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Just bought my 135i(2009) ~ 1 month ago. Current mods are: AST 4100 coilovers (550 /784 hypercoil springs), cobb stage 1 linear, vorshlag camber plates(set at ~ 1.85), hankook v12's(came with the car used)
Got a chance to take it out to auto - x with speed ventures. Had a blast learned alot about how to drive the 135i throughout the day. I believe I did pretty decent compared to the rest of the "competition" there. Placed myself about where I would expect(slower then the evo 9's, miata's etc, but fast then the mustangs, 335i's, prelude's etc.)
Impressions: I definitely felt the understeer, especially on the sharp S's. Overall felt good. This was however a LOW speed course were it was very difficult to "put the power down" at any spot on the course(yes yes I know its an autocross course). I think the fastest you could get on the course ~ 45 mph maybe 50.
As for the AST 4100, I think they were great....eliminated most body roll and the ride quality is great (even on the crazy pot holes over the 10 freeway). In fact I think the ride quality and comfort is on par with stock M suspension if placed on the softer dampening settings. For the auto x I was running stiffest in the rear and 3 clicks from stiffest in the front. Anyways had a blast, I think my camber in the front and better tires next time running will help my times alot(Rs3's!, ~-3 degrees camber)