Just finished the DIY last night, and wanted to share my thoughts.
I have been thinking about doing this for a while, but the catalyst for it, was the fact that I went on a cruise through the hills with a few friends, and couldn't hear the car, even with the windows down, and had to sometimes look at the tacho to get an idea of what was going on. That, and of course it's boring to not hear anything.
The background:
http://www.1addicts.com/forums/showthread.php?t=963343
TL;DR cut open standard 135i muffler, cover up the holes on the pipes in the muffler, in order to emulate a BMW-PE.
Now, what I found to be not very clear in the other thread, was that one pipe has two 'patches' of perforations, and the other pipe just has 3 holes.
We cut open the muffler from both sides, for ease of access.
We then proceeded to weld plates onto the large holes (top pipe, first image), then flipped it over and just tacked the perforations closed on the other pipe (second image). This proved to be quite time consuming, so we welded one big plate over the other 'patch' of perforations (bottom pipe, first image).
Here are some photos:
Now, I finished this late last night, so I have not heard a 'drive by', only from the driver's seat.
The car is standard mechanically, all it has is a JB4.
The sound after the mod, is a little louder, but it is noticeable - to me at least. I think that from the driver's seat, previously, without a confined space (like a tunnel or rows of shop fronts either side) the sound was dominated by induction noise, the kind of muffled rumble you get - not turbo induction.
Now the exhaust is at least audible, it seems to 'scream' now - in terms of pitch, not volume. The sound, I would compare like going from an Aussie V8 to an exotic V8 (Ferrari, Maserati etc.), but only to a very small extent. You will hear this 'scream' on a standard exhaust, in the right situation, such as an enclosed space, this just accentuates it a bit.
The car is not loud by any stretch. There is not really any noticeable difference while commuting, until you become at least a little generous with the throttle. Freeway at 100kph is unchanged from standard. Absolutely no drone anywhere, so far, in the rev range, although I'm yet to hit redline.
Overall, I would say, I was expecting for it to be louder, and hoping for it to be
a bit louder. I think my hope was that at town speeds it would be a bit louder (so I could actually hear what I had done to the car), but that would just be a novelty, and become intrusive eventually. So, given that I don't like loud cars, I would say, I am quite happy with the result.
That's pretty much all I can think of. Any questions, feel free. And if anyone wants to actually do this themselves, and has questions about details of how it's done or whatever, feel free to PM me if you don't get a response here, and I will help as best I can.
Cheers!