All of the differentials are opn diffs. Some of the DSMs have a viscous lsd int he rear though (there will typically be a yellow sticker that says so on the drvivers side door jamb if you have it). The center diff is open, but has a viscous coupler on it. The front diff is open. What this means is power is split front to rear evenly until one set of wheels spin (its usally the fronts, since weight trasfers to the rear). When one set spins, it get the power. Once the center vicsous coupling (VC) heats up it locks up, and you get a 50/50 split. So its an open system until one end slips, then its split 50/50. There is no other "split" designed into the system. For example, some cars have the gear ratios for the front and rear slightly different to preload the center diff/coupler, but DSMs dont do this. Some people get the center diff or VC welded (or use a solid spool) to make the car 4x4. 50/50 all the time. While this helps traction, you'll need a stack of rear axles and transfer cases laying around to replace the ones you break
