BMW diff for repowered Stag

[NZSTAG]

From New Zealand and new to the forum, I am hoping for some advice regarding gearing options for my re-engined Stag. I have owned my 1973 model for 30 years and purchased it already re-powered with a Leyland P76 engine (Australian built 4.4L version of the Rover 3500). It's mated to a Toyota Celica 5 speed gearbox. Now I would have preferred the original setup but that's the way it is. The issue is of course gearing, I realise there have been discussions on this topic but usually around small changes. The Leyland engine puts it's power out at around 2/3rds the revs that of the Triumph, so it always feels very short geared. Claimed peak power of 192hp is at 4200rpm. Not helping either is top gear of the Celica box is not really an overdrive - 0.9:1 compared to 0.8:1 for the Triumph overdrive. In a bid to help I fitted 195R14 (that's 195/82) tyres but it has always looked odd with these balloons and anyway this didn't make too much difference. I want to go back the the standard 185/80 or maybe 195/70 tyres. Fitting a diff from the 2500 saloon would help but by only around 10%. The Leyland car the motor came from has a diff ratio of 2.90:1 on the same size wheels as the Stag so you see where I need to head. I note Monarch make a conversion kit to take a BMW diff and there is a 2.92 option. However the cost of sending my sub frame assembly over to the UK is too expensive.
Has anyone done this mod themselves? I would rather leave the existing assembly alone and modify a second one. As 2500's are more plentiful, is the diff mounting subframe the same as the Stag? Any advice would be much appreciated. Thanks.

[mjheathcote]

The subframe arms are the same between 2500 and the Stag, well nearly. The difference being the Stag has holes in both arms for the exhaust. Tubes for the handbrake may be different too, not sure on that. Whatever the differences are it's minor can be modified to suit.
I know this for I bought a pair of new arms for my Stag from James Paddocks in the early 90's and they told me they were 2500 items they modified.
Regarding diffs, you hear Aus and NZ owners using Nissan Skyline rear diffs being more plentiful in your location...

[stag8manod]

Hi NZStag, The conversion you are looking at has been done in Hamilton NZ. The chaps name is Paul Zeeders. His also has a P76 in his and the Stag diff couldn't handle the 4.4s torque. Paul is probably in the phone book or contact John Parker at the Mount, he will know about it.
Cheers Ian A

[NZSTAG]

Hi Mike,

Thanks for your reply - I hadn't heard of a Nissan diff being used but will check that out.
I think I can pick up the 2500 sub frame arms fairly cheaply so can make a comparison from underneath.
Thanks.

[NZSTAG]

Hi, I will be talking to Paul over the weekend. Thanks for your help.

[bullstarz]

You could swap your Celica 5 speed gearbox to the Toyota W58 or r154. Both are come down there, or change the diff. There a couple of lads over there that have done stuff to the Stag back end.

I'll message you his email if i can find it.

I'll email him as you can't receive messages.

[jakesmig]

I've put a T5 box from a Camaro which has a 0.63 final drive ratio into mine.

011.jpg

John

[Guest]

I have just finished fitting a BMW diff into my Stag but am unable to upload photos. The diff bolts back in the same position as the original diff.
When I try to upload the photos I get this message.

[Guest]

I don't beleive it, the photo comes later when I upload the reply!
Diff 1.jpgdiff 2.jpg

[Stagdad]

Beware one of the suppliers offerings engineering is very inadequate see here. Mine wasn't the only one this happened to

http://socforum.com/forum/showthread.php?31621-EFI-1-Diff-carrier-0

[Guest]

I read your entry on how not to do this conversion(by a supplier). This has been done in New Zealand using Engineering companies involved in structural engineering hopefully not replicating your issue. Regards Paul

[Klaus Schlueter]

Hi Paul
could you please specify the BMW diff type you have used for the Stag? BMW car type or model and year, diff no. etc. Your installation looks extremely solid!
Many thanks
Klaus

[Guest]

Hi klaus
I believe it was out of a 325, E30 medium case diff with a e34 cover(ratio 3.64). I can provide more details of the project if anyone is interested.

[Stagdad]

Medium case diffs also known as 188 because of the size of the crown wheel have an 8 bolt back plate, small case or 168 have a 6 bolt back plate. Most of the conventions use the medium case as it was fitted on the larger engine models.

[NZSTAG]

I believe my Celica box is a W50 which has a 0.85:1 5th. The W58 series has 0.78:1, which is a little better but still not enough.
On reflection I feel the BMW conversion isn't the way I'l go.
If I fit the 2500 diff I gain around 9%. If by chance the 5th gears from the W58 will fit my W50 type, there is an additional gain of another 9%. Maybe that's as good as I'll get with minimal expense and effort. A drop of 18% is a lot better than present. Another possibility would be to get a new pair of 5th gears made, at a ratio of say 0.7:1. Not having seen the gears I don't know if this is feasible or not. Meanwhile I really want to get rid of the 195 tyres and fit a lower profile (which of course won't help my gearing problem), I know 195/70s will fit but might also try 205/70s. However I realise clearance to the struts may preclude this. Thanks for your help.