There are currently three types of Torsen differentials.
1. The original Torsen T-1 (Type A) uses crossed axis helical gears to increase internal friction. The Type I can be designed for higher torque bias ratios than the Type II, but typically has higher backlash and the potential for Noise, Vibration, and Harshness (NVH) issues, and requires a precise setup/installation.
2. The later Torsen T-2 (Type B) uses a parallel gear arrangement to achieve a similar effect. There is also a specialist application of the T-2, known as the T-2R (RaceMaster).
3. The latest Torsen T-3 (Type C) is a planetary type differential, in that the intended torque split is not 50:50. The Type C is available as single or twin version, the Torsen twin C differential has front and center differential in the same unit.
The Torsen T-3 will be employed as the centre differential in all future non-Haldex Traction Audi models with quattro four-wheel drive, current applications: 2008 Audi S4 (B7), Audi RS4 (B7), and Audi Q7. Alfa Romeo uses Torsen C twin differential in Alfa Romeo 156 Crosswagon Q4 and then in 159, Brera and Spider Q4 models. Also Toyota uses a Torsen T-3 in the center differential of the Toyota 4Runner and Lexus GX470, with manual locking feature, and GM has a Torsen T-3 centre differential in the (non rear drive) Chevrolet TrailBlazer SS.
How they work
The Torsen differential works by supporting a torque imbalance, the maximum ratio of torque imbalance being defined by the Torque Bias Ratio (TBR). When a Torsen has a 3:1 TBR, that means that one side of the differential can handle up to 75% while the other side would have to only handle 25% of applied torque. During acceleration under asymmetric traction conditions, so long as the higher traction side can handle the higher percentage of applied torque, no relative wheelspin will occur. When the traction difference exceeds the TBR, the slower output side of the differential receives the tractive torque of the faster wheel multiplied by the TBR; any extra torque remaining from applied torque contributes to the angular acceleration of the faster output side of the differential.
NOTE: The TBR should not be confused with the uneven torque-split feature in the planetary-type Torsen III. The planetary gearset allows a Torsen III center differential to distribute torque unevenly between front and rear axles during normal (full traction) operation without inducing wind-up in the drivetrain. This feature is independent of the Torque Bias Ratio.