BMW axel conversion

[tsecomp]

Hi
Thinking of replacing my axel with a higher ratio from a BMW
Preference is to retain the existing sub frame / trailing arms and modify to suit.
Has anyone had any experience with this conversion, any photos would be useful and is it something you can attempt yourself or best go for one of the kits available?

[MSC]

Some of our members of this forum have done this and will no doubt soon be in touch.

If its just a higher gear (lower revs) you want it is easy to fit a 3.45 Triumph saloon/estate differential. If you want a stronger diff the BMW diff is an option in varous different ratios. Either way your speedometer will need to be recalibrated. Also note acceleration will not be quite as quick.

There seems to be a couple of ways to do this:
1. Buy a kit with all the mods already done to the subframe (exchange with your old one I think).
2. Buy a modified BMW diff that can fit your old subframe.

There are companies that can do this, but I would not recommend you DIY the whole job unless you are confident you can get the correct alignment and adequate strength.

[404ROD]

I have a Weslake BMW diff which fits the rear suspension without modifications. It is expensive but I got fed up with the noise of my standard diff. I went for the standard ratio but others are available. If you go for the higher ratio you will gain by reduced engine speed but a slight loss on acceleration - the choice is yours. Don't forget to re-calibrate the speedo. Look at www.weslake.co.uk for more information.


Rod

[GDPR]

I have the higher ratio Triumph diff, and have noticed no appreciable slowing of acceleration between it and the 3.7 - in fact being that 1st on either gearbox is pretty useless, the higher ratio helps by making the gear longer.

The speedo issue is easy - I use the Triumph 2500s 140 mph speedo that the diff came from and its just fine accuracy wise - reads about 5% fast at 70.

Russ

[tsecomp]

Thanks for the info
I currently have a 3.45 axel from a Triumph 2.5 but even with a MOD gearbox & standard wheels/ tyres find gearing too low with the 4ltr engine fitted. I tend not to use 1st much and found that on a recent 2000 mile european tour was always reaching for another gear on the autoroute. Ideally I would like to be at about 2000rpm at 70mph so was looking at a 3.07 ratio which is I understand about the same as the MGBV8.
Think the conversion kit will be the way to go so any photos/ experience of the standard sub frame mounted BMW installation would be very useful.

[GDPR]

Just for info purposes, the combination of ZF4HP22 4 speed auto with locking converter, 3.45:1 diff from 2500 estate and 205/70HR14 tyres on standard wheels gives me 30mph per 1000rpm - just over 2300rpm at 70, which is very comfortable and still leaves me plenty of acceleration for quick overtakes...

[Stagdad]

Im running with the 1:3.64 bmw diff with MOD and 215,60,15's ,speedo is spot on against a sat nav with no over reading for saftey when you pass a speed camera.Also got my eye on a 1:3.46 for when i squeeze a bit more power out of the TV8 hopfully.Thats the beauty of the bmw convertion that when you have done it you can swap diffs around for about £50-£100 for quiet units with all sorts of ratios you just need to be aware that there are 3 different case sizes,I'm using the 188 medium case.The 188 refers to the crown wheel size in mm.Dont think you can swap diffs quite so easily if you use the westlake though as it doesn't use the diff cradle like others do.

Cheers Steve

[Mike Stevens]

If you are running a saloon 3.45 diff and a 'J' type overdrive, then a change in the speedo pinion in the overdrive should get the correct speedo reading. It is easy to get to as it's just behind the drive cable!

There is a number on the speedo dial, usually at the bottom just to the right of centre. This is the 'turns per mile' (I think!) and is typically 1000 for 2000 saloons and 840 for 2500 saloons (the latter should also be the 140MPH type Kryten mentions). I'm afraid I can't remember the number on the Stag one, but I suspect it'll be 840 - like the 2500 saloons. If this is the case, then a speedo pinion swap for a 2500S saloon type should give the right speedo reading!

Of course, I'm probably wrong about the Stag turns per mile....

Cheers,
Mike.

Edit. It's not often I'm right and I'm wrong again! Just checked and the Stag Speedo is a '1000' type. So the stuff I wrote above is like fetid dingos kidneys - no good. Mind you, with some checking and calculations, there may be a 'J' type speedo pinion that would do.

[Westfaling]

Mmmm Dingo kidneys. Sounds appetizing.
There's a very good site called MintyLamb - great name.
I used a 3.45:1 diff with 195/70 14 tyres to return to the correct "factory" overall gearing. Mind you the speedo error on most cars is no guarantee of accuracy through this.

http://www.mintylamb.co.uk/gearspeed/

Nick

[staginhiding]

Thats a useful site Nick..

Thanks

Sam

[Mike Stevens]

Having done some more investigation and some sums, I reckon fitting a 17 tooth speedo pinion (NKC51) will correct the speedo error with a 3.45 diff. Assuming standard tyres of course! This speedo pinion was used in the 2500S cars and is still available from a few suppliers.

I still need to check some other things - like the standard pinion (NKC49) stated at 18 and 19 teeth on different places on the internet! Also the spiral gear on the 'J' type output shaft can be one of (at least) 3 types which still needs resolving.

Still, this is not accurate science but a compromise just as the original was!

If changing the pinion does sort it out, it could be very useful as it is a relative easy mod!

Cheers,
Mike.

P.S. Fetid dingo's kidneys. That comes from HHGTTG!

[les M]

Hi
Thinking of replacing my axel with a higher ratio from a BMW
Preference is to retain the existing sub frame / trailing arms and modify to suit.
Has anyone had any experience with this conversion, any photos would be useful and is it something you can attempt yourself or best go for one of the kits available?

There are a couple other ways to fit the BMW diff in if you want to retain the standard stag set up. I am not sure that you can change out the diff case on the weslake type and you might get stuck with the wrong ratio so you need to choose wisely.

http://www.stag.org.uk/forum/attachm...7&d=1248164788 This set up requires a new prop or some sort of coupling. Or you could modify the original stag set up and fit a medium case BMW diff as I did. One of the main problems with the BMW diff is the possibility of the halfshafts pulling out of the diff under extreme load as they are only held in with small snap rings. The BMW diff is designed to be used with C/V joints and not U/J type joints as fitted to the stag. Have a look at this site goodparts.com they do a nissan up grade for the TR6 with a lot of good info. Or you could even fit a ford diff out of the escort cosworth/granada/plus these are a lot lighter and will take the power and torque.

Les.