Mostly, the rules by which each are built to comply. Like many past Porsches, the GT3 was introduced on order to homologate a version for racing (originally the GT3R).
As time has gone by, rules have changed to where we now have two different versions of the street car, the GT3 and the GT3RS. The first is used as the base version of the GT3 Cup while the later is the base for the GT3RSR.
The Cup car is very close to the road car, with all of the needed safety equipment and tuning differences you'd expect in a competition car. The GT3RSR is quite different, with a much more powerful engine (quite a bit more, even with the restrictor required for it's class), different suspension geometry, etc... but also less power assistance for braking and other differences that are all related to the rules for GT based cars at Le Mans (and it's related series like ALMS).
The Cup car was aimed at wider range of classes, with the GT3 Cup S being a recent edition to allow for teams to take advantage of a more powerful engine in those places where the rules allow it. Other than the engine, the GT3 Cup S is nearly identical to the GT3 Cup.
In terms of speed... around Laguna Seca for example, A GT3RSR is capable of a time around 1:23. A GT3 Cup could navigate around the track in about 1:32. The GT3 Cup S would be a second or a little more quicker than the Cup car.
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Mostly, the rules by which each are built to comply. Like many past Porsches, the GT3 was introduced on order to homologate a version for racing (originally the GT3R).
As time has gone by, rules have changed to where we now have two different versions of the street car, the GT3 and the GT3RS. The first is used as the base version of the GT3 Cup while the later is the base for the GT3RSR.
The Cup car is very close to the road car, with all of the needed safety equipment and tuning differences you'd expect in a competition car. The GT3RSR is quite different, with a much more powerful engine (quite a bit more, even with the restrictor required for it's class), different suspension geometry, etc... but also less power assistance for braking and other differences that are all related to the rules for GT based cars at Le Mans (and it's related series like ALMS).
The Cup car was aimed at wider range of classes, with the GT3 Cup S being a recent edition to allow for teams to take advantage of a more powerful engine in those places where the rules allow it. Other than the engine, the GT3 Cup S is nearly identical to the GT3 Cup.
In terms of speed... around Laguna Seca for example, A GT3RSR is capable of a time around 1:23. A GT3 Cup could navigate around the track in about 1:32. The GT3 Cup S would be a second or a little more quicker than the Cup car.
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Engine power if I am not mistaken.
one has an 's' on the end.