The engine is still a five-titanium-valve unit that Yamaha says it prefers to stick with rather than a four-valve configuration because it offers superior reliability. Who are we to argue? The record stands for itself on that one.
Straight from Yamaha via dirt action website. You can call Yamaha and they will tell you straight up maintence and reliability of the WR is far superior. Its meant to be riden every day of the year. Once you pass a certain power level on a 450 jug you stress out the working parts, hints the 5 valves.
All the new YZ's are now 4 valve heads.
Yamaha always had a big problem with flow restriction and cavitation with the 5 valve head, as the center port was tuned differently that the other intake ports
The 5 valve was very reliable and very restrictive.
If you ever saw the 2009 YZ GYTR head, it had 5 OUNCES of aluminum removed for porting, to make it fast enough to match the KX450f
My '08 head has MASSIVE porting done to the center port and Medium Masive to the other two intake.
I don't even have to use 1st the power is so massive down low, even geared 13/50