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As for timing itself causing it it can't. Went the key is turned off the spark to the ignition is gone so timing from that point is a none function.
Detonation or hot spots is another cause to look into.



Unless the timing is too far advanced that it's making the engine run hot, and creating hot spots in the chambers, which is causing pre-ignition/detonation at all throttle and load conditions, but he just doesn't hear it or feel it until after he turns the key off and it keeps running for a few seconds.

but, since he said he's on 87 octane with a "performance built" 360, I say first thing to do is jump up to mid-grade or premium to see if it cures it.

**Photobucket sucks**