I find fault with that conclusion. Yamaha robbed the powerplant from the xt which had a 4 valve head and EFI. The question is why did yamaha change the head design in the xt. One guess would be that an intake with 3 valves would not work well with a larger (45mm) single port as the center valve could dominate and possibly cause turbulance and flow issues with the remaining 2 left and right valves. Of course the yfz functions well with a single intake port and 5 vavles, so that brings us back around to other options. Perhaps its the size of the head or flow of the intake and throttle body. Something to ponder no less.

 

Well said Dog, and you could very well be right.
Here is my wild *** guess. EFI is more efficient air wise. You don't have a venturi with EFI so there is no restriction at the intake like there is with a carb. Couple that with the bean counters who ALWAYS want less and less to save $$$'s and you will have a head with less valves. With EFI they can do many things:

#1 Get better air flow with less parts.
#2 Same or better preformance with less parts.
#3 Customers that will pay more for COOL EFI. (More $$$'s for Yami)
#4 Advertise that they have a sport quad with EFI......oooooohhhhhhaaaaaaaahhhhhhhhhh.

Bottom line...........more preformance for the customer and more $$$'s in Yamies pocket. A win win situation for Yamie..............That is until a factory comes out with a super/turbo right out of the box. Then you can throw all of this multi valve stuff away.

 

Look where the 660 motor came from and then look at the 700R motor. The 660 started out life as a motor in a 2 wheeler racing/road bike. They still race them today, just ask Tim Barker and he will tell you all about it and how much power those motors make. The guys using them on the race bikes are claiming 74 plus HP, with stock bore and stroke and running 11:1 compression. The stock head on the 660 motor needs work, (Again it is configured for low down power in stock form), but once it is flowed right it will make massive amounts of power and flows better than any other comparable 4 valve head.

The 700R motor was developed for an off-road thumper. It started life as that on a XT. You don't need high RPM power with this motor, so engine size and use determined low down torque and power. The compact size and the way the cam and valves where placed was all done in a very compact way. All to save weight and to bring down cost. Taking it to 5 valves would have added much more weight and cause the head to be much bigger, with more assembly time. Look at the size of the cam on the 660 and the new 700 and you will see what I mean. The 700 head was not designed for top RPM power, hence 4 valves, which is suited for better low and midrange power.

All Yamaha race bikes, come out with 5 valves, and have been for 20 years or more. Yamaha was the 1st people to use 5 valve heads and has/had the patent for it since the 80's. Why would they stick with it, if it was so bad?

 

That doesn't mean anything! Explain why the YFZ still has the 5 valve head then?

 

So do I. is it that there is something "wrong" or just something that can be improved???

RoostKing...

 

I'm saying the Raptor 5 valve head couldn't have been "all that" if they changed to 4 valves for 06. Maybe they changed it because it was too expensive? Maybe they wanted to keep intake velocity up? Maybe 5 valves was too much spring pressure for a single cam and the parasitic losses were too high?

I don't pretend to know why, I just know they changed it and I'm sure they had a good reason.