Have gone through the archives and have not found the OEM specs for the coils springs both front and rear. Not interested in any guesses just if you know where to find the original springs length and their rating. Prices from vendors vary greatly and they do not know the specs of the springs they are selling.
Can’t tell you OEM specs but I have new front and rear springs made by eurospares. I’ll be glad to measure length and diameter of the new springs versus the old ones if that will help
Do you want to specify what model you’re talking about? The XJ6, XJ12, XJ-S V12, and XJ-S 6-cyl may all have different spring specs. Sportspak models will also have different springs.
Immediate need is spring information for 1987 XJ-S Coupe V12. It would be great for all of us if there where an OEM list of springs rates for all the cars you mentioned. Not all springs are created equal so knowing what company manufactured them would be of interest.