Originally Posted by MACDRIVE
Edit: I think I remember willwren telling me that you guys can run the 91 unless you've swapped supercharger pulleys; am I right?
There are no hard and fast rules..."do X if you have Y". Even the owners manual is just a guideline. And yeah, that sounds about right..the SC engines are running higher compression ratios over N/A motors, so they are more likely to run into engine knock on lower octane gas.
You run the gas with enough octane to prevent pre-detonation (engine "Knock") caused by compression or hot spots in the combustion chamber. If you really want to know if your car is running right on a particular grade of gas, look at the knock sensor counts...because that is what is going to retard the timing.
If you are still getting engine knock on a higher grade of gas, try swapping to a colder sparkplug (The leftover heat in the plug tip might be detonating the mixture before the spark even fires.)