Hi,
this is my experience with the +2 & -2 roll centre block:
+2 advantages
- Better over bumps (less driveshaft plunge)
- Rear feels more locked in
+2 disadvantages
- Less steering
- increased chance of grip roll (the rear inside wheel tends to lift, causing a jacking effect on high grip tracks)
- Although the rear feels more locked in, when it does break traction it does so in an abrupt way
-2 advantages
- More steering
- Less grip roll (i always use -2 on carpet/astro tracks as it really helps reduce grip roll)
-2 disadvantages
- Worse over bumps (increased driveshaft plunge)
- Makes the rear feel looser on the rear (good if you like an aggressive car and faster if you can hang onto it!)
Nathan tends to use +2 at Worksop but only for the first few rounds of qualifying when the tyres are bedding in. Once the tyres have scrubbed in and gripping well, he will switch back to -2 in order to gain more steering.
Hope this helps.
Craig.