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I had Koni FSDs installed on my e92 328 (ZSP) about 600 miles ago, and have a mixed reaction to them. My intent for this upgrade was to correct some instability in the rear end over uneven surfaces at speed, and when making very quick and hard left/right (right/left) maneuvers. My impressions of these dampers with the OEM springs and OEM 18" RFTs follow.
Ride quality: For everyday driving, the FSDs are significantly plusher. They do a much better job of not transmitting road irregularities. The downside is degraded awareness of the subtleties of the road surface, which is not an issue for normal driving, but can be for spirited driving.
Handling: The overall handling characteristics are better, but are still compromised. The problem with hard and quick transitions mentioned above has gone away. The problem with bumps while turning at speed is better, but is still an issue. Going over an irregularity at speed while moving the steering wheel induces a brief wallow in the rear end. This appears to be caused by insufficient rebound damping at the rear. Similarly, when going straight over a large dip, it takes the rear a bit too long to completely damp out, although this is not degraded from stock. Also, I have noticed something new that may have been there with the stock setup and didn't detect. At lower speeds turning over a bump, the rear wheel tends to skip briefly. On the improvement side, turn-in is better. Also it is easier to approach the limits on a moderate speed curve (think of a 270 degree exit ramp). Once, I looked down at the speedometer on a known curve and was surprised to find I was going 10 mph faster than I thought.
So overall for my personnel preferences, this is an upgrade, but I am disappointed at the insufficient damping in certain aggressive situations. They are also a bit too plush for my tastes. I was hoping the dual frequency aspects of the design would maintain a decent ride quality, but would also be more "race-like" for aggressive driving. In my opinion, Koni biased the tuning more towards ride quality than I was expecting. There may be another factor in this though. The dampers were apparently designed/tuned prior to the e92 introduction. My understanding is the e92 had its suspension adjusted to enhance performance with RFTs. If true, the FSDs could be optimized for other 3 series cars, just not the e92.
My personnel bias for perfect handling is reflected in this write-up. I am happy with the FSDs as nothing is worse and there are improvements, although they are not perfect for my tastes. Maybe nothing is. I am going to give them a bit more time to decide if the perceived shortcomings warrant a different set of dampers. Based on what I have seen in the forums, Koni FSDs would likely be an excellent upgrade for most users.
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