I’ve owned my 2.0 jtdm ti sw for a couple of years now. When I bought it the previous owner had fitted a duplex exhaust with deleted back boxes. My car is white and the rear picks up sooty deposits and general crap rather quickly. The tail pipes are pretty sooty and although the car is remapped I don’t suffer from excessive smoke from the exhausts under heavy load. The car is always serviced by an Alfa specialist and I never have any problems with the MOT. The DPF is also present and correct. My question basically is what exactly do the back boxes do and would reinstating them have any positive effect on the deposits from the exhaust?