Ex­hausts

The se­crets be­hind the loud­est part of any mo­tor­cy­cle

Motorcycle News (UK) - - Garage -

‘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.

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