I know this is opening a can of worms, but lets see what the consensus says...

When I travel I notice gas stations at higher altitude do not carry the 92 and 93 pump gas I normally see in the flatlands. Like up at 5K the highest I see is 89, and up at 10K its 87. So when I rode in Utah and Idaho in the past, I would jsut run the 89 pump gas in place of the 93 I usually run at sea level, with my 11.5:1 piston, and never had any issues or preignition. Been doing this for 4 years actually.

So now that I have a 14:1 compression and running 110 octane at sea level, how low can I go at 5K? I am only interested in dropping the octane for the sake of performance, its not a big deal but would be nice to know my limits, if any of you are experienced here.

 

I'm in San Diego (sea level) and the highest we have here is 91. It may have something to do those pesky environmentalists. Damn Green Peacers. LOL

 

At 5000ft then you would be running about 12.25-12.5-1. We carry 91 octane. You'd be safe w/ a 50/50 mix for sure. You might even go 60/40.. I ran my 12.5-1 motor at Glamis last week on a half tank. I only had 91octane in it and I had no issues.. I ran some race gas just to be safe for the rest of the trip.. One trip at sea level isn't worth blowing up two motors. It was more of a precautionary thing..

 

thebom told me I could go as low as 106 here at home, so I should be plenty safe around 100 at 5K, like you are saying sandgod. a 50/50 mix of the 110 and 91 pump otta be plenty good. maybe I could go lower even, but like you said better safe than sorry, and a 50/50 mix is easy math anyway.

 

It would not hurt anything to run the same Octane you run at sea level, at 5K would it. If we are talking cost, or issues with getting the fuel then great drop if you can, but lets just say money and availability are non factors...Would it be and issue to run say 110, if all you needed is 100???

 

Hightower, I run my 14-1 bike on 100 at St Anns. My 12-1 will run just fine on pump.

 

Due to the lower partial pressure of O2 at altitude, the rule of thumb is 1 octane point per 1000 feet. IOW, if you need 92 octane at sea level (mfgr recommendations are for operation at sea level), and you drive at 4000 feet you only need 88 octane (92-4). It's why "Regular" in Utah is 85 not 87. It's not the greenies. SLC is 4,700 feet. Higher octane when you don't need it is wasting your money.

 

Not to mention the slower burn rate of higher octane fuel will slow you down in many cases if it is over kill.

 

The effect of increasing altitude may be nonlinear, with one study reporting
a decrease of the octane requirement of 1.4 RON/300m from sea level to 1800m
and 2.5 RON/300m from 1800m to 3600m. Other studies report the octane
number requirement decreased by 1.0 - 1.9 RON/300m without specifying
altitude.

With your A1/A2 cams, your dynamic compression for trail riding should be just fine with the 50/50 blend.


OMR

 

So higher altitude, lower octane is fine is the general agreement it seems. Plus OMR agreed, so good as gold. (Not messing with you, just respect your comments that much).

DSNut, you said possibility of losing performance with too high Octane at high altitude. Worries me a bit, as we run 93 at sea level, with stock compression as per our recommended specs in the book. Picked up some 111 last time, and mixed it with the 93 so I am guessing the 100 range. We ran that mix at 7000 with jetting way rich (forgot to change before we went [img]i/expressions/face-icon-small-blush.gif[/img] and they ran really well. Any thoughts on that?