[Lynn Eades]

Dan, as the octane rating goes up, the flame front travels faster and so, the ignition timing requires retarding. (not advancing)This allows the piston to reach t.d.c. before the pressure on the piston builds too much.
I'm surprised that Richard F. hasn't chimed in because I faintly recall a discussion about Bedfords and Morris's that had piston problems and I don't recall the reason.