Looking to give the very quite exhaust ,a little growl, but dont want to overdo it . At the moment it is running a standard nz new twin pipe system with 3 mufflers , no catalytic converter . Have seen a 635 with 2 small expansion chambers up front in place of the first muffler , and the note was good without being too loud . Also another 635 but with 2 large expansion chambers in same spot but was way too loud . An exhaust specialist recommended changing the 2 pipes into a single large one . Would be keen to hear what others have done and corresponding noise levels ,cheers Lars .