EXHAUSTIVE QUESTION
Your kaleidoscope of bike designs through the ages in the 500th issue prompted me to ask one simple question about the evolution of the motorcycle engine, which Rick might be able to answer. Apart from looking just plain ‘Wrong’ (apart from V-twins), why has the exhaust and carb arrangement always been set with the exhaust out front and the carb hidden away from the airflow at the rear of the motor? Even looking at some fantastic BMW Kompressor engines when the supercharger is mounted at the front, the designers still chose to feed the rear of the cylinder. I appreciate the BMW engine was designed that way and it would have been more bother to change it, but can I still ask if any manufacturer has ever dabbled with the potential gain by forcing the fuel/gas mixture in the front of the engine as the bike cuts through the air?
Martin Wilkie, Reigate
Hi Martin, interesting question and I don’t know for sure, but suspect the reason originally was simply for cooling the hot, exhaust side of the engine. Reversing the ports would also make the temperature difference between inlet and exhaust even wider, risking distortion. Forcing induction simply by moving forwards is also a dubious benefit, because in the venturi carburettor, the relationship between air blowing over the jet and suction created on the fuel is not linear – ie full throttle doesn’t pick up double the fuel of half throttle, that’s why we have a slide needle and jets, to compensate for that. Riding into a headwind, for example, would probably demand different jetting to riding on a still day and road speed would also create a strange effect. All that said, I have seen reversed heads on drag bikes and customs – Triumphs having cams fore and aft of the cylinder are particularly easy to convert but I don’t know how well it works. Cheers, Rick