[jluetjen]

A couple of things to remember about springs (AKA Torsion bars) and sway-bars:

1) The job of springs is to hold the car up. If they are too soft the car will bottom out. If they are too stiff they will cause a really harsh ride. To stiff will also require a corresponding increase in stiffness for the shocks which can actually result in a reduction in traction if the result is the wheels coming off of the ground over rough surfaces. They can also be used to control roll, but doing so will often result in the springs being too stiff for the road surface.

2) Sway-bars (AKA anti-roll bars) affect the balance front/rear during steady state cornering. It is also important to remember that sway bars affect the handling of the opposite end of the car. So a rear bar will affect the handling of the front end and vice versa. Simply put, by putting on a stiffer rear roll bar you will transfer more weight across the back of the car during a corner then without. This will give the outside front tire some relief and hopefully allow it to provide more grip (thus reducing understeer). If you go "over the top" with the rear bar you can cause the outside rear tire to be overloaded which will cause oversteer and an overall reduction in cornering force.

Sway bars and/or springs can also be used as a crutch to cover up poor suspension design. For example, some suspension designs (such as McPhearson struts) work great in the center of their range but lose a lot of camber in compression, and body lean will make it even worse. Using stiff springs or shocks will prop the car up so that the suspension stays closer to it's "sweet spot".

Ideally you want a car's to suspension to be as soft as possible without it bottoming out. This should provide a pleasant ride and forgiving handling. Cars which are too stiff will often handle as if they are on a knife's edge. This sort of handling is difficult to manage for maximum speed and consistancy during a race.

That being said, the suspension design of the 911 is very good in general for most uses, especially road use. There shouldn't be a whole lot of reason to change it unless one of the parameters has changed. The reason why track and race cars have stiffer suspensions is because they are running stickier tires. As the grip of the tires goes up, the car will start to lean/dive/and squat out of the suspension's sweet spot. The fix to this problem is stiffer springs and swaybars. It's interesting to note that when it rains, race teams try to make the cars as soft as possible (softer springs, disconnecting sway bars) in order to maximize the car's use of the limited traction.

I may have been long winded, but I hope that it might clarify when you might want to change springs or bars.