So a little bit of an update and information dump.....
Firstly the Tein HA are gas pressurised according to the label on them!
Diagnosis, they are shot and will need a rebuild. Whilst it may be possible to rebuild them with new seals and re-pressurise them they will be hard at 10KG/mm 6KG/mm front rear and all internet reports suggest that rebound damping is way too high on Tein and most japanese brand coil-overs with mainly linear characteristics and not digressive as bump speed increases. What does this mean? typically that they will be crashy when travelling slow and hitting bumps with little damping. Fine for a mirror flat track but far from ideal for a pot-holed road. The shock dynos I have seen suggest that they are a little more digressive when set to hard but then are likely to be over-damped. They are single adjustable so my understanding is that only rebound is adjustable and will then be set to "too flippin hard" and risk of jacking down.
PS - it is suggested that the curves are upside down with bump and rebound reversed over typical shock dyno plots
Tein with what I believe is the range of lowering achievable for a given spring and the reccomended lowering
NOTE:- The Spring code is written on the spring and indicates rate and length so the SB100-01200 is 10 kg/mm and 200mm long.
Analysis of shock dynos suggest that many race (track) only orientated coil-overs are valved linearly. Offerings from Ohlins, Bilstein and Koni can provide the desirable digressive curve for some road/bumpy surface use. IMHO I am seeking more of a tarmac rally spec suspension than circuit racing. Some of the costlier versions offer high and low speed adjustment, most budget coil-overs don't and regularly only offer 1-way adjustment that is more of a gimmick and doesn't actually provide the shame shape curves (linear or digressive) throughout the damping adjustment due to the simplicity of the internal valve.
Valve A provides a linear curve. Valve B provides Digressive curves on both bump & rebound.
The more I am looking into this the more I am coming to the conclusion that budget coil-overs often contain inferior internal dampers than standard units!
This leaves me with 3 options at present as I see it...
1) convert my existing standard struts to height/preload adjustable by use of threaded collars and obtain adjustable Koni inserts that are known to fit and offer rebound adjustment but with digressive curves.
2) strip the teins, get them measured and use them as height/pre-load adjustable housings and seek out some adjustable inserts from Koni/Bilstein.
3) badger BC for more info on their inverted tube coilovers. (Easiest path but may put me in the same position I am in now)
At present the old Teins are being stripped of inserts and measured to determine which insert may fit.
Thoughts or comments welcome.