A Tale of Two Powerbands
A Tale of Two Powerbands
TDC
Kevin Cameron
BIG DYNO NUMBERS ARE IMPRESSIVE, sure, but they don’t necessarily make a better motorcycle.
Two riders are friendly track-day rivals, riding sportbikes that are identical save for tuning differences. Their favorite track has nine turns and one-and-a-half straightaways. Rider A, feeling that it takes power to pass, has advanced the intake cam and retarded the exhaust, thereby increasing valve overlap. A’s large valve overlap lets through strong exhaust-pipe wave action from exhaust to intake that helps clear exhaust from the cylinders and begins intake flow earlier than otherwise. This generates an extra 3-4 hp at peak.
Rider B, concentrating on early, strong acceleration off comers, has gone the other way with cam timing, resulting in moderate valve overlap that weakens exhaustwave action. B’s peak horsepower is down slightly, maybe 5 hp softer than Rider A’s.
Now let’s look at their power curves, Because A’s power curve is dominated by organ-pipe-like exhaust-wave action, its torque peak is pushed upward. But for the same reason, when exhaust-wave action reverses at lower rpm (exhaust waves naturally alternate in sign, negative-positive-negative, and so on), it works against the engine at just under 10,000 rpm, blowing exhaust back into the cylinder and delaying intake flow. This digs a deep hole in the torque curve. When these features are graphed, torque rises from idle, has a sub-peak at 8500 rpm, falls into the hole at 10,000, then climbs steeply again to peak strongly at 11,000. Peak power comes later, at 13,500 rpm.
Because B has limited valve overlap, the resulting torque curve has neither tall mountains nor deep valleys. In a word, it is flat. Because it does not “hit” at any particular point, it feels dull and, well... slow. Yet the strange thing is that despite determined riding by A, Rider B prevails in most of their track-day duels-and usually makes it look easy. Adding even more mystery is the fact that a mutual friend of A and B, standing at the end of the straight with a radar gun, reports that A’s bike is measurably faster than B’s.
Why is B quicker around the circuit? Both being keen riders, A and B have bought data-acquisition systems and spend hours after each track day analyzing and discussing their data. Looking at throttle-angle plots, they see that B is usually able to open the throttle farther, sooner during acceleration off comers. Why doesn’t A just wick it up? It’s not that simple. When A tries to get stronger drives off comers, the back end breaks away and the bike threatens to highside. In comparison, B looks almost sedate during comer exit, accelerating smoothly with hardly a wiggle.
Tire grip available for off-comer acceleration depends upon how much is being used for cornering. If the bike is at full-lean, with 95-plus percent of tire grip being used for cornering, even a tiny bit of throttle opening may be too much. As the bike rolls more upright, less grip is needed for cornering so the rider can smoothly turn the throttle up to keep engine thrust matched to the steadily increasing amount of available grip. But that assumes that turning the throttle up smoothly also smoothly increases engine thrust. Does it? Kevin Schwantz has described having to close the throttle of his Suzuki 500 two-stroke at certain points during comer exits because torque increased so suddenly with rising rpm that it would otherwise spin the tire. In a case like this, the rider must turn the throttle back to compensate for the sudden, steep rise in engine torque, then begin opening it again. And he must do this perfectly! Otherwise the tire slides.
A and B look more closely. Rider A’s torque curve with its twin peaks and a valley between puts him in Schwantz’s position described above, having to use extreme care or even to throttle back in the region of steeply rising torque in order to prevent traction loss and a possible high-side. Schwantz was able to add this task to the rest of his workload, but that doesn’t mean that he liked it. A lesser rider would, after some nasty slides and threats of doing a Wayne Gardner-style “Superman” through the windscreen, be more cautious with the throttle, and therefore lose some acceleration. Even a top rider would probably lose time because of the severe demand on his concentration that this peaky “Alpine” type of powerband imparts. Throttle match would probably be less than perfect or, if the rider concentrated exclusively on it, would divert attention needed for other riding tasks-line, weight transfer, \ front-end feedback, etc.
Seeing all this, riders A and B agree that it’s the easily controlled, flat torque curve of B’s engine that is responsible for his quicker lap times. It is the stronger peak power of A’s engine that generates higher top speeds at the end of the straight-A’s setup would be right for Bonneville. When rider A does occasionally manage to pass B at the end of the straight, B re-passes during the next corner exits. Rider B sacrifices top speed to get stronger acceleration off every comer, owing to a more usable torque curve. Thanks to its smoothness, Rider B can confidently open the throttle farther and so is actually able to use more power during acceleration than can rider A. Rider B is also able to give more attention to other riding tasks. For Rider A, steeply rising torque makes it tricky to match throttle to available grip while accelerating off the many comers.
Often during racing season we see such A/B duels played out in the news. Quite often, the quickest bike around the circuit is not the fastest at the end of the straight. At a recent AMA national, hot Yosh Suzuki up-and-comer Ben Spies showed a 5-mph speed advantage at the end of the straight, but it was veteran teammate Mat Mladin who had the quicker lap time. The tuning differences in such cases might be more complex than our example based on cam timing alone, but they would operate in similar ways.
Mladin’s crewmen have their trade secrets. □
