I'm re-posting this because my impressions have changed a great deal in 3 months.
2009 128 Sport Package 58,000 miles (3 months ago)
The car had become really loose so I did the following:
- M3 rear subframe bushings
- M3 front sway bar
- M3 front upper and lower control arms
- Koni FSD shocks
- 4-wheel alignment
Total cost (parts and indie shop labor) = $3,300
As I had expected, things tightened up quite a bit and the instability I had on the highway is much diminished. Nevertheless, the rear is still a little soft so the car rocks from side to side in tight turns and on bumpy roads. I also have some additional NVH from the M3 rear subframe bushings.
Initially, I felt that the end result was a car that feels over-sprung (over-bushinged really) and under-damped. The FSDs handle large bumps and aggressive turns well but the smaller stuff doesn't get damped as well as I would like.
Other posts dealing with progressive vs linear springs lead me to think that the rear springs allow too much initial motion and the FSDs (by design) have too much initial softness to counteract the springs' weakness. Perhaps new linear rear springs (2011+ E82 OEM) and rear guide rods (not M3)?
Now that I have ~5,000 miles on the new suspension, the harshness is completely gone. In the first few thousand miles after the installation, sharp road impacts would slam through the structure and actually rattle the dashboard. After this break-in period for the bushings, the car rides as comfortably as the day it was new but is better controlled.
Just need a little more work at the rear and I think I'll have this perfected, although I still wonder if I spent more money than necessary. Would I have achieved 80% of the improvement at 30% of the cost by just replacing the worn OEM shocks with Koni yellows? Oh well, live and learn!
Happy New Year to all.