Torque Shows

Torque shows

TDC

Kevin Cameron

WHAT IS TORQUE? IT HAS DIFFERENT meanings to different people, meanings that foster endless misunderstandings. To those with formal training in engineering and physics, it’s all very clear. Torque means, simply, a twisting force. Normally, quoted figures for engine peak torque are measured at the crankshaft, but a more useful measure is torque where it counts-at the rear wheel, in top gear. To get torque at the rear wheel, we multiply whatever torque the engine is putting out at its crankshaft, times the primary-drive ratio, times the gearbox ratio, times the final-drive ratio.

Typically, a 600cc sportbike might make 43 foot-pounds of peak torque at the crank, and in top gear, the total reduction to the rear wheel might be about 6. That gives us 43 x 6, or 258 foot-pounds of rear-wheel torque. Because the rear tire is about 2 feet in diameter, this 258 foot-pounds of torque, applied on the 1-foot radius of the rear tire, will give the bike a propelling force of about 258 pounds. That will yield a vigorous rate of acceleration.

Now consider a Harley Big Twin engine. Its peak torque is 58 foot-pounds, but its total top-gear reduction ratio is more like 2.8. If the engine is at its torque peak in top gear, the rear-wheel torque will be 58 x 2.8, or 162 footpounds; and because the rear wheel is again approximately 2 feet in diameter, that torque will translate to roughly 162 pounds of propelling force.

To some people, this is a ridiculous, impossible result. No way in hell does a little, peaky, screaming, 600cc sportbike have more torque than the Master of Torque itself, the Big Twin. Yet the numbers truly tell the tale: In formal terms, the 600 has more top-gear, rearwheel torque than the Harley.

Just in case you’re wondering, rearwheel horsepower does not also increase by a factor of 6; that’s because the rpm of the rear wheel is 6 times slower than that of the crankshaft. Since horsepower is calculated by multiplying torque by rpm and dividing the result by the constant of 5252, horsepower at the rear wheel remains the same as at the crankshaft-not counting frictional losses in the drive train, of course.

Now let’s shift to another interpretation of torque-a looser one, formally incorrect, but used by so many people that it can’t be ignored. In this definition, torque is a feeling a rider gets from an engine, a sense that, no matter at what rpm the throttle is opened, the engine will respond with the same robust forward push. In this definition, torque, or torquiness, is the very opposite of peakiness, and has nothing whatever to do with amounts of twisting force or any such textbook formalities. On a torquer, you just twist and go, at almost any rpm.

If we reconsider the previous examples in light of this definition, the 600cc sportbike is not a torquer, because a glance at its torque curve shows that it has to be spinning fairly high to give its 43 foot-pounds, and that peak figure is available in a limited rpm range. But looking at the Harley’s torque curve, we see a wide zone of rpm in which the engine makes within 10 percent of its peak figure of 58 foot-pounds. To get the sportbike to accelerate hard, you may have to tap down a gear or two to get the revs up where the engine makes good torque. But on the Big Twin, snapping the grip open almost anywhere gets the same response of willing acceleration.

Why the difference? To get big power from small engines, you have to use intake and exhaust tuning to help cram more air into the cylinders. Because neither intake nor exhaust is built like a slide trombone, this tuning only plays one note-it operates in a limited rpm range. You therefore have to get the engine spinning in that range to get strong acceleration. So long as you keep it there, the acceleration is excellent. Elsewhere, outside the “tuned zone,” it is less so.

Those same tuning techniques could be applied to a big engine, but they aren’t necessary because the extra displacement does the job better. This is why big bikes usually are easier to ride, requiring less shifting and fussing with the engine to get the desired result.

A big, mildly tuned engine, not dependent upon intake and exhaust wave action, can deliver torque across a wide band. If we were to re-tune that engine in 600cc sportbike style, with big carbs and valves, sporty cams and a race-style exhaust, it would make more peak torque, but across a narrowed rpm range, with losses in midrange and lower rpm. Like the 600, it would have to be revved and shifted vigorously to get its best performance.

Engines don’t rev on and on forever, but reach a torque peak, after which breathing falls off. As an engine revs up, it pumps air until intake velocity rises so high that friction loss stops the rise in air delivery. Beyond this point, torque falls. Mildly tuned engines are simple air pumps, and their ability to fill themselves with air is pretty uniform from low revs up to where power begins to trail off. They are torquey.

A highly tuned engine is also an air pump, but one that gets extra help from tuned devices in a narrow range. Because of this extra help, its torque is very high-but narrow. It has to be revved and shifted to get its best performance. It is peaky.

Marketing surveys have shown that riders of all persuasions generally prefer torquey over peaky, whether they are touring, cruiser or sportbike riders.

This only makes sense. After all, who’s supposed to be in charge, you the rider, or the shortcomings of your engine? Riding is easier and more fun if you don’t have to concentrate too hard on being a powerplant manager, keeping the revs in the hot zone.

Of course, the reason all engines are not built as torquers is that certain markets buy by the numbers, and success in sporting events has showroom value, too. To make the big numbers-especially from smaller engines-tuning is a necessary evil. With it comes a Matterhorn torque curve. □