Cruising at 3500rpm and boom. Turned the key off and the engine was still trying to run. Bbc 509cid.

check your timing, had same issue happen to me......preignition that i couldn't hear
in my case my distributor loosened and one tight wire pulled it counterclockwise
best .

The plug boot has seen better days, might have been arching a bunch
As clean as the ground side electrode is it looks like it was running lean. Might want to use a bore scope and check the piston

Compression measured ok, put a new plug and it runs sweet again.

dissy felt tight.

Plugs blowing out like that does happen more than you think. I've had it happen once years ago

Glad to hear that! The ground electrode felt loose when i removed it from the cylinder head. Scary part of this is that it was cyl 3, wich is usually the leanest in Bbc.

What delco plug # you using ?

mr43t plugs, 9.7 compression, timing 34 max.

Ok, the cyl 3 has 10bar compression, wich is 3bar less compared to other cylinders (13bar). Made leakdown test that measured 90%(cold engine) and got more confused, pressure seems to go in intake manifold, exhaust manifold and crakcase. Conclusion is that intake/exhaust valves are leaking, not sure about the piston rings because the large ring end caps and piston to wall clearance, due 2680 forged pistons. Took some pictures of the exhaust ports/valves/spark plugs. Removed the cmi sound eliminator mufflers, wonder if the mufflers caused reversion and caused all this. Other one of those mufflers feels tight compared to the other one. Dont know 🤥🤥

Cyl 3

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