Changing CW&P ratio

[phlip99]

I too was thinking about changing my diff ratio recently - from 3.9:1 to a 3.09:1 - Rover 3.9ltr engine.
BUT.... I then had a chat to Nigel at Nicol Transmissions in Kidderminster - highly recommended by the MG guys - and he said that they will not fit after- market diffs anymore as they have had so much trouble with customers returning a couple of months later complaining of a whining diff !! Apparently the case hardening is too thin.
They only fit new old stock or good second hand. They have no 3.09 available.
He also mentioned that the carrier is different for the 3.09 vs 3.9.
I have no experience of this stuff to date but just passing on the info. for what it's worth.

Phil.

[Homewood]

Thanks Phil I suspect your man is right from what I am hearing

[cobraboy]

I fitted a 3.08 aftermarket diff from MGB Hive many years ago, it does whine a little if you really listen, but to be honest I think the complaints come from MGBGT owners who have a closed car, with a Cobra with loud exhausts it will seldom be heard.
I think the ratio is just fine, if the weather was not so rubbish I would go and take rpm readings, but from memory I think 30mph / 1000 rpm in 5th is thereabouts. 3.909 renders first gear useless with an LT77.

[StuBaker]

Been looking at gearboxes, I can see the differences hence opinions:
LT77 SD1 V8 is 3.32 1st
T5 is 2.95 1st
UK cobras had 2.36 1st. ('L' set)
Hence no need to use 1st on LT77 with a 3.9? Unless climbing Ben Nevis with a Burger van.
AC Cobra 289 COB series generally had 3.54 diffs and later CSX cars had 3.77
All T10, T5 and LT77 boxes have same 4th gear, direct or 1:1.
T5 has 0.63 5th and LT77 SD1 V8 has 0.833 5th (hence lower diff ratio requirement for T5?)
If you have a Landrover box the ratios are different again. I'm sure I saw a 4.5 1st but was too shocked to remember what it was on. Probably a Landy.
So I guess it depends on YOUR driving style or requirements.
I suggest you find out exactly what you have and maybe get a drive in other cars before changing.
Ashcroft Transmissions seem to know what it's all about. Try them: https://ashcroft-transmissions.co.uk/ratio-calculator/

FYI - The Dragon Snakes were using 4.45:1, but top speed was only 135mph.

Some gearbox ratios:
*please feel free to correct errors and ommisions*

Cobra 'L' 2.36, 1.76, 1.41, 1.0 *generally fitted
Cobra 'M' 2.36, 1.62, 1.2, 1.0 *special
Cobra 'K' 2.2, 1.64, 1.31, 1.0 *special

LT 77 3.32, 2.087, 1.396, 1.0, 0.833 *SD V8

T5 2.95, 1.94, 1.34, 1.0, 0.63

[Homewood]

Test drive.

60 mph = 2000 rev
80 mph = 3000 rev
90 mph = 4000 rev

So with all the good advice I will stay with the 3.9 only because I don't drive it above 60mph much. I will be checking the backlash with clock dial and engineers blue to make sure it's ok.

So if it's not the cw&p making the axle noise then it must be a bearing but pinion and wheel bearings show no sign of movement and freely rotate.

[peterc]

Doesn’t sound much difference to mine. 70 = 2200 so 2000 is 63.
Noise wise could it be a UJ.
As a simple check try twisting each joint by hand. One hand on each half. Any movement at all indicates it needs replacing.
Peter C

[StuBaker]

Is your clutch slipping?
Are you numbers in the same gear?
60 mph @ 2000 rpm should be 90mph @ 3000 rpm? 120 mph @ 4000 rpm? etc.
Or may be worth checking your speedo with satnav.

My friend Mike had a clunk and I gave him my axle (un restored MGB) and it still clunked. I replaced thrust washers on pinions. Still clunked. I was stood listening to him drive by and noticed the wheel slip on the splines. Fitted new hubs. Job done.

[phlip99]

I fitted a 3.08 aftermarket diff from MGB Hive many years ago, it does whine a little if you really listen, but to be honest I think the complaints come from MGBGT owners who have a closed car, with a Cobra with loud exhausts it will seldom be heard.
I think the ratio is just fine, if the weather was not so rubbish I would go and take rpm readings, but from memory I think 30mph / 1000 rpm in 5th is thereabouts. 3.909 renders first gear useless with an LT77.

Many years ago the aftermarket stuff was probably much better quality.... old fart alert!! > in my experience quality of repro parts for the big volume MGs/TRs etc has generally got poorer rather than better in the drive to keep prices down (or profits up, if you want to be cynical!).
Phil.

[phlip99]

I too was thinking about changing my diff ratio recently - from 3.9:1 to a 3.09:1 - Rover 3.9ltr engine.
BUT.... I then had a chat to Nigel at Nicol Transmissions in Kidderminster - highly recommended by the MG guys --------------------------------------------

Phil.

By coincidence, this afternoon I was scanning an old copy of Sports Car Mechanics from August 1985 - I have masses of old car mags so they are all on their way to be recycled but I have a quick read before taking them - and came across an ad from Nicol Transmissions, Kidderminster.
So they have been around a while & definitely should know what they're talking about!!
Rebuild of your MGB rear axle now is quoted at £800 - £1000 for comparison !!

Phil.

P1230945.JPG

[StuBaker]

Apologies,
some erroneous data from myself... I'll correct previous post if I can.
I have 39mph/1000rpm in 5th and about 24mph in 4th. (T5 and 3.07 diff)
Don't know why I write such rubbish sometimes, will have a go at my pwoof raider.
Did 160 miles yesterday, just feels uncomfortable in 5th between 50 and 60.
I still think 3.54 is a good ratio for the diff...