I have a 98 S90, and I've always been curious about the sport mode. What exactly does it do? does it actually add power of any kind? I've played around with the winter mode while driving on the beach and it takes off VERY slow, its great. But any input? I'm really curious.

 

Comparatively speaking, sport mode holds off shifting until the higher rpms. If you've ever seen a powercurve (think of a dyno sheet) you will remember that an engine produces more power with higher rpm. By doing so, it helps out with accelerating and passing. You might also notice firmer shifts while in sport mode. By contrast economy mode is opposite. Winter mode starts the transmission out in 2nd (or 3rd?) which leads to less torque/wheelspin and slower acceleration off the line. That's the quick and dirty.