The Honda VTM-4 system is quite good in it's intended operating environment - which is operation during winter weather. It uses electromagnetic clutches in the

The Honda VTM-4 system is quite good in it's intended operating environment - which is operation during winter weather. It uses electromagnetic clutches in the rear differential housing to direct torque to the rear wheels with left/right independence. This gives you the effect of a locking differential, up to the friction limit of the clutch material. There's no center differential in the system, and the front differential is open (but Honda includes a TCS, which is cheap because once you have the ABS sensors & pump, it's just software). Even during good weather on dry roads, the system has benefits by engaging the clutches when accelerating from a stop, so all four wheels are driven when you need to get going. The clutches are disengaged after about 20 mph so you get the fuel economy of a FWD vehicle.