Peak torque is reached when the engine runs out of air or loses its ability to breathe better. That is the point of maximum cylinder fill. An engine can continue to make more horsepower even when torque is falling, as long as rpm is increasing faster than torque is falling. Therefore, if maximum torque is the point of maximum cylinder fill, then maximum horsepower is the point where torque is falling off faster than rpm is increasing.