I live in Mt at 5000 ft. regular at this altitude is 85.5 octane. Most cars that call for 87 octane run on the 85.5 without noticable problems. Can a mini S use the mid grade @ 88 octane because of the altitude,or does altitude play any role with the octane rating?

 

MINI recommends 91 octane gas for the Cooper and Cooper S. You *can* run lower octane but the computer will retard timing and you'll be loosing power and mileage.

 

dont go cheap on the gas...it does make a difference and even at prices today....its not that much more in the overall picture.

 

Normally thats true. That is not true in the mountains. Many mountain communities you can only get 85 octane, maybe 87. Depending upon altitude its the same thing as 87 reg, 91 premium at sea level. The effect of the altitude is to lower the compression ratio of the engine (and hence all boost is lower)

When we traveled extensively in the mountains in both a supercharged and turbocharged car I was very concerned about 85 octane gas but found no issues whatsoever using it.
The higher you go, there is a tremendous loss of boost in a turbo from what I remember, something where your glad when your back at sea level.

 

MINI recommends 91 octane at least. 85 is way to low. You can run the MINI of any UNLEADED gas. I accedentally put lower octane in mine, and there was a very noticable difference. I dont recommend it, but its your choice. Plus if your in the mountains, wouldnt you want a 4WD car?

 

Altitude makes a difference in the required octane of the car. Higher altitudes means less compression, which means less prone to detonate, which means you can use lower octane.

 

That is simply not true. The higher the altitude, the lower the compression ration. Go google octane and altitude to understand.

 

It is a little bit more nuanced:

1. The density of the air at altitude is less, hence the lower effective compression ratio (less air is being compressed, therefore lower effective compression). With a lower effective compression ratio, there is less need for octane to retard pinging.

2. Regarding altitude and loss of boost from a turbocharger, there is some reduction in boost with a turbocharger, but less than is experienced with a supercharger. A supercharger is a fixed boost device (a supercharger's compression is a function of engine revolutions, with less dense air to compress at any given engine revolution, less air will be forced into the compression chamber ); a turbocharger's boost is a function of exhaust gasses spinning the compressor blades, so the limit that is faced by a turbocharger is due to the reduced density of the exhaust gasses exiting the combustion chamber. Typically the limit is experienced by greater (longer) turbo lag.