What is the difference between a helical LSD and Torsen® LSD? Torsen® is a brand name for a helical gear type limited slip diff produced by JTEKT corporation (formerly Gleason and Zexel). Other similar gear type limited slips are produced by Eaton, Quaife, GKN, and some other companies. Although there are some variations in the gear tooth profiles used, they are some form of helical or worm type gear profile and all function in a very similar manner. Helical/Torsen® LSD's are torque sensitive limited slips with a torque biasing ratio. This is the result of the friction generated by the thrust forces on the gears against the housing and within the gear mesh between the pinion and side gears. The torque directed to the high traction wheel is equal to the torque available at the low traction wheel multiplied by the torque bias ratio which is generally in the 2 to 3 range depending on the application.

Subaru uses a Torsen® T-2 type LSD in the rear of the Impreza STI starting in 2007 as well as the rear of the Legacy Spec B (6MT version). The front helical LSD's used in both 5MT and 6MT models are, as far we can tell, GKN models.

Is this a 1-way, 1.5-way, or 2-way? These gear type LSD's are usually designed to work in both drive and coast directions equally and therefore could be considered a 2-way LSD although this type of terminology is usually associated with clutch type differentials to describe the ramp angles in drive and coast directions.

Do these LSD's require special fluids? The LSD manufacturers recommend a GL-4/GL-5 gear oil of high quality. However, there are no specific friction modifier requirements for smooth operation like there normally is with a clutch type LSD. Any compatible gear oil for the Subaru will be fine.

How do gear type LSD's compare to clutch type LSD's? The gear type LSD is a maintenance free item that will maintain a consistent torque bias ratio for the life of the unit. However, because the bias ratio is a multiple of the torque available at the low traction wheel, if the wheel is off the ground then no torque can be biased to the high traction wheel resulting in a total loss of forward traction from that end of the car. A clutch type LSD will lose some of its torque bias ratio as the clutch plates wear, however, it can be tuned to provide a very aggressive lock ratio. Its initial torque also allows it to transfer torque even with one wheel completely off the ground. Gear type LSD's are relatively noise free whereas friction plate chatter is normal with a clutch type LSD.

Can a gear type LSD be set up with an initial torque? Special belleville friction washers can be used to preload the pinion gears on some motorsport specific LSD's such as Quaife ATB but this is not an option for these factory LSD's.