Project NSX: Part 2 – Tuning a Limited Slip Differential

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The cross shafts for the spider gears on the OS Giken diff end with these cams in the carefully shaped holes in the pressure ring.  When torque is applied to the diff carrier the cams force the half’s of the pressure ring apart putting pressure on the LSD’s clutch plates increasing the locking force.

A Salisbury LSD allows good differential action (different wheel speeds) when there is minimal torque load; like when in the middle of the corner with a little throttle. As the throttle is applied more aggressively, the locking action is increased in proportion to the amount of torque applied to the ring gear. Thus there is more clamp load applied to the clutches locking the differential together under heavy acceleration and less tendency to spin the unloaded wheel with lower traction.

Tuning of a limited slip diff is little known subject and is critical for handling. The difference between a properly tuned differential over one off of the shelf can be several seconds a lap. We selected the OS Giken Super Lock differential for Project NSX because it is one of the most tunable differentials on the market and we are going to exploit its tuning capabilities to the maximum.

The first setting that we needed to select was the differential configuration. The OS Giken differential can be ordered in a 1-way, 1.5-way, or 2-way configurations. A 1-way differential’s cross shaft cam and the ‘ramps’ it sits in is angled to increase the clamp load on acceleration force while hitting a perpendicular wall under deceleration and thus not adding additional locking force under deceleration. A 2-way has identical ramp angles under both acceleration and deceleration which applies load equally when the torque is applied by the throttle and when engine braking applies load on deceleration. Finally, a 1.5-way applies more clamp load on acceleration than under deceleration.

The shape of the cam and the slot determines if the diff is a 1 way, a 1.5 way or a 2 way.  On our 1.5 way OS Giken diff the drive side of the slot has a more gradual ramp giving the diff more locking force in that direction than the flatter deceleration side of the slot.

On this 1 way Nismo diff on a FWD car, you can see the deceleration ramp of the pressure ring is flat providing almost no wedging force.

All of these configurations will significantly change the way the car feels. The 1-way LSD provides all the benefits from its locking ability on throttle while acting more like a stock open differential or Quaife/Torsen differential off throttle. 1-way LSDs are commonly used on FWD cars or on the front wheels of an AWD car. In this respect it causes the least amount of lock under deceleration and off throttle turn-in and entry into corners. When equipped to a RWD car it also has less understeer off throttle with more rear grip and stability.

The explanation of power flow through the diff is easier when looking at this picture of half of the diffs internals. The engine’s torque spins the pinion gear which in turn spins the ring gear bolted to the differential case.  Slots in the ID of the case drives tangs on the pressure ring and the driven clutch disc’s. The pressure ring spins the spider gears through their cross shafts.  The spider gears are meshed to the bevel gears which are connected to the rear wheel axles through splines on their ID.  Teeth on the OD of the bevel gears are engaged to the driven clutch disc’s in-between each each drive disc. When the bevel gears are spinning at different speeds as when the car is going around a turn, when power is applied,  the load on the cross shaft wedges the halves of the pressure ring apart, applying pressure to the clutch disc’s and causing resistance to the wheels turning independently.  Whew!

A 2-way differential will lock equally under acceleration or deceleration. This makes the car drive more like a fully locked “spool” or go-kart rear end. A lot of rally racers and drifters prefer this sort of feel because the wheels don’t want to travel at different speeds and will either cause the car to go straight or break traction and slide easier and longer. 2-way LSDs often have too much deceleration lock for road racing/grip driving.

Case to ring gear bolts have been known to come off and when they do, the results are catastrophic.  Thus careful case prep is needed to ensure proper tightening of the bolts.  Here we deburr and chamfer the holes for the case bolts in the diff.  You can see in the picture the light chamfering that is good.

Here Steve Mitchell chases the threads in the case with a tap to ensure clean threads for accurate torque readings.