Thinking it just detonated and busted a chunk out. No sign of wall scoring in the pix, like the ring seized up. I've pulled out a piston before pretty much like that; detonation broke the top edge out. Broke the ring up too, and some ring chunks worked up through the new hole. (But it ran a lot longer with the issue going on...)
10 psi and 9 SCR is not tons of pressure and SCR; I've run a lot at a true 8.2 with 14-15 psi. So the idea on the timing is probably a pretty good guess IMHO.
OP, how is your boost being controlled? Just the size ratio of the charger and engine? Any other control on that, and any gauge?
