4 THINGS YOU NEED TO KNOW…
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.