Engine Detonation
 

Hi All,
I have a big block 454 that has just been tuned up. plugs,wires,rotor cap, rotor and modual. I am suffering detonation when I turn off the engine.
Some times longer than others. When it seems to go on for a while I switch the key back on let it start and then turn it off. Usually that fixes it that time.
This cannot be good for the motor.

Any ideas?

Thanks in Advance