The original reason Yamaha went to a five valve head was an elegant if simple solution to a difficult engineering problem.
The key to increasing rpm for a given engine size is to reduce the weight of the reciprocating components in the drive train. Anything that moves back and forth in the engine, pistons, connecting rods, and valves. The faster you spin the engine, the faster these parts have to go to keep up. The faster they go the inertia they carry and more stress developed on parts when they accelerate and decelerate.
The problem with valves was the valve springs. Back in the eighties when Yamaha first came out with the 5 valve, manufacturers were limited by metallurgy and expense when making valves for production bikes. When rpm got too high the valves were carrying to much inertia for the springs to provide enough force to keep the valves/shims/buckets in contact with the camshafts. This caused what is known as valve "float" and deformed valves and seats as they crashed together.
In order for the valves to be as efficient as possible they need to cover as large an area as the cylinder head will allow. The bigger they are and the longer they are open the more fuel/air can be drawn into the cylinder, the more power can be made. So rather than 4 big heavy valves, Yamaha went with 5 smaller lighter valves. This resulted in even more area for flow while the valves were open and higher rpm limits and more power for a given engine size.
The problem though is a huge limitation on valve angle, the angle at which the valves are set in the head. In order to only use the standard two overhead camshafts (DOHC) the valve angle had to be widened. This resulted in a pentagonal shaped combustion chamber and put severe limitations on compression. By 2004 they'd reached the limits and just couldn't get any more compression out of the 5 valve heads.
Since the inception of the Genesis engine metallurgy has improved enough for production valves to be much lighter than in the eighties and increased the strength and flexibility of springs. So much so that the current large valves in the R1's 4 valve heads are actually lighter than their smaller 5 valve head predecessors from the nineties. Going back to 4 valves has allowed them a shallower valve angle in turn allowing them to reduce cylinder head dome height and increasing compression.
In the eighties and nineties the 5 valve was better, not so anymore. Too many advantages with the 4 valver.
R1-013 LUCKY 13th SQUADRON
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