If you get your car mapped on the "ragged edge" as mike put it, then you dont ever want to run a lower octane fuel that it was mapped on.

If you dont use your car often, bear in mind that couple of month old super is only around the same octane as normal unleaded!

I get it on my mini cabrio when i dig it out after the summer, the ignition curve is fairly aggressive and on old fuel it dets, but as soon as i fill up again its all back to normal, scared the shit out of me the first year it happened as it never occurred to me the fuel had lost so much during the layup!

so thats one possibility.

Poor flow on that injector compared to the others is another (even brand new they are NOT identical IME, and with age they get worse)

Could be an oil problem on that cylinder, like valve guide or rings, as if oil gets into the chamber it effectively lowers the octane rating of the fuel.



Surprised its happened on Stu's mapping in particular though as i beleive he is normally fairly conservative with his mapping for all the above reasons (ie he will sacrifice those last few bhp you could get right on the edge of det in order to give you extra reliability as thats what a roadcar needs), so i doubt he brought it nearer to det than about 3 degrees, so whatever it is causing it i doubt its a very trivial thing as if it was i would still expect it to be safe!