Definitely detonation. Pre-ignition would have melted the center out of the pistons.
Here is an excellent article describing the differences between the two and some good reading for basic tuning principles as well. Probably stuff you all ready know Barge, but it was a nice read for me since it simplifies a lot of stuff.
Keeping that in mind, I may drop to a 2 step colder plug for my next build along with a 9:1 ratio. The only other thing I wonder about is if there is any easy way to calculate the optimum compression ration given the specifics of the chamber design and anticipated fuel octane. It appeared to be complex fluid dynamics used to calculate optimum performance on their part.
Rick
