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World Superbike configurations

exhaustsAt the recent Monza World Superbike round, I asked Marcus Eschenbacher, crew chief to Cal Crutchlow at Sterilgarda Yamaha, about their choice of exhaust configuration. As many who work in Formula One or study it will know, the exhaust systems on each side of the engine are arranged in a 'four-into-one' collector with four primary pipes coming together in a single collector on each side. The arrangement is often abbreviated to '4-1', denoting four pipes converging into one larger pipe.

There are various reasons why Formula One teams choose a 4-1 arrangement - the fact that the packaging can be easier, the peak performance is better and the mass of the exhaust system is lower are all important factors.

The arrangement was popular in motorcycle racing but has largely been supplanted by pipes being coupled in pairs before the two secondary pipes are brought together in the final collector. This system is known as four-into-two-into-one (4-2-1)

Eschenbacher confirmed that the titanium exhaust systems supplied to the Sterilgarda Yamaha WSBK team do not follow the same pattern as current Formula One systems, and they use a 4-2-1 arrangement; their particular arrangement has the final pipe splitting into two before the twin silencers (4-2-1-2) which exit underneath the seat unit.

It's acknowledged that this 4-2-1 style of exhaust costs a little performance in terms of absolute peak power, but Eschenbacher says the overall effect of the 4-2-1 type of system is positive for the Yamaha compared to the 4-1 arrangement, owing to a wider spread of torque and improved rideability. Even for tracks where top speed is felt to be important, the 4-2-1 system is retained.

The Yamaha R1 motorcycle used for World Superbike racing has a different firing order to many 'across-the-frame' four-cylinder machines, owing to its twin-plane crankshaft. This has been pioneered in recent years by Yamaha after successful results in MotoGP using the same configuration.

Most four-cylinder machines of this type have a single-plane crankshaft, and for 4-2-1 exhaust systems the tuning has traditionally relied on the pairs of primaries being linked in each case firing at equal intervals of 360 crank-angle degrees. So, conventionally, we would link primaries 1 and 4 together so that the wave reflections to each cylinder of the linked pair are the same in terms of magnitude and timing.

But even for conventional four-cylinder engines with a flat-plane crank, it can be advantageous to link primaries together that don't obey the traditional pairings (1 & 4, 2 & 3) to achieve good results. We should not therefore assume that the Yamaha suffers greatly in terms of exhaust tuning owing to its unique firing order, compared to conventional 4-2-1 systems on four-cylinder engines with equal firing intervals. The unusual firing order also leads to a unique exhaust note - certainly among in-line four-cylinder machinery currently racing - with the machine sounding much more reminiscent of a four-cylinder vee engine.

As with its MotoGP programme, Yamaha has had recent World Championship success with its unconventional superbike engine. The 2009 World Superbike champion Ben Spies rode this machine before moving to MotoGP this year.

Fig. 1 - Rear views of the exhaust system from Ben Spies' 2009 machine

Written by Wayne Ward

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