high altitude motor build recommendations


4 replies to this topic
  • rfx125

Posted February 08, 2011 - 09:39 AM

#1

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.

Kirk

  • grayracer513

Posted February 08, 2011 - 10:47 AM

#2

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:

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  • rfx125

Posted March 08, 2011 - 10:07 AM

#3

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.

  • primerib108

Posted March 08, 2011 - 10:36 AM

#4

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.

  • grayracer513

Posted March 08, 2011 - 11:44 AM

#5

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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