Hi Fred
I am not sure if you say you want to change both damper and springs but will assume both and if wrong you can ignore the comments relating to the part you don't need!
If I remember correctly the factory standard fast road spring rates were 250 lbs front and 325 rear. The standard dampers were the gas Spax you seem to already have.
To upgrade the dampers the factpry used to recommend the Trax Spax damper. I had these for a while but was dissatisfied with them as they always seemd to get problems requiring a rebuild. I do not beleive I was alone with suffering from problems. If you go to Trax Spax you need to ensure you get the correct rating as they have three different types for differing spring rates. You need the heavy duty ones which cope with 500lbs springs. However I wouldn't use them on my car again. Other people may have had better experiences.
I upgraded my spring rates to a very stiff 450 lbs rear and 500lbs front. This is too stiff for road use but feels ok on track. I am thinking of changing this set up. A popular choice at present on other cars is to use variable rate springs made by Eibach (or others) with a rising rate. Starts off low say 300lbs and then rises to 800lbs or something similar. I do not know the full spec. Not cheap compared to a constant rate spring though. This is the route that appeals to me.
On dampers I use Koni twin adjustable dampers. These are more expensive than Trax Spax at about £1600 a set but I have not had any problems with them and once set, they have proved faultless. Just expensive to get initially. The Koni Technical Centre in Mytchett, Surrey, UK measured my car and supplied a group of us a few years back. The dampers are available under job number D100 if you want them. I assume that is still the case.
However if you are playing around with this area of your car you need to take into account your anti-roll bar situation. My springs rates are high but I have soft (ish) anti-roll bars. This is not ideal and can be improved upon.
I think that to get better handling out of a GTD you need to go as stiff as you can find comfortable on springs for your main use of the car be it on road or otherwise, and then to reduce the roll of the car with better stronger anti roll bars. Better dampers can cope with the extra strain and if multi adjustable can give more room for personal set up.
Everyone I know has their own opinion on setup and each has their own budget levels. To maximise your car you need to put time in and play around with the options. Easier said than done.
Malcolm
PS I have kept off corner weights and ride heights but these too all affect the cars handling. All compromise really!
