A few more thoughts and details, in no particular order...
- Notice how in the fender side, the bottom edge of the flange lines up with the other fender screws. This makes it easy to position and to drill the hole through the fuzzy undertray and lends strength to the entire assembly.
- The natural (straight) position of the duct gets it pretty close to the fender liner hole - all I had to do is poke my fingers inside and bend it a little bit, then catch the bottom hole with the screw.
- The hose is pretty solid and stiff, rated to 500F if I remember correctly. This should help, since it touches the U-shaped end pieces of an oil cooler. Oil should not get hotter than 300F if everything works well. Just to be on the safe side, I wrapped those U shaped end pipes in thermal tape - so it doesn't touch the brake duct directly.
- Pic below shows the airflow - goes inwards and up a little and curves outwards towards the brake area. The path has been subject to much deliberation - initially I wanted to replicate the path that was posted on the initial brake DIY post, which would have had it exit still in the undertray, towards the interface with the fender liner. That would have made the installation simpler, since the tray would have been more or less self-contained - and should things not work as anticipated, I can always switch to that layout.
- Fender liner pieces are not expensive, around $50 per side - so I had no fear cutting it.
- So why go up and come back at an angle? Well, since I won't be installing backing plates (at least for now), I wanted the airflow to be directed towards the brake area as much as possible. Ideally, it would come perpendicular to the wheel near the suspension arms - but that's impossible, that area is full of important-looking bits and has no space to route a duct.
- By scientifically eyeballing the exit angles, I picked one that still flows towards the brake area, as high as practically possible (there are other things encroaching in the area, such as the oil cooler end tubes). But at the same time I decided to anchor the bottom edge of the flange in a way that would be easily reproducible and not make installation a total nightmare. End result, catching the duct behind the fender liner was not too tricky (but still took about 5 min per side).
- Besides cleaning up the excess goop in the fender liner area, I also need to trim the fender liner hole a little bit - it's not perfectly aligned to the flange on the other side and has a slightly smaller diameter.
- I have the templates for the front/rear ducts in case someone else is brave enough to try to duplicate the setup. I also have some silicone duct hose left, enough for another brake job in case someone is interested.
- It's easy to remove them - I can cut a flat cover piece out of fender liner material (ABS plastic) and screw/glue it in place on top of the openings. Or I can order the fender liner pieces (approx. $100) and even the undertray (approx. $175 if not mistaken).
- Those bowden cables are there, I think, to hold the undertray more rigidly in place at high speeds. Due to the curved nature of the undertray profile, I think at speeds it may bow outwards a little due to the pressure differential created by the airflow. Mine are currently disconnected, just as well as I don't regularly drive at max speed on the Autobahn. If I did, I would have found a way to keep the cables connected - perhaps to figure out a route that goes inside the V of the cables and comes out at the bottom of the undertray.
2009 E90 M3 ED
2014 X1 28i ED
Last edited by adc; 06-27-2010 at 02:12 PM.