diffs, all you need to know (updated)
Seeing as many of these questions get asked multiple times I thought I’d put this together for future reference.
history
The Borg-Warner diff was first fitted to later model VK’s, it had a 2spider center with 25 spline axles. On the VL’s they fitted 4 spider LSD centers and 28 spline axles for the VLT (turbo 6 cyl) with 3.45 ratio and V8 models with 3.08 ratio. From VN to VS and VT series 1 all BW 78 diffs had 4 spider centers with 28 spline axles, the standard ratio is 3.08:1 for all models, the V8 models had shot peened gears (crown wheel and pinion).
In the VP range Holden first introduced the Independent Rear Suspension (IRS). It is not possible to change a commodore fitted with semi trailing diff to IRS as the entire rear section of the floor pan is different, different mounts etc. Limited Slip Diffs (LSD) were fitted as standard equipment on SS models and optional on all other models.
also note that although sedans, wagons and utes all use the same borgwarner diffs the wagons and utes use different mounting points for the shocks and panhard rod so diffs from wagons and utes are not usable in sedans, all the gears and centers are still interchangable
Diff gears (crown wheel and pinion) are interchangeable between all BW 78 diffs including all IRS diffs. You can also fit a LSD center to any BW78 with open center, it’s a straight swap, just fit your original crown wheel to the new LSD center (unless changing ratio at the same time). IRS LSD centers are interchangeable with semi trailing diffs, however it has been suggested that for later model IRS commodores were fitted with a different offset centers and these would not be interchangeable with earlier models, however I have never seen one and I’m not sure about this. There is a slight difference between IRS and solid axle LSD units. The IRS LSD centres have a groove machined in the splined hub so that the stub shaft/hub is retained, the solid axle models don't have these, there for the IRS LSD can be used in a solid axle but you can't use the solid axle LSD in an IRS centre as the stub axle/shaft hub will not be retained.
Borg Warner has has a few different names over the years, the are currently owned by Dana ( a rather large US company that builds very tough diffs used in many US truck applications) and were also know as BTR at some stage
Diff ratio's
For those interested in other diff ratios for the BW78, the Nissan R31 Skyline and Pintara also used the BW78, these were fitted with 3.73 (manual and very rare) and 3.89 and 4.11:1 other ratio's also available are 3.27 (VP GTS). Ford also used the BW78, i'm certain that the XD and XF falcons had the BW78 but not certain what ratio's were used. i've heard 3.23:1
Latest info I have is that Ford used the BW78 diff centres from XF to FG model Falcons
edit:I've since discovered that the Ford Falcons have used various ratio's. XR6/8 models seems to have been fitted with 3.45 ratio LSD units. XD/XF Falcon utes have 25 spline centres with 3.23 ratio where the sedan models have a 2.77 ratio (this would explain why they seem to be slower on the road when they tended to have a lot more power then the equivalent Commodore model).
Note of interest.
The VN GRP A commodores were fitted with diffs from the VLT with 3.45:1. They used these diffs because they are slightly narrower so that they could use wider wheels (different offset) under the rear of the car.
VT series 2 onwards they fitted the BW80 diff, which has a larger crown wheel and pinion, and also different ratios. Parts from these are not interchangeable with the earlier BW78.
What this means to you is that the mounting points for the diff from the first VB commodore sedan all the way to the VS sedan(not including IRS of course) are the same, so it's possible to fit any BW78 diff into a early girl commodore. however keep in mind that the VN-VS diff is wider (40mm) then the VL which is wider (12mm) then the VB-VK diffs. also note that the BW78 uses flange arrrangement to bolt the driveshaft to the diff were as the salisbury diff the rear universal on the driveshaft bolts directly onto the diff joke using 'U' bolts. to fit the borgwarner to a early girl commodore you will need to use the rear half of a driveshaft used with a borgwarner diff
Diff upgrades
However it is possible to fit the entire IRS setup from a late model commodore (VTs2,VX,VY) into a earlier model IRS commodore but you will need to use the entire diff assembly/cradle as well ass the drive shafts and hubs from the donor car (STEALTHY has done this in his VP international)
The Holden LSD is fairly average as fitted by Holden and have a tendency to single spin when abused. it is possible to have these diffs tightened where the clearances are reduced and fitted with better springs. this is a good idea to have done if fitting a used LSD as it's probably seen better days before you get hold of it. approx $300
Probably the best upgrade to get in my opinion would be the Harrop/Eaton True-trac centre
http://www.harrop.com.au/drive_detail.php?prod=99-TTRC9837-00 . All the benfits of an LSD without any clutches etc to wear out but fairly costly at approx $1280 (2012) pricing
Another option is to get a KAAZ center fitted, these are about the best LSD available but expect to pay for it. you won't see much change out of $2k
it is also possible to fit mini-spools in these diffs. mini spools lock both axles together so that both rear wheels always turn at the same speed. these are great for drag racing and doing skids. to fit a minispool you need to have a open center as they do not fit inside LSD centers. minispool $80-200 (supply only)
the next option is to fit a full spool. these fit in place of your standard centre and your crown wheel bolts straight to it. these are stronger then mini spools but would be considered overkill for even a performance engine, really only suited for track use.
Lastly, there is the 4X4 Systems Lokka centres
http://www.4wdsystems.com.au/index.php?id=29 these are a bit like the mini-spool in that they are installed in your standard open diff carrier in place of the factory spider gears. On info I have recently received the 4x4 lokka is a lot quieter and more progressive in operation when compared to the Eaton style Detroit lockers of old.
if you are after a slightly different look under the rear of your car you can use the aluminium diff cover from a XD/XF falcon. you will need to remove the stud used for the watts linkage, other then that it's a straight bolt on option (you will require slightly longer bolts) and will need to slightly modify the bracket that holds the brake line under the 2 bolts on the side of the cover. these aloy covers can be highly polished and have the added advantage that alloy dissipates heat faster then steel thus helping to cool the oil in the diff faster (my diff guy recommends these are fitted to all diffs with higher ratio's as they produce more heat) a picture of one of these covers can be found in my ride (link at bottom of post) post 15 (page 1) or you could get TA Performance diff girdle
http://www.ebay.com.au/itm/TA-Perfo...r_Truck_Parts_Accessories&hash=item51a236f5c5 Both of these diff covers/girdles strengthen the housing and help maintain the crown wheel to pinion clearances and the TA girdle has the added benefit of providing more support for the bearing caps.
overview
VB-VK small and large salisbury diffs (large for 308 only and optional on other models), 2 spider center, optional LSD
VK 1/2 BW 75, 2-spider center with 25 spline axles, 3.23:1 ratio (6 cyl only, V8 continued with large salisbury)
VL BW 78, 2-spider open center 25 spline axles, 3.45:1 ratio (6 cyl), BW 78, 4-spider LSD center 28 spline axles, 3.45:1 ratio (6 cyl turbo), BW 78, 4-spider open center, 28 spline axles, 3.08:1 ratio (8 cyl)
VN-VS BW 78, 4-spider center, 28spline axles, 3.08:1 open and LSD, V8 have shot peened gears. VP onward Holden introduced the IRS as standard equipment on Calais and optional on other models
VT s1 BW 78, 4-spider center, 28spline axles, 3.08:1 open and LSD (same as VN-VS)
VT s2 BW 80, 3.07:1 (auto) 3.46:1 (manual) LS1 and L67 models, V6 models continued with the BW 78
VB-VK small and large salisbury diffs
VK borgwarner same width as salisbury diffs
VL Borgwarner 12mm wider then VK
VN-VS borgwarner 40mm wider then VL (52mm wider then VK)
Diff Gears and Centres Identification
It is easy to identify diff gear sets and centres by the numbers stamped on them, the centres will also have the numbers cast into the housing.
Gear sets will be stamped as follows
0575 - BW75 series
0578 - BW78 series
0580 - BW80 series
0575 gear sets are used with 0578 centres ( VN - VT s1) but are not compatible with the later 0580 gears or centres.
once you have fitted new diff gears your speedo is going to be reading incorrectly. the simple solution to this problem is to fit a Jaycar speedo corrector kit, this How-to
https://forums.justcommodores.com.au/threads/install-speedo-corrector-kit-vn-vp.60651 should cover all the information you need to correct this problem.
if your keen on getting your hands dirty and want to source a set of diff gears then read
https://forums.justcommodores.com.a...our-own-diff-gears-remove-them-yourself.79929 written by Wagon wheel
if there is anyhting i've missed or you believe is incorrect please add it beneath and i'll update this post as info becomes available
