I am going to effort welding some very short body 400cell cats right to the merge collectors. If it works right, I should have an output flange about where the factory one was, right after the sway bar.

Have you read or tested 2.25" vs 2.5" pipe from the cats back? There might be more selection in 2.5 (and the cat bodies might be shorter), but if it doesn't do anything worthwhile, I'll make up a new midpipe with the Borla in it at 2.25.

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2001 E53 3.0 5pd