Hey Guys,
Been a while since I have checked in. This is one of my favourite subjects, so I'll help out.
I performed a KYB adjustable shock conversion on my ST165. Used front struts cartridges in all four corners. I got help selecting springs from guys in the local rally club who built their cars about the same time as me. I believe my springs ate 350 lb/in X 12" front, 225 lb/in X 10" rear.
Now, back in the old GT4 Onelist days, David C. posted an email that he got from Bruce Neale at TTE, the post was dated March 1999:
"Our spring and damper specs for ST165GroupA are probably not relevent as we used very progressive springs at that time and the damper rates relate only to the springs we used. I can give you the base spring and damper settings for ST185, which used linear springs. These are relevent to our rally cars weighing around 1200 KG, but would not be suitable for road use, as they would be too hard:
Gravel front: 75Nm spring, 400-220 damper
Gravel rear: 40Nm spring, 260-240 damper
Tarmac front: 80Nm spring, 430-200 damper
Tarmac rear: 75 Nm spring, 280-220 damper."
I believe that the above units are in fact supposed to be N/mm, and the above email has typo. This would put a gravel set at 425lb/in, 225lb/in. This is extremely close to my set up (350/225 from memory). My springs are stiff, even though my car is likely around 3000 lbs (stripped with roll cage). Keep in mind, my springs are already 2-3 times stock rates. If you are building a true stage car with anticipated air time, you would want to kick it up a notch. Dampers play a huge roll in rally, and my KYB shocks would have nothing on what real rally teams use.
There is some leeway here. I changed the fronts from 250 to 350, and did not notice much difference. Unless you are doing severe racing, and have access to lab type testing and datalogging, once you get into these ranges, you will not notice much difference in spring changes. I would budget on changing one set, either front or rear, after the fact to try and fit your driving needs. IE, too much understeer-maybe soften front or stiffen back...too much over steer-maybe soften rear or stiffen front. It is really hard to gauge this without access to legal test area (race track, parking lot, etc).
I noticed Bilstein has started selling dampers ready for customization. You would need to weld on tabs and custom machine the tops. The info is all on line.
I have a pair of used Eibach 12"L, 2.5ID, 250lb/in springs. If someone wants them, make me an offer. I am in Edmonton. I can get them to Vancouver as well.
Good Luck.
Marc