Originally Posted By: VeryNoisyPoet
Originally Posted By: Globalksp
Thanks for this. Curious to learn more about that octane "issue". Being the car it is, I always fill up with "regular" which I believe is 87 octane here in VA.
"Regular" is normally 87, but up at high altitude, I saw a few stations with 85 as "regular", 87 as "plus", and 91 as "premium".
At high altitude, air is thinner. This lowers peak combustion chamber pressures, so knock or pre-ignition doesn't happen as easily. Apparently it is cheaper to produce lower octane fuels, which can be sold at those high altitude places (for more profit $$$).
A naturally aspirated engine will do just fine running 85 instead of 87 at high altitude, but may encounter problems if it descends to lower altitude and thicker air without refueling to boost the average octane of the tank back up. Cruising will be fine, but accelerating to pass or enter the highway will be where the problems arise. Best case is poor power and fuel economy as the ECU tries to compensate, worst case could be damage if the engine is forced to run under sustained heavy loads with inadequate octane.
A turbocharged engine is a different animal, since the turbo system will compensate for thinner air and will maintain relatively high chamber pressure (and therefore power). Use of turbochargers and variable superchargers for altitude compensation as well as power boosting dates all the way back to aircraft in WW2. Turbocharged and high performance vehicles should always use the octane rating recommended by the manufacturer regardless of altitude since the engines are under more stress and operate closer to the point of pre-ignition by nature.
You should be fine with either 85 or 87 if you need to refuel in the mountains, but in your position I personally would pay a few bucks more and get 87. I forked over for the 91 when I did my cross country trip, since my car does take a power and economy hit on 87.
Thanks for this!
