[jluetjen]

"Coil Overs" are coil springs mounted over (actually around) the shocks. Most cars on the road today use a version of coil-overs with their strut suspensions. For years coil-overs were the norm for formula cars until the advent of inboard suspensions, etc. Note that coil-overs are not legal in NASCAR so they still have the their springs mounted separately from the springs. In general the big benefit of coil-over springs is the reduced space requirement since they are mounted concentrically.

In the case of 911's, most were not designed for coil-springs, but rather used torsen bars (T-bars). For extreme track use (read big sticky tires and lots of HP) you just can't get T-bars strong enough in the required size to hold the car up in the corners. As a result Porsche started to mount coils over their shock around the time of the Carrera RSR. By the time they got to the 935, they dispensed with the T-Bars all together. Newer 911's use coil-overs for the same reasons as other cars. You can do a search on this BBS for coil-overs or T-bars and read considerable discussion regarding the merits of each style of suspension.