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I'm just wondering what effect high altitude has on a sled's top speed.

I have 2 real-life experiences:

1) I just bought a 2004 Polaris 550 fan. It has a 136x1.25 track. On hard-packed trail, at 9000 feet, I can barely hit 50MPH on it. My old Cat 440 (fan) Panther 2-up (136x.75) was easily able to get up to 65MPH with 2 people on it. That was at 600 feet, give or take a few.

2) My son is still riding my old ZL500... I had clocked it once at 86 MPH (gps), at 600 feet... Put the same kid on my buddy's Ski-Doo 600 HO, and he can barely touch 65MPH at 9000 feet.

The 440 was 100cc smaller, but went 10+ MPH faster. The Doo 600 HO is supposedly a more powerful engine than my Cat's 550, but it's 20+ MPH slower at high altitude.

Why the huge difference? I'm guessing it has something to do with altitude, but I'm not sure. Anybody know what's going on? Should I expect drastically lower speeds because of the high altitudes I now ride in?
 

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You are supposed to rejet your carbs based on elevation. The owner's manual will tell you the jet size you need for your height.
 

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at 9000 feet, a NA engine will have only 50-60% of its normal power at sea level, IF it is jetted perfectly. If you didn't change jetting, that is going to drop even further. Having the jetting exactly on won't get you past the 50-60% though, its simply a matter of less oxygen.

(here is a calculator if you want to play around with it. At 9000 feet the barometric pressure in in/hg is about 23 or so)

Relative Horsepower Calculator
 

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Exactly at alt, you only get 50 percent! That is why so many want as big a cc as possible and even turbo them! EFI realy shines, esspecially when you are going up and down in alt!
 
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