The secrets behind the loudest part of any motorcycle
‘Negative pressure helps pull the fresh charge in through the inlet valve on overlap’
An exhaust uses the negative pressure of the exiting gases to help suck more fresh fuel/ air into the cylinder. It has to complement the type of engine and its number of cylinders and changing the design can alter where the power is delivered in the rev range, affecting drivability and other characteristics.
When an engine’s piston rises it pushes the exhaust gases out of the open exhaust valve as a pulse, and this gas pulse is ridiculously hot and travelling at an incredibly high speed. Just like a Motogp bike going down the straight, this creates negative pressure behind it which helps pull the fresh charge in through the inlet valve on the overlap.
Unless the engine is supercharged or turbocharged you can’t fill the combustion chamber to its maximum, so the exhaust valve is still slightly open when the inlet valves open to let fresh air/fuel in – this is called overlap. The charge is sucked in by the negative pressure from the exhaust when the exhaust valve closes. But the charge still continues to come in at its original speed, filling the chamber more than it would otherwise.
Getting the most from an exhaust is a science involving an intimate knowledge of how engines work, and must also take into account desired power outputs, packaging, sound, as well as material strengths and weaknesses. And the whole thing’s finished with intricate, perfect, welding.
Who knew so much work went into a simple exhaust system? 4 1 3 2
THE EXPERT MHP Exhausts. Ex-racer Mark Hill makes bespoke race exhausts, as well as development systems for the likes of Hesketh and the Gemini T3 projects.