Think Valve-Head Periphery!
New WheelBase member Larry Cavanaugh, like me a devoted motor head, has asked (via the Internet) about the comparitive advantages of four- and five-valve cylinder heads. It's a good question, as Yamaha has employed five-valve heads on production engines, yet in racing the four-valve head remains supreme.
Part of the answer to Larry's question comes to light if we ask another question: Why do we bother to fit more than two valves when we have only two processes, exhaust and intake, to be provided for?
Get ready for a labyrinthine discussion, which is what you always get when you ask simple questions about complex subjects such as the internal combustion engine. And somebody remind me to tell you sometime about my favorite oddity, the rotary radial engine used in WW1 aircraft. These engines had pistons, which did not reciprocate; they had crankshafts that did not turn; and they had one-valve heads.
You may have imagined that four-valve heads have more valve area than those with two valves. In fact, at any given valve angle the flow areas at full lift are virtually identical. Where the four-valve arrangement shines, in terms of air flow, is at lifts less than 25% of valve head diameter -- which is to say nearly all lifts less than wide open. At lifts from 0.0% to about 25% the air passes through the annular slot between the valve and valve seat. Therefore, real flow area is tied to total valve circumference, not area. And there is more circumference around the diameters of two intake valves than is provided by a single valve of equal area.
The problem with multiple valves is that the narrow spots between valve seats are hard to cool. Worse, the port tunnel becomes more complex and less efficient as you go from one intake hole to two or three.
Part of the bifurcated and trifurcated ports' efficiency loss is due to increased wall area. We talk about "thin air," but the stuff has thickness and viscosity (the reason flying insects have the shape they do and not a shape we readily identify as "streamlined"). Air sticks to the port walls like syrup, completely stopped at the wall and exerting drag on the passage of air through the port. The more port wall area you have, for a given cross sectional area, the greater the drag and the lower the mass flow.
Another problem is the confusing effects a branched port has on sonic wave activity, which plays a large role in charging the cylinder. It is impossible to join even a one-into- two duct, much less one-into-three, without creating a large change in cross section area. Such abrupt changes cause sonic waves to reflect furiously, breaking up and losing energy.
I suspect it is some combination of the above factors that have kept Yamaha's five-valve layout from more general use.