What Pete says is true but is also incomplete.
The car does ride and handle fine when lowered as long as the suspension geometry is properly changed to accommodate it .
The factory spec a-b is more a measure of available compression travel in the shock
a is the axle height
b is the t-bar height which is the truest measure of ride height
the center of the t-bar just behind the front wheel, down where your feet are, to the ground is b
here's a survey of 911 specs, 930s isn't listed but would be similar to a 911
a - b of 108mm has ~3.5" of compression travel+/-, as a gets bigger or b gets smaller you lose compression travel, the limit is when the shock rod is riding on the bump stops.
You can have a - b at an infinite # of different ride heights as measured by b alone.
another thing that happens at the same time is that the roll center of the car is lowered this lengthens the lever arm that cornering forces use apply to the car increasing body roll and lastly the relative height of the steering rack outer end( c) and inner end( d) changes, d - b is fixed unless the steering rack height is shimmed up. so as b goes down c - b goes up, this is a measure if the bump steer present in the suspension geometry.
the more the car is lowered from its stock height the less travel(a -b), lower roll center and the more bump(aka roll, c - b ) steer is present, at some point all will need to be compensated for.
to fix the travel and roll center issue there are multiple options, all or a combination may be used
1. cut the bump stop down
2. shorten the shock rod and wiper seal
3. raise the spindles
raising the spindles also raises the outer end of the steering arm, c, this makes roll steer(c - b) worse ,to compensate for roll steer issues lower the steering knuckle height c, some use and o/s steering knuckle but a better solution is aa double sheer cage that lowers the knuckle.
