As for more agressieve sound: I think I would look into a tap like a bypass but always there. If you weld on a bung for an O2 sensor before the back muffler so there is a solid fixture that will not start to immediately self destruct I think that could achieve your desire.

Actually to test my theory, hook up your mufflers but space the connection open with some washers like 1/16" all the way around.

If you get the desired effect, put some form of seal like some lead plate on just the top to force the leaking exhaust only down.

I want to replace my rear sub mounts and with exhaust in the way I may try this myself.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)