Detonation can be caused by carbon build-up inside the combustion chamber, which effectively raises your compression ratio, which can cause detonation. GS-R engines have a knock sensor to retard timing to help prevent that from happening (which is what it does if you use less than 91 octane fuel), but it may be too much for the sensor. I'm not real familar with how the system works, though, so I can't be sure. You might try some fuel system cleaner to help get rid of any of that stuff. I use Valvoline's SynPower fuel injector cleaner every 3K miles (every oil change), and every 12K miles I substitute that for their fuel system cleaner. After using it for awhile on my old car, it allowed me to return to a lower octane fuel without any knocking or pinging problems. I use it regularly on my GS-R as preventative maintenance.