Suspension of AE86 "86"

[NoHachi]

I know for sure that Autosportparts in Delft kan order custom springs to your specifications. This is also possible via Chiel Bos in the eastern part of Holland, though he is expensive. In the end he just orders the springs form the merwede factory and they make a custom run. You could try it directly from the factory.
Remember one thing: Big numbers do not equal Big performance. 8kg/mm is right at the edge of the koni's capability. When they start to age the damping will go down, leaving you with no adjustability to cure the now underdamped state of the car. That sucks, especially for drifting (highsiders/tankslappers R us..) and is just dangerous on the track.
Why do you want such high rates? Do you plan to put racing slicks on the car? If you stay on street tires then I would not go any higher then 7kg/mm for the front. That rate is still more suited towards semi-slicks..6kg/mm is a nice rate for performance street tires. That is still a higher rate than most aftermarket springs. You can calculate the ideal rear rate from the front rate. 3kgR 6kgR is the combo to have for track racing. It will understeer some, but also put down a LOT of power to the track. Rotating the car must be done with masstransfer during corner entry (essentially a drifting technique, only the car is setup so that it still grips like a nutter). For more low speed or tail-out stuff bring up the rear springrate.
Remember: oversteer comes not from springrates but from the front to rear ratio in springrates. the standard setup is front biased. Al the high springrates will do for you is reduce grip if you dont drive on slicks. I'd much rather do this through a set of crappy tires and/or high tire pressures.