A lot of people fit either a decat or sports cat downpipe to the M135i because not only do they make a brilliant noise, but it also allows us to tune the car to around 400 bhp. Because of this, not a lot of people fit a cat-back system as well as the downpipe.
Recently, Cobra have developed a new system and offered one to us for some testing. We had both the resonated centre section, and the none-resonated centre section. At first, with the car having a decat downpipe fitted, we were going to fit the resonated system. We decided to fit the non-res system first though, to find out just how loud it actually was.
To our surprise, the car sounded brilliant. Under load, the "raspy" BMW 6 cylinder gained a lot more of a bass tone and the owner reported back saying there was very little drone on the motorway. Not something you'd expect from a turbo-back system with a decat and no resonators!