I have Koni sports on my '89 with Eibach springs, and I'm looking to replace the Eibach springs with a GC kit (car is primarily an auto-x and track car, with some street driving).

I'm not looking to go with extreme spring rates...in fact I'm looking to use maybe a bit softer rates than most. I don't plan on lowering the car very much compared to stock, only about .75". I'll probably go with 7" front and 8" rear springs based on feedback I've seen in various places. Any thoughts on where I should be for rates? I'm considering a range between 275/175 on the low end and 350/225 on the high end. The car does not yet have an LSD, which is part of the reason I'm looking at slightly softer rear rates.

 

When looking at those soft rates you should make sure that they've got enough stroke for your needs and that they won't coil bind on you. This is a real danger in going with the really soft springs.

I'm running approximately 400/275 and the ride's not too bad and I don't think wheel spin will be too much of an issue with something like 350/225.

 

First thing you have to make sure, is that the shocks are strong enough for the springs

otherwise you are in for a bumpy ride. The car will bounce around like crazy

 

They should be good for springs up to about 450/350 or so, so that's not an issue here.

 

So would a 7" front be enough length with a 325? The standard ones are 375 IIRC, and they use 7", so I'm thinking I may need to go to an 8" spring for the front. An 8" in the rear should be OK with a 225 spring, which is only 50lb/in off of the stock GC rate. If not, I'd need to go up to a 10" spring since the 9" don't appear to come in rates that low.

I guess I need to put in a call to GC....

 

From what I gather with GC sleeves on a FC3S, even a 7" spring won't net you much if any drop without camber plates or racing beat lower strut top mounts.They are a pain since the spring is short causing it to flop around when the car is jacked up and they make noise over bumps ,uneven road .Adding tender/helper springs adds greatly to the cost as well as exasperates the lowering problem with stock type shocks.The sleeve collar sits at the worst spot causing reduced inner susp. to tire clearence.That's the problem with those things,by the time you add all things to make it even a mediocre setup, you may as well buy a true double adjustable coilover package.
Here's a possible suggestion. Since you already have koni inserts, there are places that will modify the stock front bottom strut tubes for a reasonable price (60-70 per tube IIRC) with a sleeve and then you can run a longer spring at a softer rate and avoid the possibility with coil bind and also have the sleeve collar lower so you can maximize inner tire/wheel clearence. You can run standard add-on sleeves in the rear with a decent spring length. You can probably run something like a 300/225 lb. combo which should give you a decent all around ride.
Good luck with GC staff, when I called I must have gotten the biggest ******* that works there on the phone.

 

The standard rate is 400/275. I'd go for at least an 8" spring in the 325 rate if that's the rate you want. I've read that the 7" springs put the car low even at their highest setting, and you say you want it at nearly stock height, so with a softer spring you'll be really low. 10" is too long in a 275 in the rear, it might be ok with the 225's, but 8" is probably about right in a 225.

Don't take our words for it though, talk to GC and see what they say about it, since they're the ones that do it all the time.