More on dynamometers: Horsepower vs. torque

Publish Date: 
Jul 17, 2014
By Tracy Martin

Before we discuss how torque and horsepower are related, let’s review. Work is the application of force over a distance. Power is the rate at which work is accomplished within a finite time. Torque is the twisting force applied to an engine’s crankshaft. Torque is always listed at specific rpms because no work or power is produced unless the engine is turning. Once an engine is turning fast enough, the force exerted against a load (like accelerating the drum on an inertia dynamometer) and speed at which its work is being accomplished can be measured.

Keep in mind the connection between torque and horsepower in this example: Two identical V-twin cruisers, with riders that weigh the same, are trying to pass a truck on the interstate. At 60 mph, both riders are in fifth gear, with their engines running at 2,500 rpm as they pull into the left lane to pass the 18-wheeler.

The first rider, Dick, believes that acceleration is caused by torque, and he knows from reviewing a dyno chart for his bike that torque increases from 2,500 rpm to 4,200 rpm. He leaves his motorcycle in fifth gear, whacks open the throttle and starts to pass the truck.

The second rider, Jane, believes that horsepower, not torque, causes acceleration, and she downshifts into third gear, and opens the throttle all the way. When Jane downshifts, her engine is turning at 4,000 rpm, 1,500 rpm faster than Dick’s engine. She has looked at the same chart and knows that torque from Dick’s engine will decrease when it reaches 4,200 rpm, but her engine’s horsepower will increase as the engine turns faster.

When passing the truck, Dick accelerates at a constant rate as his engine torque increases. Jane accelerates at a faster rate than Dick, even though her torque is decreasing as her engine turns faster, because her engine’s horsepower is increasing. Which rider passes the truck first?

Even if you don’t ride a V-twin cruiser you probably know the answer -- Jane, who downshifted into third gear and watches Dick trying to catch up to her. To make the most power, an engine has to turn at a higher rpm, and Jane starts to pass the truck with her engine spinning at 4,000 rpm vs. Dick’s engine that is only running at 2,500 rpm.

Horsepower is the rate, or rpm, at which work is performed, and more revolutions per minute equal more horsepower.

Almost all motorcycle reviews list hp and torque numbers. When riders bench race, these same numbers are thrown around with bravado as each owner brags about his engine modifications. Manufacturers use the numbers to sell performance, image and technology.

Because there are so many variables when performance testing a motorcycle, it is not easy to make true comparisons. The same motorcycle tested on different days, measured on different dynos with different correction factors, tire pressure and dyno operators can generate different numbers.