4 THINGS YOU NEED TO KNOW…

Motorcycle News (UK) - - Garage -

More valves, more revs…

The en­gi­neer­ing rea­son­ing be­hind a three in­let/two ex­haust valve lay­out 33 years ago was that it al­lowed the bike to rev higher be­cause the valve train was lighter. Back in the 80s met­al­lurgy and cost meant that pro­duc­tion valve springs strug­gled to cope with the valves’ in­er­tia at higher revs, so you’d get ‘valve float’ and valves, seats and pis­ton crowns might crash to­gether. Go­ing for more valves meant they were smaller and lighter.

…and bet­ter flow too

On pa­per a five-valve lay­out gives more valve area to flow the charge through. How­ever, to main­tain the cor­rect com­bus­tion cham­ber shape with th­ese larger valve heads, the valves are set at nar­rower an­gles in the cylin­der head. The cen­tre in­take valve is set at 9° and the two outer ones at 17°. Ex­haust valve an­gles are 13°.

More power and torque

Yamaha en­gine tests at the time found that the mo­tor’s out­put and torque were bet­ter across a wide range of en­gine speeds com­pared with con­ven­tional four-valve en­gines, with over­all power 10% higher and fuel ef­fi­ciency 5% bet­ter. Valve main­te­nance is also in­creased to once ev­ery 26,000 miles.

Get­ting more swirl

To push the power ever higher you need the fuel/air charge to tum­ble and swirl but when I stripped stan­dard mo­tors you could see there were ar­eas un­touched by com­bus­tion. Swirl al­lows the charge to flow into the com­bus­tion cham­ber for longer as the pis­ton comes up and you get more atom­i­sa­tion, which al­lows you to run more ad­vance with­out wor­ry­ing about det­o­na­tion.

I dis­cov­ered an Amer­i­can tuner (mo­tomanusa.com) who filled in 40% of the in­let ports on an en­gine to in­crease the charge ve­loc­ity. In­spired by him, in 2000 I tuned an R1 en­gine that way and slot­ted it into an R7 chas­sis for Michael Rut­ter and he won the Ma­cau GP on it. The smaller ports gave Rut­ter’s bike more drive out of slow cor­ners and the same top-end.

20 valves in to­tal for the ground-break­ing 1998 Yamaha R1 A five valve Yamaha R1 en­gine was slot­ted into Rut­ter’s R7

The FZ750 led the way when it came to the five-valve head

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