FC Diff Specs

[FC 304 wannabe]

Hi Guys,
I haven't posted before...
I was wondering if anybody new the Diff specs required for a Borg Warner diff conversion into my FC project.
The diff shop has asked if the specs are the same as an FE or a HR ? I believe my FC has a "HR rear end".
Any help would be great.
Thanks.

[FATBOY]

what size tyre are you running what rear brakes are you running what backset on the rims are you running what  ride height are you going to be having Once you have worked all that out you can decide what length you need to cut your diff down.Mines been cut down to 1345 mm disc to disc im running 31 spline billet strange axles 3.9 diff gears mini spooled.commodore rear disc brake set up.Rims are 16x7 with a 3.5 inch backset

regards fatboy

[FC 304 wannabe]

Thanks for the quick reply, but I have been told that to make my conversion legal in Qld I must modify the diff to be exactly the same width as the original factory diff.  Then I suppose I can work out what wheels and ride hight are required...
The diff will run VN disc brakes and commodore stud pattern.
So, really what I was asking for was the original diff specs.  The diff in my car is a HR unit I'm told.
I don't want to pay for a conversion only to be told it is not legal because I gave the shop the wrong information....

[mcl1959]

HR and FC diff width is the same

Ken

[FC 304 wannabe]

Thanks Ken.
I have just started an FC project where I am fitting the driveline from a VN V8 Calais including diff, brakes and 304 EFI engine.  The front end will have the collapsable steering column from a VK commodore and the VN discs fitted (done by Rods Racks in Beaudesert).
 I am told this is a fairly standard sort of build, but I'm sure to have plenty of setbacks along the way....

[fe350chev]

Wouldn't it depend on what tyre size u want, whether standard or wider and what offset etc? So you might opt for narrower. I think I get what your asking.

[Harv]

FC304wannabe,

What the guys are trying to point out is that the advice you have been given has probably been worded poorly by the engineer. Some engineers/rego authourities want the rear track to be the same in the modified car as the original. Track is nrmally measured to the centreline of the tyre, not the diff flanges.

For example, in NSW this is specified by the RTA guidelines as:
"If an axle assembly is shortened then the track width limit is taken as the axle manufacturers original track dimension, less the amount the assembly has been narrowed, plus 25mm."

Track width thus includes both the diff and the wheels. This is where it gets harder, as the Conformodore wheels are very different than the original early Holden ones. For example:
Commodore  6" wheels (152mm) are wider than original early Holden wheels (114mm or 4.5”).
Commodore offset varied from 26-43mm (positive offset in all cases), much bigger than the early Holden offset (6mm positive).

The process the guys are suggesting is:
a) work out what wheels you want (rim width and offset). Aim for standard Commodore dimensions if you can so you don't have to buy custom mags later. This can get even harder when the engineer/rego authority has wheel size limits. For example in NSW for an EK wagon: The vehicle was built in 1961, and hence was not built to comply with ADR 24 (prior to 1973 for passenger cars). With a kerb weight of 1183kg, the maximum permitted rear rim width is 204mm (8 inches).
b) work out where the tyre centres will be relative to the old ones due to the change in width and offset.
c) work out how wide the diff needs to be so that you meet the engineer's track requirement.

These are the NSW guidelines (imagine QLD has similar):
http://www.rta.nsw.gov.au/registration/downloads/vsi/vsi09_rev4.pdf

This post has some info where I went through the NSW guidelines for my EK project:
http://www.fbekholden.com/forum/viewtopic.php?f=27&t=14204

Hope this helps.

Cheers,
Harv.

[FC 304 wannabe]

Thanks harv,
I understand completely what you are saying .   But I have double checked with my engineer and he states that the diff length must be equal to the oem diff.
He says wheels are owner assessed and do not require his certification.
Guess qld rules are still a little different.....

[Harv]

Not trying to be funny (and apologies in advance if it sounds that way), but I think your engineer may be misreading the code. I get interested in codes and legislation as I'm trying to learn what I can get away with with my own vehicle  .

From what I understand, the Queensland guidelines are here:
http://www.tmr.qld.gov.au/~/media/b3ea8c53-b6a1-489e-933d-dac4054f21ba/codeofpracticelightvehiclesreductions.pdf

It looks like he is applying LD1, which requires for diff replacements that:
"to ensure the correct wheel track is maintained, the width between the axle flange faces of the replacement or modified axle must be the same as the dimension on the original axle fitted to the vehicle being modified". (my bold)

It also requires retaining the vehicle's original wheel stud pattern, and retaining the original brake type 

LD2 allows wheel track to be up to 26mm wider (with original brakes and stud pattern). This sounds close to NSW regs.

I haven't read the full document in detail, but it sounds like the intent is very similar to NSW regs. Might be worth a read through and a discussion with the engineer.

Cheers,
Harv.