Hi guys, about getting some nice sound from 6+ cylinder diesels, the DPF is a real obstacle.
I've got some experience both on inline 6 BMW and V6 VAG engines.
The best solution, both for performance and sound, is to remove every silencer or DPF, and leave just a small silencer on the centre or rear section.
In the case of the 335d, the stock system, as you'll know, is composed by:
Downpipe including DPF and Ceramic cat:
The front Y-pipe goes from 70 to 2x55mm each (2x60mm on the 335i, and they include two cats)
The centre section is 2x55 before the silencer, 2x60 after (2x60 all the way on the 335i).
The muffler have 60 mm pipes, and a pressure balance connecting pipe in the middle. These are taken directly from the 335i, and that pipe is there to reduce droning on the petrol engine...no need on the diesel!
This is the view from below the car:
What I would do is fit the Supersprint downpipe, this way you'll remove the DPF and have the main difference in sound.
The only issue is that this way you need to have a ECU reflash to remove the regeneration cycles, and tune the turbo pressure and EGR to get a good power delivery and not too much smoke.
Another issue is that the car will emit COx and NOx, so a solution is to fit a cat on the exhaust line...the cheapest way is to cut the front pipes after the Y-pipe, and fit the
OEM 335i front pipes that include two ceramic cats!
These will greatly reduce emission, smell and also add a bit of backpressure which is not bad for turbo spool up. They also remove any metallic sound given by long uninterrupted pipe sections. Find some used ones from people who went decat on their 335i...
You can also use the x-pipe from the 335i, removing the 335d centre silencer.
Once you have those you can fit just the racing mufflers from Supersprint (part. number 046124 + 046144)
You'll end up with a very free flowing exhaust line, sounding real nice, with low emissions and not too expensive!
Obviously you can ask Supersprint for a front Y-pipe with their metallic sport cats (very expensive) and their x-pipe, but you'll spend much more for the same result.