I always find it a bit amusing, and sometimes frustrating, when people use a peak horsepower number as the absolute measure of performance. Bike A has 150 horsepower, bike B only has 140 horsepower, so bike A must be more powerful.
But that peak number only tells part of the story, and can be very misleading. In the real world, it's quite often torque that makes one bike feel more powerful than another, regardless of horsepower; for the full picture, it's necessary to look at both values, over the entire rev range.
Torque is a measurement of twisting force, calculated by multiplying force by the distance from the centre of rotation. When you try to loosen that rusty bolt, you can either push harder on the wrench for more torque, or use a longer wrench. Note that when torque is applied, there may or may not be motion; the bolt doesn't necessarily have to turn. Horsepower requires motion to take place, and is a measure of torque over time - how fast you can turn the bolt while still applying the torque.
For a better understanding of the relationship between torque and horsepower, we can use the analogy of pushing your motorcycle. The strength of your push can be used to represent torque. Once underway, we can consider that you are making horsepower; how much is determined by not only how strong your push is, but also how fast the bike goes. Note that maximum horsepower depends on how hard you can push as well as how fast you can run, just as a real engine's peak horsepower depends on both torque and maximum rpm.
Back to the real world, an engine with fewer, larger cylinders will typically make more torque than an engine with more, smaller cylinders. A multi-cylinder engine can typically rev higher than a single or twin of the same displacement, making more horsepower even though it makes less torque. So a breathed-on big twin will make a lot of torque and give you that big push, but only in short bursts and at low speeds. Conversely, a four-cylinder middleweight can feel pretty lethargic at low speed, but put it on an open, fast road course and watch what happens.
Consider the two motorcycles shown here, the Honda CB1000R naked bike and the Yamaha YZF-R6 supersport machine. Both make the same peak power, just under 110 horsepower. The big difference is in torque and rpm: The R6 makes just 44 foot-pounds of torque and revs to an astronomical 15,500 rpm to make that horsepower number, whereas the CB revs to just 10,500 rpm but makes 65 foot-pounds of torque - almost 50 percent more than the R6.
Clearly torque, rpm, and the spread of both torque and horsepower are characteristics just as important as the peak horsepower number. While the differences in those values may not be as pronounced among bikes in the same category, it's often enough that one bike may feel considerably different than another, even though they have the same peak horsepower.
Part of my interest in peak horsepower numbers and how they relate to "real" power lies in the Mopar Canadian Superbike Championship rulebook. Currently, bikes in the Superbike class are limited to 190 horsepower as measured on the series official dyno, while bikes in the Sport Bike class are limited to 125 horsepower. Savvy tuners know that peak horsepower is just part of the equation, and will work diligently to boost torque and horsepower across the rev range without going over that magic number. In this manner, a bike can have a significant power advantage on the track, but still be safely under the class limit when it comes to post-race inspection.
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