Feul additives for higher compresson engines.

If you're unwilling to use race gas, you can try toluene - it's supposedly around 114 octane. See this link:

http://www.elektro.com/~audi/audi/toluene.html

We used it on a turbo demo car at work at the PRI trade show to make sure it didn't suffer any detonation during dyno pulls. There wasn't anything to tow it home with if something broke.