Limited Slip Differential Installation

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LSD: a mechanical gear device which used to ensure that some power is always applied to each of the wheels, even when one is on a slippery surface. Power is transfer from the slipping wheel to the non-slipping one. This is done extremely fast. Lexus has electronic slip control (slow, inefficiently, rob all of your power)

 

(Lexus has NO mechanical slip control, the factory open differential gear ratio for GS4xx is 3.26:1)

- RMM or Supra TT Auto LSD: (both are the same, made by Toyota) Gear 3.76:1, Torsen LSD type (use Automatic SupraTT only)
Toyota p/n 00647-41301-203)

ie 3.76:1 meaning: drive shaft spin 3.76 turns to turn the wheels 1 loop

- TRD LSD (standard type LSD in that it uses clutch packs)

Here are the reasons why I chose the Supra TT 3.76 gear LSD over TRD

 

RMM or SupraTT LSD 3.76

TRD LSD 3.26

Cost

-Cost: $1300(RMM), ($800) SupraTT LSD or used one ($300-700)
Paid: $500 shipped, from 98 supraTT

-$800+ (list for $1100)

 

Installation

-direct bolt on unit, can be done at home, ~$200 to get a shop to install it (~2hrs)

-requires remove the rear end, disassemble it, separate the ring gear from the stock LSD, re-attach the ring gear to the TRD unit. Professional install only (6hrs cost) ~$400

Picture

Description

(from various supra technical articles and user feedback)

Torsen is a Torque Multiplying type of diff with a multiplication ratio of around 5:1. What this means is that whatever torque can be sent to the wheel with the least traction, it can send up to 5 times that torque to the wheel with the most traction. Its biggest advantage is that it can do this extremely quickly and doesn't ever lockup the ratio; continuously "sensing" and adjusting the torque between the two wheels. Its only real disadvantage is that when one wheel has no torque available (say it spinning in mud), then 5 times nothing is still no torque to the other wheel, so you can get stuck just like a std diff. Once the car is moving however this cannot happen. Get larger wheel in the back to guarantee some traction to at least 1 wheel at all time.

The TRD LSD is a standard type LSD in that it uses clutch packs to transfer torque from one wheel to the other. Transferring torque via clutch packs is a slower process than the Torsen transfer method and it will absorb more energy from the engine when doing so. However we are probably only talking a few hundreds of a second difference here and maybe a few hp. The main disadvantage of the TRD diff is that it can "lockup" the ratio between the two wheels at 1:1. This means both wheels will then turn at the same speed and if this happens on a corner, it would be exciting at the very least. This "lockup" is also its only advantage over the Torsen diff as even when one wheel is in the mud, the other wheel can provide traction to pull you out.

Require no special gear oil, any standard/synthetic 75W90. In theory, it will last "forever" which is truth for most supraTT

Require special gear oil with added friction modifier to prevent accidental lockup. Recommended 85W140. Clutch packs will wear out depend on your usage and will need rebuilt

Application

Stock or light modified car. seems to work fine on SupraTT auto, with upgrade HP. Highly recommended for street performance car.

Recommend for high HP application, rally driving or Drag racing.

Gear ratio

-3:76:1 (higher gear) gives at least 15% more torque
-Highway cruising rpm is a little higher
(cost a little more gas (~1 mpg) (number from my car))
-May hit the rev limit due to the extra rpm when you have stock 16,17 wheels and stomp on the gas pedal. The engine RPM rise up a lot quicker than the stock engine computer is expecting (I don't see this problem with my car)
-If your car is N/A, this behavior is exactly what you want (more torque + slip control). Not recommend for force induction or NOS powered vehicles
-"Like" getting a limited slip differential with a free SRT intake (due to the higher gear ratio, the torque increase can be felt in all the gears)

- 3.26:1 (stock replacement)

-Same as stock

 

 

 

-Only provide slip control, nothing else
-No change in torque or RPM speed

Special tool needed: 10,12 mm hex wrench, 12mm socket, transmission lift or any standard hydraulic lift, some thread-locked compound for the bolts during re-installation.
Notes: The LSD weight 100lbs. Don't be under it. Have a stand-by helper at the whole time.

 

Time: 4 hrs, with 2 helpers

Bolt on job.

See diagram and pictures. Basic steps:

Step 1: Lift the car rear ends up on the ram or something secure (I used 2 hydraulic jacks), remove exhausts and the exhaust pipe
Step 2: Remove drive shafts(2 sides, 1 main shaft in front) (10 mm hex bolts for the sides x 6 bolts, 3 14mm?? bolts for the main drive shaft)
    - Hints: use the parking brake to lock the rear wheels so you can remove bolts holding the drive shaft to the wheel. Put the gear to Neutral to turn the main drive shaft to access any hidden bolt, put it back to Park to lock the drive shaft in place, to help remove the bolts.

Step 3: Put another hydraulic jack under the diff, Remove 5 bolts that hold the LSD unit (2 18mm bolts front,3 12mm hex bolts back) see diagram
Step 4: Reinstall the new LSD in the reverse order, the bolts torque specs is shown in the diagram
It's a bit hard to put the new diff in, since it's heavy, get someone to help you out.

I also re-sealed the valve cover of the differential, since the diff is used. I don't know if it's leaking or not. Get the Toyota Sealant gasket gel (cost $16 for a small tube). open 8 bolts, torque set 34lbs/in to reinstall.

Used 1.5 qt Amsoil 75W-90 differential gear lube.

 

Update: I just found out that the diff I have does not have the torsen or TRD LSD gear, only the 94-95 Supra TT auto has them.

The 97+ diff does NOT come with LSD from factory. Trying to contact my seller now Ben Phillips and Jarret at Powerhouseracing.com to see if they can give me discount  for this unit or have another unit for me to exchange.



Special thanks to Dale (CL) for giving me some pointers on his LSD install.

Diagrams are taken and edited from www.mkiv.com, figure for illustration only.

 

Picture of the Supra differential (Auto) 3.76 gearing

See update, I found out that this is not the torsen or TRD  LSD gear, bummer =(