Dynamometers: everyone talks horsepower, but how do you measure it?

Publish Date: 
Jun 19, 2014
By Tracy Martin

The term “horsepower” is often discussed at a dealer. “How much horsepower does this model have?” “How can I get more horsepower from my bike?” “These two engine are the same size. How come one has more horsepower than the other?” This article will provide a brief history of horsepower as well as the tools used to measure it.

Defining horsepower has always come down to who you ask and what machine (dynamometer) was used to measure it. To complicate things further, there are many definitions of horsepower, including gross, brake, shaft, effective, indicated, relative, SAE Gross, DIN, JIS, ECE, ISO, shaft, watts, kilowatts, advertised and rear wheel. To understand the term -- and be able to explain it to your customers -- horsepower needs to be put into historical perspective.

How much power an engine produces has been a subject of controversy since the advent of the steam engine in the late 1700s. The dollars and cents of measuring engine power is easy to understand. For example, if an engine made by one company makes 100 horsepower and another manufacturer makes an engine that produces 104 horsepower, and both engines sell for the same price, which is more desirable?

This logic applies not only to engines, but also anything that can be added to an engine to increase horsepower, like exhaust systems. Many aftermarket exhaust manufacturers advertise that their systems will increase the power output of an engine. If two similar systems claim different power increases, one will have an advantage in the marketplace. This is also true of motorcycle manufacturers. Honda, Harley-Davidson, Suzuki, Kawasaki, Yamaha, Ducati, BMW, Triumph, KTM and other manufacturers all are trying to sell products, and if horsepower is a factor in the equation, more can only be better.

Read the latest review of any motorcycle and more than likely rear wheel or crankshaft horsepower will be listed as a means of comparison between similar bikes in a particular class. Horsepower is measured using a device called a dynamometer, and while these machines don’t produce power like an internal combustion engine, they have something else in common. When it comes to advertising horsepower numbers, more is always better.

Every company that manufactures dynamometers has a practical reason to steer potential customers away from their competition by pointing out why the other guy’s dyno produces inflated or inaccurate horsepower numbers. Messing around with the numbers that calculate horsepower has been going on for a while. In 1712, Thomas Newcomen designed the first commercially successful steam engine, but it was not very efficient and had limited uses, mostly pumping water out of deep mines.

A Scottish mechanical engineer, James Watt, came up with a vastly improved version of the steam engine in 1764 that used 75 percent less coal than the Newcomen engines. Watt’s business plan was to collect royalties from his customers based on the savings in coal, which worked for customers that had existing steam engines and could track their use of coal. But mine operators that still used horses to get their work done need a different way to calculate what they would pay for this cutting-edge technology -- the steam engine.

Watt’s plan to entice mine owners to purchase one of his steam engines was based on how many horses the owners could replace. But a question had to be answered: how much work can a single horse accomplish in a given amount of time?

Watt reasoned that if a horse could hoist a bucket of coal weighing 366 lbs. up a mine shaft at the rate of one foot per second, in one minute the horse could raise the bucket 60 feet. With this information, Watt calculated that the horse could raise 21,960 lbs. one foot in one minute (366 x 60 = 21,960 foot-pounds per minute). Other engineers at the time placed the amount of work a horse could do at 22,916 or 27,500 ft. lbs.

Watt experimented further, and in 1782 found that a brewery horse (a large breed) was able to produce 32,400 ft. lbs. of work per minute. Watt rounded that number up to 33,000 and that became the standard still in use today.

Continued