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masterraptor

New YZ450 4-valve motor?

7 posts in this topic

The 5 valve design was maxed out for flow... by going to a 4 valve design it leaves more room to play with porting and other changes to improve efficiency/power output.

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I am basically a quadder switching over to 2 wheelers.

One thing I love about this 450 (and 250) motor is the high revving power. On the Raptors, yamaha switched over to a 4-valve head with fuel injection in 2006. Killed the top end. Totally took the fun out of the bike in my opinion. The older 660s had a better power range.

Another thing I like about 2 wheelers is they are good to go out of the box. Don't really need mods. Quads, you gotta sink a couple grand into them to make them worthy. Would hate to buy a new wr450 and imediately start thinking about what I gotta do to it to make it fun, like install oversize valves, port the head, and a install larger TB and controller. Not sure if this is a problem with the 450 though.

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So, how much power did the R1 loose by going to 4 valves? You do know the YZ450 has more power with the new head than with the old, right?

The 5 valve head is actually a limiting factor in power production in peak terms. The advantage it was originally supposed to have, and actually did, was that it could give a high performance engine better performance at lower speeds. But in peak power terms, the triple valve intake presents a number of major problems in trying to build a high performance head. You have two port dividers to flow around instead of one, and three valve guides rather than two. Plus, the flow from the ports into the chamber with a center port between the two sides creates a variety of challenges. Then add to that the fact that the space taken up in the head by three intake valves can accommodate two intakes which together have a larger total area than the three do, and you see the 4 valve having the clear advantage.

Ron Hamp has been turning out 65 HP Honda 450's for a long time. It took him years to get a YZ450 over 60 HP, and the head was the reason why.

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