So you just did a bunch of high performance engine work to your ride and everybody wants to know how much horsepower your scooter’s got. Although it’s become the benchmark when people talk about how much power their bike makes, horsepower isn’t the whole story. In a dyno shootout, it’s the deciding factor, but in reality, that horsepower number doesn’t necessarily make one bike faster than the other. There are so many other variables that affect the ability of a motorcycle to be fast and they aren’t always taken into consideration. Although horsepower is a handy factor for comparison, it’s more of a mathematical by product than anything else.
Most people are so fixated on horsepower that they can’t see the forest for the trees. The truth is, horsepower can’t exist without torque and revolutions per minute (or RPMs.) To understand the relationship between the two, we should focus on torque first.
By definition, torque is the measure of a force’s tendency to produce torsion and rotation about an axis. When we measure this force for an internal combustion engine, we typically refer to it in ft./lbs. A foot-pound is equal to the twisting force that is exerted on an axis by one pound on a one-foot lever. So if you stand on a one foot breaker bar and you weigh 175 lbs, you have just delivered 175 ft./lbs. of torque to the bolt you’re trying to break loose. The problem is, the bolt is frozen and 175 lbs. isn’t enough force. Even though the bolt didn’t move, you delivered a measurable amount of torque.
Remember that torque has a “tendency” to produce rotation, but that doesn’t mean it always will. You can increase the torque by adding more weight and/or increasing the length of the lever. So now you get a guy named Moose and a three-foot breaker bar to deliver 900 ft./lbs. of torque and the bolt just snaps. Although you’ve delivered a significant amount of torque to the job, not even one horsepower was produced. How can that be? It will all make sense after you understand how horsepower is made.
Scottish engineer James Watt used horsepower as a way to compare the power produced by his steam engines to that of draft horses. He calculated that one horse could produce 33,000 ft./lbs. of work in one minute. The standard formula used to calculate mechanical horsepower in an engine is torque x RPM 5252. So horsepower is a measurement of torque and motion over time. When you’re talking in terms of your engine, that motion over time is measured in RPMs. So how come Moose didn’t make any ponies? Because the breaker bar never moved. No RPMs = no horsepower, just torque. But if the bolt had moved and Moose could have spun that breaker bar at 30 RPMs, he would have made 5.14 hp.
Because RPMs play such a large roll in producing that horsepower number, it’s very possible for one engine to have much higher horsepower than another without actually being more powerful or capable of doing the same amount of work.
When someone refers to an engine being a horsepower engine vs. a torque engine, what they’re saying is that a horsepower engine is one that makes more horsepower than torque (high RPMs) while a torque engine produces more torque than horsepower (low RPMs). The line where this crosses is that magic number: 5252 RPMs.
Let’s compare a horsepower engine to a torque engine by looking at a V-8 sports car engine and a diesel truck engine. The car engine makes 390 hp at 5500 RPM and 375 ft./lbs. of torque. The truck engine only makes 350 hp at 1850 RPMs but it’s capable of producing 1000 ft./lbs. of torque. If you revved the diesel up to the same number of RPMs as the car engine, it would make over 1000 hp! But you wouldn’t expect to swap motors and have the small block V-8 haul 100,000 lbs down the road at 70 mph. Nor would you expect the heavy, low revving diesel engine to pull the car down the quarter mile in 13 seconds.
Comparing the two is like comparing a gymnast to a power lifter. A gymnast (horsepower) can climb a ladder quicker than a power lifter (torque) but if you weighed them both down with 100 lbs. the gymnast would take significantly longer and the power lifter would get it done in about the same amount of time.
Now let’s get back to motorcycle engines. Let’s say a given V-twin makes 100 ft./lbs. of torque at 5252 RPMs, and 100 horsepower. If it could produce that torque at 10,504 RPMs (twice as high) the horsepower would be 200. Cut the RPM in half and the horsepower would be 50. No matter what the horsepower, the torque determines what it can move!
The bottom line? When you’re talking about moving a heavy American V-Twin down the road with a portly pilot and matching mate, you need torque and lots of it! Join us next time when we’ll take a look at some dyno charts and talk about what works on the street.
Dan Pike owns FTF Cycles in Randolph, MA and campaigns an XR1200 race bike. His tech columns also appear in The Motorcyclist’s Post.
BY DAN PIKE, FTF CYCLES
FTF Cycles • Randolph, MA
781-961-9100 • www.ftfcycles.com
IronWorks April 2013