A Brief History and Definition of Horsepower

Several centuries ago James Watt put a draft horse to work in the hope of standardizing the notion of “power.” His seminal equations have stood the test of time. CB Tech Advisor Rich Burgess summarizes Watt’s numbers and how we came to have the goal of “more horsepower”.

Horsepower is the standard by which everyone from salesmen to racers (especially bench racers) relates to how a bike can perform—the exception being V-Twin aficionados, many of whom prefer to talk torque. In point of fact, horsepower and torque are mathematically related and have been since the late 18th century when James Watt worked out an equation to compare the output of steam engines against the power of draft horses.

The story goes that he took his horse to a local quarry to get some baseline numbers to use for comparison against his “new and improved” version of the steam engine. (It really was a huge improvement.) But, as John Stringer, my colleague at the Southern Alberta Institute of Technology where I’m a millwright instructor likes to point out “he could have used a donkey.” In which case, today we would be talking donkey power.

There is some debate on the numbers he came up with. Was it a small horse? No. That would have been one way to make his engine look better but apparently he decided to be conservative and leave no doubt folks were getting better than promised performance. It would be nice if that kind of marketing were still widely employed.

He needed a number that would come close to the work an average horse could do all day long, as his engines could. Some research I did said a good horse could do almost 15 horsepower for a very short burst and some humans could do one horsepower or more for a very short time.

I actually witnessed that phenomenon many years ago at a Christmas party at Performance Cycle and Auto in Calgary. As a party game a 10-speed pedal bike was strapped to their DynoJet dynamometer. The late Dirty Harry, a regular customer, ace welder, pro drag racer and all-round good guy pulled the best of the night, I think it was about one hp if memory serves; he was a bit wobbly when he got off the bike while I was too wobbly to get on.

Ultimately Watt rounded up the numbers and decided that his horse on average could lift 550 pounds up a mineshaft one foot in one second or roughly 33,000 foot-pounds per minute (550X60).

Another measure is the Watt, (not a coincidence). One horsepower equals about 746 Watts (or 735.5 in metric terms). The standard was set; work could now be measured. And it could be compared to familiar work animals—people who lived in the era of the early Industrial Revolution would have been able to visualize the performance easier than most of us can today. They could see weight moving through time. In Physics if there is no movement no work is being done.

The numbers stand to this day: One horsepower equals 33,000 foot-pounds divided by one minute.

Power-to-weight ratios are very telling: less weight makes a huge difference in acceleration. Knowing both power and weight makes drag strip times at least slightly ballpark predictable, assuming similar wind resistance, traction, gearing, skill and luck.

The other big player is torque, or how much outright force can be delivered to the rear wheel. Torque happens without regard to time, and that’s what makes it different. Mechanics do it every day (torque that is). The act of applying force in a turning movement is the definition of torque. Wrenching always applies at least a little torque.

A “torquey engine” pulls effortlessly up a hill where an engine with more horsepower may need a downshift or two if you are not going quite fast already. It is mathematically related to horsepower this way:

Torque = 5,252 X HP ÷ RPM

BHP = Torque x RPM ÷ 5,252

Because of this relationship HP can be measured on a dynamometer. The lines on the graph should cross at 5,252 rpm where the numbers are the same. The dyno has an internal resistance device that can measure the turning force (Torque) and a tachometer to determine RPM and the rest is the computer doing math.

It’s interesting to note that an engine gets its best torque at the rpm where it gets its best volumetric efficiency. That means gear ratios play a big part in going down the road both effortlessly and with good fuel mileage at the same time. An engine with high torque may need a much larger, stronger clutch than an engine with more horsepower. I can remember a few years ago the Formula One cars were burning clutches beyond use if the race had to be restarted. Those clutches could last the race delivering high hp but the high torque of a restart could turn them into smoldering, slipping, expensive garbage.

Manufacturers spend a lot of time and money building just the engine package customers want. Harleys and the other big cruisers have lots of torque for effortless cruising while sportbikes in general go for horsepower with one of the exceptions being the Suzuki Hayabusa, which has lots of both. Adventure bikes try to get the power down to match traction and so on. You don’t really need to think about it but you are being taken care of quite nicely.

I can vividly recall the dyno sheets that showed the testing results from an old Harley Evo I built. There was lots of torque and I noticed how the lines crossed at 5,252. (It was an honest dyno at Performance Cycle and Auto in Calgary, thanks to Mike Briggs.) This engine was built with a mild cam, high compression pistons, a good pipe and careful assembly. The result was lots of torque and decent horsepower, but I could see from the graph it all ended early at about 6,000 rpm.

When I looked at the dyno sheet with results from my Evo alongside the bikes owned by two of my friends—Dan’s Honda CBX, and Ron’s Kawasaki Vulcan 750—I could see different power characteristics, but the common thread 5,252 was always where the lines crossed. Dan’s CBX six-banger showed torque and horsepower building smoothly and gradually across a wide rpm range. I remember how impressed I was with my CBX the first time I rode it on snow-covered roads. The lack of low-end torque enabled me to start off without spinning the tire at all, yet the power would build to an exciting top end rush above 7,000 rpm—you would definitely want dry pavement for that. Very different from the Harley.

Ron’s Vulcan showed a 750cc bike somewhere in between the other two, a V-Twin able to rev quite high although with less authority. To be fair it was pretty much a match for a stock EVO in terms of horsepower. More modern engines will have all kinds of characteristics and bigger numbers but the basic math remains.

I wonder if James Watt would be surprised to see his standard still in wide usage today. No doubt he’d be glad to have not taken the donkey.

by Rich Burgess Canadian Biker Issue #314

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A Brief History and Definition of Horsepower

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