Good Morning. We have just purchased a 2006 Mini Cooper S. After reading the Manual and stopping to re-fuel for the first time, The placard on the fuel cap door says’ to use a min of 91 octane fuel. Locally, most of our pumps have 87, 89 and 93. I was just in Denver and some of the pumps there started out at 85. What are the detrimental effects of using lower octane fuel than the 91? Would alternating between 1 tank at 89 and 1 tank at 93 be a alternative? Are there re-tuning choices? Like different Plugs, or re-programming the ecm? Any other concerns? Thank You.

 

I have some experience with this. Using an octane too low, especially in warmer weather, will allow predetonation to occur (ping) that can cause damage to the engine. Sometimes, you cannot even hear the ping because it is so mild, but it can still do damage over time. If you are accelerating under load (onramp, etc) you are more apt to have it occur. You spent good money for your car, don't go cheap now to save 10 cents a gallon.

 

The MINI ECU won't let pinging happen, it will pull ignition timing and your power will drop off and you'll get horrible mileage so in the long run, it's cheaper to use 91+

 

My Mini is really succeptable to what Adam is talking about. Shortly after I bought my Mini, I tried 87 Octane and the car's performance and milage fell on it's face. I recently put in a tankful of 89 octane because the station I stopped at was out of premium. I figured what the heck, let's see what happens. Again performance and milage went in the toilet. I am not sure how the ECU works, but it seems to take about a tankful of premium to get the performance back up to snuff.
I wouldn't recommend anything less than premium, even in a normally aspirated Mini like mine. And I am a real tightwad who wouldn't spend any more for gas than he has to.

 

When the ECU sees knock it pulls a lot of timing really fast to prevent damage then slowly adds it back in if the knock is gone. So yeah, it should take about a tank or so of premium before your timing returns to normal.

 

Does the base Cooper (i.e. not an S) also require 91 octane?

 

very cool, thanks for the info! I knew toyota had a knock sensor and did that, but none of my current cars (9 of them) have that function.... well, one does now.

cheers

 

Yes, it does