Front Tension Arms
One of the upgrades that is most talked about for the E8X/E9X is the OEM M front control arms - the tension arms and lower wishbones. Unfortunately, in the class I am competing in, the longer M wishbones are not a legal upgrade, but the tension arm is, so I decided to do the upgrade.
Top is a 67k mile stock tension arm shared by any non-M E8X/E9X. The bottom is a TRW 1M/M3 tension arm. The arm and ball joints are identical (although the new ball joint is a nice thing to have, my old ones seem to move smoothly and show no signs of wear) but the reason these are an upgrade is the bushing. The top bushing is the standard soft-as-heck bushing that has considerable give in each direction, while the bottom is a much stiffer bushing providing for less deflection during cornering and braking.
The install is easy - this thread has some great images:
http://www.e90post.com/forums/showthread.php?t=778726
A few things that that thread does not do well though is understand how to remove the balljoint nut. What worked well for me was a 21mm on my impact gun to loosen it initially, then a 21mm wrench and a T40 socket on the end of the bolt. To reinstall just did the opposite substituting an impact gun for torque wrench.
Don't forget to not torque the bushing end until the car is at ride height - in my case we lowered the car (on its front wheels) onto ramps which allowed us to still get underneath and the access is actually quite easy at that point.
The first impressions were not hugely different, but the biggest reason I wanted to do this mod is to help the car not behave strangely under braking, so I look forward to testing it at autocross.
One other thing we checked was the the noise I was getting from my front swaybar. Despite less than 10k miles and no real winter driving, it turned out upon undoing the end links, that the swaybar was not only making tons of noise, but actually binding to the point that it was almost not moving. Some grease into the handy zerk fittings and cycling through its range of motion has it silent and now moves much easier - this was a huge difference on the drive home and makes me think that the car may have really been struggling. Well worth triple checking regularly even when it's not making noise.
-Mark