Small issue with my car.

If i run 100% gas 91 octane the car has noticeable detonation on the highway.
if i run 91 octane with ethanol, it mostly goes away, but not 100%.

luckily for me, they just put a new gas station in down from my work that has 93 octane! i run it and the car runs awesome and great. more power and better mpg. i'm not going to quit running it, but my question is, shouldn't i be able to run 91 and have no issues with it?

2003 cooper s jcw w/ 88k miles.
new plugs, new wires, new coil, promini cai, ddm intercooler diverter


thanks!

 

I would run the 93.
The car runs best on that.
I run 93 only in my R50.
That's what is the best for the car.

 

are you running SHell 91 when its detonating?

 

seems like any 91 will do it, but to be honest, I haven't noticed the stations. I know i have tried a shell before and it did it