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As mentioned in an earlier thread, I purchased a performance exhaust from United BMW of Gwinnett. Very clean deal, discounted to $735 with shipping included. Arrived yesterday (May 29th) as promised. What arrived is a replacement muffler and a really trick clamp. This installation requires cutting the original exhaust (take care). I believe the exhaust is the same for the 135 and 128.

The installation:
Allow about 1 to 1.5 hours depending on level of ability. I spent at least 1/2 hour measuring to be sure there were no errors. The good news is that this is very easy and the result is great. This exhaust does weigh less and the outlets are larger than stock so it looks really nice and noticably different when complete.

Jack the rear up for access. Spray the two rubber hangers at the rear of the original muffler with silicone spray to allow easy removal. Also spray the supports on the new muffler for easy installation. Wrap the exhaust tips with a towel, the reason will be explained later

Measure approximately 2 and 1/8 inches from the muffler housing by the exhaust pipe inlet along the exhaust pipe. Scribe a mark on the exhaust pipe, this is where you will cut. Again this is the distance from the muffler measured on the exhaust pipe. Wrap tape around the pipe to insure a cut that is perpendicular to the axis of symmetry. If there is some error the provided clamp with allow for this.

Place a support under the muffler but dont apply pressure. Cut the exhaust with a hack saw or sawzall. I used a hack saw to allow for a slow process. There is room for either a hack saw or sawzall. Be careful not to hit the plastic under covers. When the cut is complete the muffler will want to fall and pivot towards the rear of the car. This could make the exhaust tips scrape or crack the rear valence (plastic bumper). By supporting the muffler this wont happen. The towel will be extra insurance. Dont forget to remove the vacuum tube that connects to the outside of the original muffler. That will be closed off by a solid piece on the new muffler.

Take a large flat head screwdriver and begin pushing the rubber hangers off the stainless supports. When both are about to come off, remove the outside hanger first. This will allow the muffler to rotate away from the muffler tips and prevent damage to the valence. Pivot the muffler and remove the second hanger. Notice how heavy this muffler is. Compare to the new muffler and you will see the weight savings.

Using 200 grit sandpaper smooth the edges of the exhaust pipe where cut. Notice that you could very easily replace the original exhaust if you choose.

Take the included, very trick, clamp and slide onto the existing exhaust pipe. Push on as far as possible. This will be to the point where the pipe bends. It will not be flush. Take the new muffler and rotate into place. First slide the muffler inlet into the clamp then slide the rubber hangers onto the muffler supports. The order for the rubber hangers is not critical but be careful not to hit the valence. The new muffler comes with packing over the tips and I would leave that on until the installation is complete. Now push the clamp back towards the muffler. Dont push all the way to the muffler as this will be too much. You want to be where the cut is in the middle of the clamp.

Before completely tightening the clamp, check to see if the tips are level. You can modify their position by rotating the muffler then tighten the clamps. Dont forget to connect the vacuum line from the car to the stainless rod on the new muffler. This may stretch the vacuum tube but there is some slack available.

The sound and look is really an improvement. You will notice the sound at almost all rpm's but most noticible when under load. The engine seems to rev more easily. There are other posts that have wave files of the sound.

Good luck !