Project NSX: Part 2 – Tuning a Limited Slip Differential

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We WPC treated our differential’s clutch discs which gave them this lustrous silver color.  WPC treatment eliminates the rough ratcheting that LSD diffs have in low speed cornering and greatly increase clutch disc life.  The smoothness also helps improve traction on corner exits.

Finally the OS Giken Super Lock diff has a unique feature. In the pressure ring there are negative springs, that push the half’s of the pressure ring together, counteracting the wedging action of the cross shaft cams. These springs control the slope of the lock up curve or how quickly and smoothly the diff responds to the torque wedging of the cross shaft cams.

Road race cars with a lot of power need the differentials lock up forces to come in more gradually and smoothly, this reduces tire shock, helping the tires find traction on the exit of the corners better rather than snapping sideways. Stiff springs will work best here.

Drag and drift cars might want a steeper slope and a more aggressive lock curve to lock the rear end together immediately. If this is the case choose a soft spring. OS Giken has spring rates of 18kg, 28kg and 38 kg available. We used the stiffest 38kg springs for the smoothest and most gradual action.

Billy removes a pressure ring spring retainer, replaces the spring, putting loctite blue on the retainers threads and reinstalls the assembly.  This is repeated until all of the spring are changed.

We feel that these settings are the best for a car that sees both street and track duty. For a track only car with huge sticky tires and aero downforce we would probably set up the diff with more locking ability by not deactivating any clutch plates. For a pure street car, with more pedestrian rubber and less power, we would deactivate more clutch plates.

Billy places a cone spring into the differential case.  On OEM type limited slips without optional cone springs, you can shim the stock cone spring tighter by placing a shim made of shim stock under the cone spring.  Typically you can make a cone spring shim from 10-20 thousandths of an inch thick to improve the differentials clamping action, the thicker the shim, the more clamp.  If we were going to tune the stock diff, this is how we would have done it.