I am about to undertake the 135i > 1M quad exhaust swap because I don't care for the selection of 135i quad exhaust muffler styles. Something about the half-assed tiny tube that connects the passenger side tips (I'm looking at you Remus and Eisenmann..)
I know that there's a Hartge diffuser which is supposed to have the same exhaust tip spacing as the 1M that fits the 135i M-sport bumper, but it's NLA.
There's also a specific model Rieger diffuser which has the same 1M tip spacing, and fits the 135i M-Sport bumper, but it's approximately $1,800US.
I know I can swap my M-Sport bumper out for a 1M style aftermarket rear bumper, but looking to keep the OEM M-Sport if I can. Has anyone done the 1M exhaust swap used their M-sport rear bumper? If so, what method did you use to create a diffuser that fits?
1M stock exhaust outlet:
135i stock exhaust outlet (spacing is different on the left side exhaust outlets):
