Running standard pro-tech factory suspension, Just measured up my rear springs to find that they are in fact 7.5" springs rather than 8" (which I thought was the standard MK length. As a result the spring runs out of movement before the limit of damper/wishbones movement. Effectively with the car rear jacked up i get about 10mm of vertical spring play. Is this a bad thing, should I upgrade the springs to a longer length to fix this? Thanks.

Ive just been researching new springs for my car and i never saw any listed at 7.5in, they went up in inch increments although i did see some 10.5 ones. Your springs sound too short, with the car jacked there should be still some pressure from the springs to keep the seat in place.

Have you not contacted MK surely they would know the exact springs you need?

How much preload is required on the spring unloaded is dictated by the stiffness of the spring, the weight of the car and your desired ride height. Nothing to do with spring length. If you fitted an 8" spring of the same rate, you would still have the same problem.

Have contacted MK but other than specifying a 300lb spring rate, they have been unresponsive to further questions. I measured the rear springs myself and they are definitely 7.5" snd 1.9" Id. The protech website does list a 7.5" 300lb spring, which I think must be what is fitted. If I kept the spring bed In the same place the 8 inch spring would take up the slack, albeit with a higher rear ride height I would have thought. Unless I am having a complete brain fart there doesn't appear to be a way to adjust the spring pre-load with the protech shocks, merely ride height via the bottom spring beds.

The goal of this all really is to fit some stiffer rear springs - currently the car during higher speed corners squats down onto the back too much under power, resulting in power understeer, so am looking at fitting either 325lb or 350lb springs. While other things can cause this, the car's body control is also fairly poor without maxing out the dampers. I I fit stiffer rear springs this will also go some way to tackling this, I hope in theory anyway!