These clutch type limited slips from STI were developed in cooperation between Cusco and STI for rally use. They are homologated under VO for N14 and newer models but suitable for any application requiring similar performance characteristics. They feature a ramp option pressure ring set to allow the LSD to be set up for a wide range of surfaces and driving styles. The front LSD provides a 55 degree 1-way ramp, 45/20 degree 1.5-way ramp and a 60/45 degree 1.5-way ramp. The rear LSD provides 3 different 2-way ramps: 25/25 degrees, 45/45 degrees, and 65/65 degrees.
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Can you explain the difference between a 1-way, 1.5-way, and 2-way limited slip? This describes the drive and coast sides of the pressure ring ramps. The pressure ring compresses the LSD clutches when torque is applied. The percentage of applied torque that is directed toward the clutches is dependent on the ramp angle. A 1-way LSD has a ramp angle only on the drive side (acceleration) of the pressure ring. On the coast side (deceleration) the ramp angle is 0 degrees so no pressure is applied to the clutches. A 2-way LSD has equal ramp angles in the drive and coast directions and therefore will have significant diff locking pressure when decelerating. A 1.5-way LSD has unequal ramp angles whereby the coast side angle is steeper and provides reduced diff locking pressure when decelerating.
When should a 1-way, 1.5-way, and 2-way LSD be used? Driver preference is a big part of it. More diff locking pressure on deceleration helps with braking stability but negatively affects the handling balance at corner entry. When a 2-way LSD is installed at the front of the car there will be an undesirable understeer characteristic. For this reason a 2-way LSD is rarely used at the front of a car. Typically a 1.5-way LSD is used on loose surfaces and a 1-way LSD used on tarmac surfaces. In the rear diff a 2-way LSD can be used on a wide range of surfaces without severe handling issues. However, a 1.5-way LSD is tyipcally used on tarmac and a 1-way may be preferred in low speed courses with very tight tarmac corners (such as autocross and some types of gymkhana competition). The amount of initial (breakaway) torque may also influence the type of LSD that the driver prefers in certain locations.
What type of oil should be used with these LSD's? For optimum performance and reduced wear and noise these LSD's require LSD-specific oils. For rear diff applications we suggest using either Motul Gear FF Competition or Motul 90PA. For front diff applications Motul Gear FF Competition is the preferred oil but Motul Gear 300LS may also be used. Synchronized Subaru 5MT gearboxes are very sensitive to oil selection. There is a tradeoff in synchro performance with most oils that feature limited slip additives. If you are experiencing unsatisfactory synchro performance and longevity it may be necessary to run Motul Gear 300 (Non-LS version) and accept a small loss in LSD performance and additional noise.
Is there a break-in process with these LSD's? All clutch type limited slip diffs will function with better performance and less noise if they are properly broken in. The break-in process generally involves running in a constant low speed figure-8 pattern for approximately 30 minutes followed by changing the oil. This process smooths and burnishes the clutch plates and ensures even contact between the friction surfaces of the plates.
What does "N11", "N12", "N14", etc. mean? This is a Prodrive designation for their Group N specification STI vehicles. It is commonly used by Subaru Group N competitors as a reference to a specific homologation. N11 = MY2005 STI, N12 = MY2006 STI, N12b = MY2007 STI, N14 = MY2008 STI.
