Changing CW&P ratio

[Homewood]

Hi

I have a standard 3.5 Rover SD1 V8 with 5 speed gearbox and 3.9 Mgb axle.

It whines and needs rebuilding.

The car drives well I don't use 1st gear ever and it cruises fine revs well down.

Do I change the ratio of stick to 3.9 ?

Has anyone replaced their CW&P themselves, it looks pretty straightforward.

Thanks

[cobraboy]

V8 Ratio 3.08 :1 all day, every day.
Setting up a diff is for men in brown workshop coats.

All the V8 ratio repro gear sets used to whine anyway, don't know if they still do.
Make your exhausts louder !

[David Large]

When I put a late block 4 litre engine in my BRA 289 about 20 years ago I put in a 3.08:1 diff. It took me a while to get used to the different driving style but I wouldn't swap it now, it is great.

David

[StewbieC]

As already mentioned. Special tools and collapsible spacers are needed to replace Pinion gears. You even need to be careful replacing the pinion seal, returning the nut to the exact same place when refitting. Heathrow transmissions come recommended.

[StuBaker]

I think 3.08 is much too high.
I have 340bhp SBF with T5 and 'standard' ratios with 3.07 jag irs which gives me about 39mph/1000rpm in 5th. That is just above idle at 40mph!! Makes changing to 5th interesting. Certainly no lack of torque but...
I'll be swapping to 3.54 as soon as funds allow. This will be more like 33mph/1000 rpm in 5th and put 4th to same as original AC Cobra 289.
I'm guessing you're getting around 23mph/1000? I think you would regret changing to a much higher ratio, especially with a 3.5 RV8.
You may as well have the acceleration as I doubt you'd need the top speed. And you don't use 1st? Where do you live? Have you never done a hill start?
Anyway, each to his own. This is just what I'm doing.
I have rebuilt a B diff myself but wouldn't do a Jag.
Hope this all helps,
StuB

[peterc]

It also depends on what tyres you use. Slab sides usually have narrower / smaller profile than an FIA or 427 set up.
I have a 3.31 Jag axle and get around 32 per 1000 but I run with 245 x 60 x 15” rears.
Peter C

[Homewood]

Thanks I am going to check the revs at 60,70 and 80 mph.

[Homewood]

Thanks I am going to check the revs at 60,70 and 80 mph. My original diff was making noises so I replaced it. Both need new thrust bearings. I have today examined my old axle and the cw&p teeth all look good just a tiny bit of wear, as I have a brown coat I bought some blue marking and will check tooth contact when it arrives. There is a very small amount of play if I hold the crown wheel and turn the pinion but I think this is normal perhaps a dial gauge will confirm ?

[David Large]

I think 3.08 is much too high.
I have 340bhp SBF with T5 and 'standard' ratios with 3.07 jag irs which gives me about 30mph/1000rpm in 5th. That is just above idle at 60mph!! Makes changing to 5th interesting. Certainly no lack of torque but...
I'll be swapping to 3.54 as soon as funds allow. This will be more like 25mph/1000 rpm in 5th and put 4th to same as original AC Cobra 289.
I'm guessing you're getting around 23mph/1000? I think you would regret changing to a much higher ratio, especially with a 3.5 RV8.
You may as well have the acceleration as I doubt you'd need the top speed. And you don't use 1st? Where do you live? Have you never done a hill start?
Anyway, each to his own. This is just what I'm doing.
I have rebuilt a B diff myself but wouldn't do a Jag.
Hope this all helps,
StuB

Stu,
In my car 60mph = 2000 rpm, my engine idles at 850rpm and 3000 rpm = 90mph.
When we built the car in the 1980s we used an MGB automatic diff of around 3.5:1 and I never used 1st gear unless on a really, really steep hill. When we had to replace the diff beacause of excessive whining we found we could only get a limited selection and we went for the MGB GT V8 diff at 3.08:1. The driving experience was very different but I use all 5 gears and it is fine.

David

[StuBaker]

Hi David,
It's interesting to hear that. I could probably drive around in 3rd all day but I can't imagine not using 1st.
I'll have a go next sunny day,
Looks like an error - read 'just above idle at 30!!' I'll try to edit that out.
Cheers,
StuB