[Yetiman]

There are many factors involved, but it depends partially on how much ignition advance you want to be able to run, and how hot the engine gets.

That said though, if your running all the advance you want to, and have no detonation issues when the engine is as hot as it will ever get, there's no reason to up the octane any further.

Also, if you run Av gas, i wouldn't go telling people that.