MotoGP exhausts

The 2013 season of MotoGP promises to be very exciting, with the prospect of seeing Valentino Rossi on very competitive machinery alongside MotoGP debutant Marc Marquez compensating to an extent for Casey Stoner’s retirement. The top class of motorcycle racing is for four-stroke machines up to 1000 cc, having a maximum of four cylinders; previously the maximum engine capacity had been 800 cc. The increase in engine capacity and the 25% increase in torque has helped improve the spectacle, although electronics play an increasingly important role in taming excess torque.

With the increased engine displacement came a new rule limiting the engines to an 81 mm maximum bore. In pursuit of performance it is likely that all the engines will be taking advantage of the full 81 mm allowance in order to keep the stroke as short as possible. The short stroke allows the highest possible engine speed and therefore, if efficient breathing is maintained and friction kept under control, the maximum power output.

The 800 cc machines were not limited in terms of bore size or number of cylinders, so they had higher engine speeds. One result of the change to 1000 cc, and the lower engine speed, is a change in exhaust tuning. The time taken for a pressure wave to travel from the exhaust valve to the end of a given exhaust system or junction in the system is, for set parameters such as temperature and gas composition, a fixed quantity. The 1000 cc machines will therefore need an exhaust system that is tuned to suit the lower speed range. If we wish to increase the time taken for a wave to reach the exhaust system end (or junction) to be reflected and to make its way back to the exhaust valves then the length of the exhaust system (again assuming fixed gas temperatures and so on) has to increase, as do the distances from the exhaust valve to any junctions in the exhaust.

With an increase in exhaust length comes an increase in mass and a different challenge in terms of packaging. There may be difficulties in  packaging the required system length on the bike without resorting to design features such as adding extra curved sections. The Honda RC211V (the original 990 cc five-cylinder bike from before the 800 cc era) and the RC213V (the new four-cylinder 1000 cc bike) notably struggled to accommodate the required pipe length from the rear bank of cylinders. The pipe is therefore contorted to accommodate the extra length, much like a single coil from a helical spring, there being a complete 360° ‘ring’ in the underseat section of their exhaust systems. This type of design feature may be specific to the bikes with vee engines that use an underseat exhaust exit on the rear bank of cylinders, as the underseat exit leaves little room for sufficient pipe length. At present only Honda and Ducati have vee engines, but in the past the Ilmor and Suzuki bikes were also V4s.

Having to resort to adding extra curves in the exhaust will lead to increased exhaust back-pressure and a possible small loss of performance. If the curves are too tight then the flow can separate from the walls of the system, giving rise to a very significant increase in pressure loss and performance.

Written by Wayne Ward