DIY PE Mod exhaust
Bought a spare stock N54 135i exhaust from the forums, with the intention on modding it to the PE mod:
http://www.1addicts.com/forums/showthread.php?t=963343
Bought it at the end of 2015 and only got around to doing it the last few weeks. Installed it on the car a few days ago.
Fortunately I saw
woosh for sale thread with pictures of the 135i muffler. The N54 and N55 135i exhaust is different. The Y pipe internally is perforated on the N54 but is solid on the N55. This also explains why the exhaust flap on an N54 135i is pretty ineffective, as both pipes are muffled regardless of whether the valve is open or closed.
To do this mod properly, I had to cut open both sides of the muffler and weld up all the little holes in the Y pipe. The Y pipe is perforated on the top and bottom, quite hard to access.
The 3 big holes in the middle pipe I patched up with some small piece of stainless. The 3 big holes are connected up to the exhaust flap. The other pipe in the middle is perforated and I left that one alone to make the exhaust a bit more quiet when the flap is closed. I replaced all of the fibreglass packing.
Then welded it all back up and ground everything flat. I underestimated the amount of work required for this mod, this was my first welding project and was a bit of a learning curve. Also grinding, and smoothing everything took forever. Ended up polishing it all up to make it look a little nicer.
I am very happy with the exhaust. When the flap is closed it is louder but still pretty quiet in the cabin with the windows closed. The exhaust note is a bit more higher pitched and is much louder when under power. The crackles and pops are much clearer and crisper but are not over the top. Blocking off the exhaust valve in the engine bay (effeictly golf tee mod) has a much more pronounced effect, much louder driving around at low loads, but I prefer the automatic exhaust flap control via the DME so i'll leave it connected.