Horsepower Versus Torque: Who Is Right?

© Provided by Bonnier Corporation   Cycle World

By Kevin Cameron, Cycle World

Our readers are talking about two different things.

Torque and "torquey" have two very different meanings. Torque is a physical measurement with a precise definition. But when riders say a particular bike feels torquey, what they mean is not that it produces high torque on a dyno, but that its engine can accelerate the bike strongly at almost any rpm in its operating range.

The opposite of torquey is peaky, which means instead of having torque uniformly distributed across its operating range, its best torque exists only at higher rpm, where it can make a lot of horsepower. When you ride a peaky bike in its midrange or low-end, snapping the throttle open gets you only moderate torque, resulting in weak acceleration. But if you tap down two gears, putting the engine up at 9,000 to 10,000 rpm, the bike rockets forward because now it is operating up where it makes its best torque.

The result is that if two bikes—let's say a Harley-Davidson Big Twin and a Suzuki GSX-R1000—make a top-gear roll-on from 3,000 rpm, the Harley pulls smartly ahead until the Suzuki can rev up into its rpm range of best torque, at which point it evens the score and then pulls away rapidly.

These two kinds of engines represent the achievement of different goals by differently slanted compromises.

A touring or cruiser engine needs strong torque at low rpm to start and accelerate a heavy bike, so it is given quite short valve timings with almost no (or in some cases, negative) valve overlap. Such short timings run out of breath as the engine revs up, but because nobody tours or cruises at 150 mph or tries to break 10 seconds in the quarter-mile, it doesn't matter. It's a compromise. The touring rider needs and likes bottom torque and doesn't mind if the engine runs out of breath as it nears its peak rpm of 5,500.

A peaky engine results from the longer valve timings that can continue to fill cylinders well at higher rpm, such as 12,000. Short valve timings close the intakes before the cylinders have had time to fill at higher rpm, causing an engine to run out of breath. To overcome this, intake closing is delayed after bottom dead center for 50 or 60 degrees, and the intakes are made to begin opening 20 or more degrees before top center. At low and mid rpm, this late intake closing allows the rising piston to pump back out some of the intake charge it has just taken in, reducing torque at those engine speeds.

At higher rpm, this pump back does not occur because intake velocity is then high enough to just keep coasting into the cylinders even though the piston is rising on compression. As a result, peak torque is moved to higher rpm. It’s a compromise: giving away bottom-end and midrange to move the torque to high rpm where it becomes high horsepower.

In physical terms, torque is a force, tending to rotate something around an axis. It is measured as a force—ounces, pounds, kilograms—acting on a lever arm of a specified length (inches, feet, meters). This gives us the familiar pounds-feet or kilogram-meters in which torque is specified in manufacturer brochures or road tests.

When Cycle World tests a bike on its dyno, the results are presented in the form of two curves on a graph—one for horsepower, the other for torque.

© Ducati   Big speed requires big power: Ducati and Honda MotoGP bikes ridden by (from left to right) Andrea Dovizioso, Marc Márquez, and race winner Danilo Petrucci surpassed 215 mph this past June on Mugello’s 1,100-meter-long front straight.

The ideal toward which all manufacturers strive is to have high torque, constant over a wide range of rpm, resulting in a nearly horizontal line on the graph. Indian’s dirt-track racing engine, the FTR750, achieves this—flat, constant torque—from 7,000 to 11,000 rpm. Because horsepower is just (torque x rpm)/5252, the horsepower curve resulting from this flat torque is a sloping straight line, rising from left to right.

Real life is not ideal, so actual torque varies because such things as intake and exhaust-pipe resonance and airbox effects exist. They cause local variations in the height of the torque curve. In the case of a traditional touring or cruiser engine, the torque rises to a useful amount at as low as 1,200 rpm, peaks somewhere in the usual range of 2,500 to 3,000, and then slopes gently downward after that. Why? This is the “running out of breath” effect mentioned earlier. As the engine revs up, there is less and less time for cylinder filling, so the short valve timing and moderate port sizes of such engines cause cylinder filling to become less and less complete as the engine revs up, so torque falls.


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Automotive Magazine: Horsepower Versus Torque: Who Is Right?
Horsepower Versus Torque: Who Is Right?
Automotive Magazine
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