Ok, after dynoing my Nitrons and installing them, I dynoed my Bilstein Track Packs just for the heck of it since all I can gather were mostly anecdotes and no actual data.
I've had these 5 years, 25K miles of 60:40 street/track miles on them.
I've been told all along that it's rebound only adjustable, or on the rare occasions, that it's mostly rebound and some compression adjustable. Welp, I can safely say that it's COMPRESSION ADJUSTABLE.
The two stiffest settings slightly affect the rebound side, and the two softest settings don't do too much.
Overall build quality and consistency throughout the range is pretty good.
The hysteresis has similar quality to the Bilstein LSS dampers.
Something that isn't apparent on the graph, but according to the guys who dynoed my dampers is that these had fairly high gas pressure, 200~250psi.
Maybe someone more qualified can share their observation.
I also dynoed the springs because I couldn't find a straight answer.
I couldn't measure it the way I wanted to(1 inch preload, measure load every 1/4 or 1/2 inch and divide by displacement to get lbs/in at displacement), but this is what I got.
Ideally, it would've been nice to utilize the damper dyno machine equipped with spring tester to get more precise measurement, but I didn't have access to such setup.
What I have so far is:
FRONT(1 inch preload): 195lbs/in at 2", 200lbs/in at 3", at 4" I'm at full bind and hitting 300lbs/in so it would be somewhere under there, but you'd be hitting the bump stop around then.
REAR(1 inch preload): 270lbs/in at 2", 300lbs/in at 3".
I don't trust the numbers too much, because the the quality of the measurement wasn't as consistent as I like on repeated tests.
However, the numbers is along the lines with the data floating around that the spring rates are either 271/371(unknown progression), or 180~214lbs/in front 266~371lbs/in if you squint your eyes hard enough.
Or maybe I should have used Eibach's own measurement procedure since they are Eibach springs:
It's suggested by the spring dyno operator that these might actually be dual-rate springs since the coil spacing is consistent in the narrow and wide section, achieving similar effect of having tender springs. Progressively wound springs tend to have varying spacing and widths throughout the range.
I want to find better info on the springs some day.
The ID of the spring is 1.88", the spring rates are available up to 400lbs/in 600lbs/in front and rear so replacing them is an option as well as revalving.
On the side note, I've recently been experiencing some handling issues, which gave me more reason to replace my dampers. The car would gyrate up and down during hard right handed corners.
Dynoing my Bilsteins made the issue apparent, the nitrogen has bypassed the divider piston. It has been corrected since.