high altitude motor build recommendations

My '06 YZ450F motor needs a bunch of $$ thrown at it; new cams, crank set, piston kit, cam journal repair, the valve job has 93 engine hours on it, I expect to throw WR gears in it and maybe a WR crank & electronics for lighting.

I ride high country single track at elevations between 8-12,000 ft and accumulate 100-150 engine hours a season; I want a motor that will perform reliably during the season with the performance to handle long, techy climbs (think Timberline backwards).

I'm soliciting ideas for how to spend my money; stock displacement vs bored/stroked, oem vs aftermarket cams, ported head, stock vs OS valves, carb mods, etc.

Thanks for your input.


For the riding you describe, I would build an entirely stock engine and forget about any sort of high performance mods with the single exception of an increase in compression ratio as a compensation for your altitude. In the interest of longevity, select a piston with two compression rings.

The WR trans conversion should cost between $4-500. The WR crank and electrics will be much more expensive:


Gray, thanks for such a quick and definitive reply, no one else has anything to add. Word on the dirt here is that the high compression pistons make kickstarting problematic, so I am inclined to skip that option. I did however, find 2 torque cams for stock YZ450F motors @ Web Cam Inc. (grinds # 297/49 & 297/192 (w/more exhaust lift & duration)). The person I spoke to couldn't qualify what the extra exhaust lift & timing would do for the powerband. Gray, might you recommend a torquey cam for my trail riding when paired with the WR gears.

I use my YZ anywhere from 3K - 8k feet.....messing with the jetting is about all you have to do but I did notice a little bit of an increase in performace (low end) at really high altitudes going from 12.3:1 to a 12.5:1 compression ratio piston. Vertex makes at piston that you can use stock yamaha rings for the next rebuild.

Extra lift would be one thing, but the added duration will most probably not help low speed power. The other thing to consider is the lobe centering. Generally speaking, a smaller lobe center split (the distance between the intake and exhaust lobe centers in degrees; smaller meaning closer together, so more overlap with any given grind) biases the cam toward a higher RPM. You might consider trying the exhaust cam from an '03-'05 YZ450 if it's really lower RPM power you want. Another cheap trick could be the CDI box from an '04 (NOT an '05). I can't recommend any aftermarket cam grinds that I know of for the purpose you describe.

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