not good..sorry to hear it.....that is why I always suggest going up one notch on octane in an offshore engine ...if it says 87 I run 89.

my .02

My pistons are true flat-tops with one relief, with Merlin heads, I think I am at 8.9 : 1 compression ratio. I am running a 50/50 mix of 89 and 93 (91 octane). My timing is conservative, I shouldn`t have had octane issues.

Unless you got a tank of bad gas...and that does happen...lots of crap going on these days in the gasoline buiness.

many years ago I had a similar condition on one of my bbc's One plug tip was missing and circulated through out the motor. It ended up that the cyl.head had a small crack in it and the spray of cool water cracked the plug. Probably not your condition but it may pay to check out the heads real close anyway!! good luck keep us posted!!

Did you check the rod bearing on the cylinder with the broken plug?? Detonation can flatten rod bearings due to the extreme forces as the piston is forced down as the rod is pushing up. Are you running inconel exhaust valves? If not I would invest in them this rebuild.

Investigate the fuel pressure when you get the motor back together. The loss of rpm was likely running out of fuel which caused detonation due to the high a/f condition caused by low fuel pressure. Check the fuel pump, the fuel filters etc. and make sure the fuel pressure stays greater than 4 psig at the carb with a NA motor.

All rod bearings look great.....

I`m replacing the fuel pump with something more substantial.

I have my fuel pressure set at 7psi on a carbed engine. Anymore than that, and you will overwhelm the needle/seat.

I took my engine parts to machine shop this morning.
Here`s the verdict:
Engine ran lean, comb chambers very hot, #1 cylinder dropped exh valve seat. Seat bounced around through engine, damaging tops of pistons.
I need new pistons, rings, rod brgs, mains, gaskets, new exh valves, valve seat, mill heads, clean block, hone cylinders, valve job, valve seals, polish crank......about $1500 plus my labor.

I would double check your intake valves. I bet you have some tulips!

I`m having this shop inspect EVERY engine part carefully. He knows it`s in a boat and that I run it hard.
This guy is the guy who built all of Eddie Hill`s engines back when he raced drag boats and Top Fuel dragsters. He knows his stuff.