With a car as powerful as the citroen GT, you will be racing in the high end rpm ranges a lot. So always stage 3 is the better choice unless you're racing... cote de azur ?Stage 3 gives optimised performance when you are racing at high rpm's whereas stage 2 gives best performance in mid-ranges rpm's.
So you could buy both and change between each one depending on the characteristics of the track your going to race on.
Hope that helped?
Maybe it makes the torque/power curves smoother, maybe it makes more torque.
This is something the tuning shop needs come to think of it; it should display the torque graph so you can see what horsepower modification does what.
why is everyone talking turbo's???? he said engine upgrade.