z31maniac wrote:
4v > 5v, ask Yamaha why they went back to 4 on the R1.
I don't know the math, but I had always heard that a 5v was mathematically ideal for flow, which was why it was used on the 20v 4A-GE, and several late-model ferrari motors. I emphasize that I don't know the theory behind that, and if it's false, I'm not going to argue elsewise.
I figure Yamaha would go back to a 4v due to complexity and weight reasons, not because of flow or lack thereoft. I would guess that that 5th valve would have to have it's own special rocker arm (whereas you could use the same rocker for all 4 valves in a 4v), and it would have it's own timing issues, both of which increase the cost of the bike and add failure points. I would also guess that it's because advances have allowed 4v motors to flow just as well as 5v motors for all practical current uses, and the benefits no longer outweigh the costs.
Kinda like how desmondromics was awesome for high rpm, until the metallurgy caught up and allowed you to get desmondromic-level rpm with a valvespring. Desmondromics got dropped because it's just extra complextiy and no longer offers enough gain over a valvespring equipped motor in terms of performance to make up for it.
Chevy would probably be a better example than Yamaha, though. Rather than use fancy-pants OHC at all, they've stuck with 2v pushrod motor. And the LS1 crapped all over the Ford DOHC V8 made at the same time. Ford had to add a supercharger on top of that 4v motor just to keep up with a pushrod!
