Someone else before me also observed that the U.S. E90 Halogen has a 'hood' in front of the lowbeam H7 bulb while the european halogen does not.
See this euro pic
I know the US and Euro reflectors are probably also different, but that 'hood' must be a DOT thing. And since it is a DOT thing, it is most likely stupid...
Anyway, this mod will remove the 'hood'. It does not change the beam pattern or the light cut-off, and it may improve the light output slightly, since you'll remove an object in the light path, but I certainly cannot tell any difference.
So this mod really does not do anything, other than being fun to do.
WHAT YOU NEED:
T10 Torx bit
MAGNETIC screw driver (VERY IMPORTANT!)
Small flat-head screw-driver
0) Make sure your headlight bulb is cool. Also, make sure you want to do this. If any parts/screw or your tool fall into the headlight assembly or into the area between the front bumper and the wheel well, it will be very difficult or impossible to recover!!!
1) turn your front wheel all the way to the right to access the passenter-side headlight panel in the wheel well. Use the quarter to open the panel. This is very clearly stated in the manual so I won't elaborate.
Put the flashlight on the front wheel for better illumination.
2) Unclip the cover of lowbeam headlight, remove the power plug, and then unclip and remove the H7 bulb. Put the bulb on someplace clean and safe. Look at the base of the metal clip, you'll see something like this (this is driver-side):
3) Use the T-10 bit to loosen the two screws holding the base in place. BE CAREFUL not to drop the screw. You can either remove them by hand or if you trust your magnetic screw driver then pull them out very slowly.
4) With the screws removed, pull out the whole base and the 'hood' that is attached to it. Something like this:
The 'hood' is held to the base by three metal tabs. Use your flag-head screwdriver and needle-nose plier to open them, so you can remove it.
5) Now reverse the steps to put everything back. The base will now be a bit difficult to re-attach without the 'hood'.
6) Completed headlight
7) Don't forget your flashlight on the wheel! I did that and luckily, the only damage was several scratches on the flashlight
Start the car, make sure the light is on, then turn the front wheels all the way to the left and do the driver side.