My understanding is you lose one point of compression for every 3000ft in elevation.. So at 5000 ft you are losing about 1.8lbs in compression bringing your compression from 14-1 to about 12.3-1..

 

Corrected Compression Ratio = CR - [ (altitude÷1000) x 0.2 ]

Thus:
14 - [ (5000÷1000) x 0.2 ] =
14 - [ (5) x 0.2 ] =
14 - 1 = 13



Remember that temperature, humidity and altitude are key to jetting.
If you normally ride in low altitude, warm and humid air conditions;
then when you go to high altitude, cooler and dry conditions; you must correct by
each of the three stated elements.
Higher = lower octane and jet leaner
Cooler = jet richer
Humidity = Drier is richer jetting required
By running higher octane at higher elevations without rejetting, you must have cooler and less humid conditions to get by. Sometimes you can luck out.
Bests policy is to carry your jet kit with you when you travel.
Make a good judgement call as Hightower is doing and adjust for the conditions of the day when you are at your final destination.

 

Wow, very good to know. It wsa a clear day, at about 40 degrees and very dry. That would be why they ran pretty good then. Jet kit is almost always with us, some dummy forgot the entire tool kit and nobody realized it till we were there.

I am not sure how to word the next question on this, but basically at the Oregon Dunes cooler and higher humidity usually go together. Would it be safe to say in that aspect they probobly cancel each other out in effects, and now it is just an elevation unit, or does one effect more then the other? I hope that makes since...

 

Here's a good ratio to go by:
For every 30 degree change - change one jet size. Example 175 to 180 Cooler is richer
For every 3000 feet of elevation - change one jet size Higher is leaner
For every 30% change in humidity - change one jet size Drier is richer


With elevation the atmospheric pressure is lower. Thus the need to control pressure thru the carb. You could advance your timing to compensate, but that is not easy. So by running lower octane you can burn the fuel faster and increase internal vacuum signal.

 

And since my dad and I are both running aftermarket CDIs that would have helped out our condition also (RWR and Vortex)?

 

Also, I would have thought that a higher humidity would increase the air density much as the cold does. But per your example it is the oppisite. Why is that?

Man I love learning new stuff.....

 

Humidity—When the percentage of humidity in the air increases, the engine draws in a lower percentage of oxygen during each revolution because the water molecules (humidity) take the place of oxygen molecules in a given volume of air. High humidity will make the air-fuel mixture richer, so you should change to smaller jets.

Also, the water in the combustion chamber will cause excessive cooling and thus the reduction of cylinder head temperature will result, output horsepower will be reduced.

 

thanks guys, you've confirmed some things I've always thought, and tought me some new stuff with the octane formula. I know its all theory on paper with the math, but it all makes sense and is the best place to start, letting trial and error runs fine tune from there.

when I go from home in Indiana to Utah or Idaho annually, I always jet down. Knowing that temp and humidity effect jetting as much as altitude, its always a general guess since I don't know what the weather will be til I get there. Trial and error runs up Sand Mt. or Choke Cherry hill always end up putting me about 2 jet sizes under what I was running at home. Cause its very humid in Indiana in the summer, but not as hot as the desert, but the desert is very dry, and indiana is near sea level where utah and idaho dunes are 5K, all 3 things considered the general rules of thumb mentioned by OMR apply very well and validate my experiences looking back in retrospect.


I will run 100 octane, cause it seems to fit the equation well, and as mentioned its easy math. There are a few reasons I wanna run minimum octane requirement - performance, price, and it'll be that much less fuel I have to pack with me. I burn about 30 gallons in a week long trip riding my ***** off putting many hundreds of miles on the DS in just 5 or 6 days. There is no place within an hour radius to get race gas from L.S. utah that I am aware of, but I can get pump 90 right down the road. So I only need to pack 3 cans of race gas instead of 6, all the way across country. Makes it that much cheaper too.

 

Rocky Mtn ATV carries 111 octane. It comes in 5 gallon cans at 40 bones.. You know that's on the way to LS Utah..

 

holy crap!! $8 a gallon! I'm paying $4 for sunoco 110 leaded here at home!
but yes its nice to know if I do run out I can make a trip up and get some, its kinda fun running up to Rocky Mt. after being out there for a few days anyway, thanks Sandgod!