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Definitely could be a cat issue. Unless! You need to go one step colder on your spark plugs. Food for thought.

In a nutshell, this is right on.

Yeah dude, they're bolted on. Sides of the block, directly in the center. They're either a yellow/tan or black. Round units with a single two/three wire plug.

I see stock internal G8's with...

Hehe, they're on the bottom (sides) of your block. The headers primary pipes run right by them.

Looks false to me, like vibration induced. I run 19-21 degrees, with forced induction, on 93 octane. 10.8:1 Compression.

Check the mechanical factors around the knock sensors.