The dampers don't determine ride height, the springs do that. The front springs for the CDTI have a higher spring rate than the 1.8 to cope with the engine weight, I suspect the rears are also different in order to balance with the front. The dampers have different damping parameters to suit the spring rate and the mass being supported by the strut assembly, Using 1.8 dampers with cdti springs may lead to a very bouncy ride (i.e. underdamped). Whether or not that would still be better than the aging dampers in place?
