I was using 91 non ethanol but have been backing that off just to 87 as that's what I built it for at about 9.3:1 compression.…
As I recall I was at about 16 initial and 34 all in with no vacuum advance.
The plugs were looking great until this happened..
Again everything was working running and operating absolutely perfect until this happened..
The tops of the Pistons in the spark plugs weren't showing any signs of detonation....