Quote:
Originally Posted by Olgeezer1
Either Koni or Bilstein will be an improvement. 1 series shocks/ struts, being BMW'S cheapest car, did not get their best components. All factory BMW springs work well with what used to be called H.D's (B6 ). If you lower more than 25mm. (1"), which Performance springs don't, then you want the B8's. Been on B6 with stock Sport Package springs for years, no issues, work great.
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IIRC states B8s are for "lowering springs" since the length of the damper is shorter. Jury's been out for years on Bilstein defines as a lowering spring and when asked directly they've given conflicting answers. The fact that there are sport and non-sport springs only adds to the shitshow.
Eibach springs list a drop of 1.2" (from non-sport springs), which ends up being less than 1" (and varies based on the spec of the car) if swapping from sport springs (which all 135i came with AFAIK), yet Bilstein still went with the B8s for the B12 kit. So if we apply the >1" lowering rule we should use the drop from non-sport ride height? Or are the B8s appropriate for a <1" drop like the other half of Bilstein marketing material claims? No one, including Bilstein, knows apparently. Same applies for BMWP springs, which claim a (significantly more consistent) 25mm/10mm drop for non-sport/sport respectively.
I dug through years worth of threads full of bickering over this topic when I was looking for dampers to go with my BMWP springs. There's no clear answer and I found plenty of people running both B6s and B8s w/ BMWP springs. I went with B8s because I found more reports of people happily running B8s, and at the time (mid-2020) it was damn near impossible to find B6s or B8s outside of the B12 kit. When I could find either option they ended up being about the same cost as the B12 kit, so I figured I might as well get some free springs to resell.
@OP, I wouldn't run stock M-sport dampers with BMWP springs as they'd likely be too compliant with the stiffer spring. For the Bilstein options, I don't think you can go wrong with B6s or B8s.