Differential FAQ
What diffs came in the R31?[edit]
The R31 Skyline and Pintara came with either a BW75 or BW78 differential depending on year and model. It is reckoned those cars that came with factory LSD used the BW78 diff, along with the last 1990 built Series 3 examples. The two diff types are ostensibly the same, except for the crown-wheel/pinion mesh geometry. This change requires modification of the BW78 pinion, to make it fit the BW75 housing, but it works fine. All other parts are interchangeable between the two types. The 4.11 ratio diff fitted to the Pintara is a BW75, the BW78 wasn't manufactured with this ratio option.
There is no visible way to determine a BW75 from a BW78 externally, and even gear set part numbers are common for both types. Another consequence of the mesh helix change, is a slight shift in effective drive ratio.
The Skyline had a 28-spline, 4-pinion center.
The Pintara had a 25-spline, 2-pinion center.
The Skyline center is naturally therefore stronger than the Pintara center.
2 pinion diff
4 pinion diff
What diff ratios are available?[edit]
The Skyline came from the factory with these 2 ratios depending on whether the car was manual or automatic:
- Manual = 3.70
- Automatic = 3.89
This is correct for the BW75 diff. The equivalent ratios for the BW78 are 3.73 and 3.91 respectively.
The only exception to this rule was the GTS2, which had a 3.89 ratio regardless of transmission type.
The Pintara came with a single ratio of 4.11 regardless of transmission type.
A wide variety of ratios were available for the BW78 diff. These include:
- 2.73 (Ford Falcon)
- 2.90 (Ford Falcon)
- 3.08 (Ford Falcon, Holden Commodore)
- 3.23 (Ford Falcon, Holden Commodore)
- 3.42 (Mitsubishi Sigma)
- 3.45 (Ford Falcon, Holden Commodore)
- 3.70 (Mitsubishi Sigma/Nissan Skyline R31 manual)
- 3.89 (Mitsubishi Sigma/Nissan Skyline R31 automatic)
- 4.11 (Nissan Pintara)
What diff centers will fit in the R31 housing?[edit]
During its reign the BW78 diff was a very popular diff, being used in almost all locally produced family vehicles. Examples include:
- Ford Falcon - XD to AU (6 cyl or V8)
- Holden Commodore - VL to VT (6 cyl, V6 or V8)
- Chrysler Valiant - Late models* Mitsubishi Sigma (2.0 or 2.6)
All of the above mentioned centers will fit inside the R31 housing. Longevity requires correct adjustments made to the new center. IRS versions of the commodore center are also interchangeable.
What L.S.D centers are available?[edit]
The R31 came with the optional BW78 Cone type L.S.D: 28 spline, 4 pinion.
This diff is very effective for providing limited slip in slippery conditions, however the design does not lend itself to durability. Many users experience a loss in the locking ability of this center between 3 months and 1 year of regular hard usage. These centers can be rebuilt (reshimmed) but will have a similar lifespan.
Most L.S.D centers that you will find out of other BW78 cars will be based on this design.
They are available in the following configurations:
- 25 spline, 2 pinion (Ford Falcon - XD, XE, XF. Holden Commodore - VL)
- 28 spine, 4 pinion (Ford Falcon. Holden Commodore - VL turbo, VL V8, VN onwards)
KAAZ diff centers are available for the BW78, and is quite possibly the best aftermarket L.S.D available. They are a much better design, based on clutch packs, rather than cones.
The most available KAAZ center is what is know as a 2 way diff, which applies lock under both acceleration and deceleration.
The following video demonstrates the difference between a 1.5 way and a 2 way LSD: YouTube
Available in 28 spline, 4 pinion
Eaton does a Torsen based center for the BW78, it is a very sturdy design. Its based on another set of gears inside the center, which under slip, adds torque to the wheel that has more grip, based on a multiplication factor. The Eaton/Harrop branded 'Truetrac' center as mentioned above, is very strong and should be used in high power applications.
You will also find a 'Mace' branded Torsen center, that cost around the $600 mark. These are not for high power applications. These centers have been designed as a cost effective/longer lasting replacement for the 'cone' type LSD as fitted standard. That said, these Mace LSD centers are only designed to hold stock engine output power, and work fine in this duty (but can shatter the case in high power usage).
Other locker options[edit]
If you feel the need for some locker action, you have three main options.
Minispool, which is a locking device that replaces the spider gears in the center with a solid metal cylinder. Hard locks the two axles together.
Welded Center, which is what it says, a center that has had the spider gears welded to the centers housing. Hard locks the two axles together, durability is questionable as opposed to the minispool as it depends on the quality of the welding.
Full spool, which is the big brother to the minispool, it replaces the diff center with a solid carrier that only holds the crown gear and has two sides for the axles to enter, will hold stupid power levels. Hard locks the two axles together. Used by V8 Super Cars.
Contributors: Session, Mainyard, solder