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Handeling
Suspenstion
I think everybody upgrading suspension should have an understanding of exactly what their products will do for them. Reading these could help you decide what you want.
Two questions...
what is the car used for? Daily driver? Weekend car? Track car?
How much are you willing to spend?
I couldn't see anybody using 400-500+lb springs on a daily driven car...so assumeing its daily driven, I would stay under 400lbs for springs and remember to get the rear a bit softer (less weight to hold up...possably other reasons as well)
Koni's seem very hard to install if you plan on doing it yourself, and I've heard some shops don't even want to mess with it since they are just shock inserts. Also, you have to take them off to adjust them. I don't know about you but once I had mine on there I'd be to lazy to ever adjust them if I had to take them off to do it.
I'm not sure if they make them for the s13 but GAB super HP are supposed to be THE best shocks around, easily adjustable 8way front 4 way rear, but will set you back ~$700...you get what you pay for.
So if you got those shocks with the GC coilover's (~$400) you could have bought a tein coilover system with height adjustment like the GC coilovers and 16way dampening adjustment. If thats too much $$$ then most people who have KYB AGX shocks are happy with them.
Don't forget anti-roll bars either!