2.88:1 Diff

Baron von Marlon

Baron von Marlon

Member
I've found a Jag 2.88:1 LS Diff for reasonable money - does anyone have any practical experience of what impact this will have on the performance of a V8 P6 fitted with an LT77, please?

Plugging the numbers into the following link:
http://johnmaherracing.com/calculators/ ... alculator/

It drops 70 from 2400 to 2250 RPM, which will make cruising slightly easier, but will it have much of an impact on acceleration? I'm guessing not, purely by looking at the numbers, but so often theory is different to practice :D

I'm planning on using it with the 3.5 engine initially, with the ultimate goal being a 4.6, which is the rationale behind doing the swap in the first place. The 3.5 currently puts out extra torque with the 3.9 heads and cams, but will it be enough to cope with the lower ratio? I know that the 4.6 will have no problems whatsoever.

One last question - if upgrading to a 4.6, would it be worth upgrading the brakes? If that's the case, I might forego this particular diff, in favour of finding an XJS diff, complete with discs and calipers, instead.

Any advice welcomed - all my mechanical experience so far has consisted of fitting parts that were designed to fit the vehicle concerned :)

Cheers,
Martin
 
First of all, the XJS final drive and calipers are fundamentally different from those on the Rover. They fit in the Jaguar IRS cage and are secured by 4 bolts at the top of the cage. I cannot think how you could attach the Jaguar setup in the Rover. Secondly, what is the purpose of going for ever bigger engines, only to strangle them with ever higher final drives ? You need to go away and think, carefully, what your objectives are. This is particularly so considering that you already have substituted a 5 speed box for the original 4 speed. And in answer to your question about acceleration, of course it will have an effect. If you increase the gearing by , let us say, 15 %, you reduce the torque at the wheels by about 13% , and you reduce the surplus torque, which is what provides the ability to accelerate, by much more than that
 
There are guys on here who have fitted the Jag diffs to Rovers. There is even one (at least) that has fitted an entire
Jag rear end, so that is not an issue if one is technically up to it.
As for the ratios, a lower ratio WILL make acceleration slower, there's no getting around it, but it will also drop your
cruising revs down as the calculator shows.
You need to decide if you want to go from traffic light to traffic light with the Yobs in their Vectras or if you want to
do your thing in the comfort of your Rover.
 
viewtopic.php?f=21&t=17146&start=0&hilit=302

Course it will fit 8)

Cars of the P6 generation were always a bit low geared. My Audi is spinning at 1,800 revs at 70mph, but the Rover, even with the LT77 is still around 2,500.

I have toyed with the idea of a Jagg diff and would certainly go for the 2.88.

Richard
 
Baron Von Marlon wrote,...
One last question - if upgrading to a 4.6, would it be worth upgrading the brakes?
Click to expand...

Hi Martin,

I have been running a 4.6 in my Rover since 2007, with over 113,000 Miles (182,000 km) covered since the installation. The P6B in standard form has excellent brakes, and I have never found them wanting, mind you, I don't go flying at top speed into corners and the like.

The 4.6 will deliver torque beyond what the LT77 is designed to handle, but if you drive it gently, then you should be ok.

Ron.
 
quattro said:
http://www.classicroverforum.net/viewtopic.php?f=21&t=17146&start=0&hilit=302

Course it will fit 8)

Richard
Click to expand...

When you read the thread Richard quotes, you will see that it will not fit without an enormous amount of re engineering of the mountings
 
christopher storey said:
quattro said:
http://www.classicroverforum.net/viewtopic.php?f=21&t=17146&start=0&hilit=302

Course it will fit 8)

Richard
Click to expand...

When you read the thread Richard quotes, you will see that it will not fit without an enormous amount of re engineering of the mountings
Click to expand...

I think that is fairly obvious, it won't just bolt in. Well worth the effort if you're increasing engine power/torque though.

Richard
 
I do wonder who made the Rover final drive. If it Salisbury, as is the Jaguar, then it seems likely that it uses the Dana 44 gearset. If that is the case, then it might be a easier to use the appropriate dana gearset ( taken from the Jaguar casing ) in the Rover casing which would save a great deal of trouble
 
I think the P6 diff is Rover's own, and I doubt the casing would be big enough for Jag internals even if they were from the same manufacturer. ISTR the Rover diff castings have "Qualcast" cast on them.
 
Hi, it's not the diff that is the weak point. It's the half shafts in my experience,
and the input shaft in other peoples experience.

Colin
 
Thanks everyone.

Ron - Thanks for the tip about the brakes, I'm not planning on flying into corners at top speed either, so will stick with the standard brakes. That'll make the job slightly easier! What transmission are you running behind your 4.6? There seems to be a whole range of opinions on limit of torque for the LT77, ranging from quite low, to really quite high.

I had been thinking of an R380, but they seem to be difficult to get hold of with ratios suitable for the V8 - the van variant is relatively common though. One of my other lines of enquiry, on which I'm still digging, is the feasibility of combining the gears from a 4x4 box, with the tailshaft, etc, of the van box. No-one seems to have done it, but that doesn't mean it can't be done :wink: I'm coming round to the idea that it is probably possible, as long as the two boxes have the same suffix. That's probably something for another thread though!

Christopher - I'm aiming for a fast GT style, really. I want to reduce the cruising revs, I just wasn't sure what impact that it would have on acceleration. The ultimate goal, with the 4.6, is something that will cruise at nice, gentle revs, but will also get there quite quickly. I'm not after a sprint car for drag racing, etc.

The Jag diff is physically larger than the Rover unit - I have done a bit of research into stronger gears for the Rover unit, to make the job a bit easier, but there doesn't seem to be anything available/suitable. I found this thread which details fitting one of the type that I've got a lead on, which is some of the inspiration:
viewtopic.php?f=21&t=5288

It's still quite a big job, but not as big a job as fitting the XJS unit, outlined in the thread that Richard linked to.

i think I'd better go and have a look at this thing!
 
colnerov wrote,...
Hi, it's not the diff that is the weak point. It's the half shafts in my experience,
Click to expand...

Hi Colin,

I am sure that you meant to say the (internal) drive shafts :wink:

Harvey wrote,...
ISTR the Rover diff castings have "Qualcast" cast on them.
Click to expand...

It does indeed, made by the company that was making lawn mowers at the time.

Baron Von Marlon wrote,...
Ron - Thanks for the tip about the brakes, I'm not planning on flying into corners at top speed either, so will stick with the standard brakes. That'll make the job slightly easier! What transmission are you running behind your 4.6?
Click to expand...

Hi Martin,

My Rover's transmission is an Australian spec BW M51.

Ron.
 
Baron von Marlon said:
One last question - if upgrading to a 4.6, would it be worth upgrading the brakes? If that's the case, I might forego this particular diff, in favour of finding an XJS diff, complete with discs and calipers, instead.
Click to expand...

From what I've seen, there are very early Dunlop brakes, and regular Girling brakes, no others, so XJ6, XJS etc as far as I can tell are all the same. I experienced a heavy rear bias on my brakes, so pursued upgrading the brakes to fill my 15" wheels. I can throw it at corners with confidence, but there are a lot of other modifications that have additionally been done so it is a completely different beast.

Baron von Marlon said:
Thanks everyone.

I had been thinking of an R380, but they seem to be difficult to get hold of with ratios suitable for the V8 - the van variant is relatively common though. One of my other lines of enquiry, on which I'm still digging, is the feasibility of combining the gears from a 4x4 box, with the tailshaft, etc, of the van box. No-one seems to have done it, but that doesn't mean it can't be done :wink: I'm coming round to the idea that it is probably possible, as long as the two boxes have the same suffix. That's probably something for another thread though!
Click to expand...

Having messed about with both rwd and 4wd variants of the LT77 and R380, there is a reason why people generally don't do what you are contemplating, the main difference is the mainshaft is entirely different so would be an extremely involved job. I don't think it would be impossible if you were to find the same suffix boxes, but maybe with the taller final drive the lower 1st, 2nd and 3rd may not be as much of an issue?

Baron von Marlon said:
Christopher - I'm aiming for a fast GT style, really. I want to reduce the cruising revs, I just wasn't sure what impact that it would have on acceleration. The ultimate goal, with the 4.6, is something that will cruise at nice, gentle revs, but will also get there quite quickly. I'm not after a sprint car for drag racing, etc.
Click to expand...

This is something only you can decide, as it usually goes down to personal preference and driving style.

Baron von Marlon said:
The Jag diff is physically larger than the Rover unit - I have done a bit of research into stronger gears for the Rover unit, to make the job a bit easier, but there doesn't seem to be anything available/suitable. I found this thread which details fitting one of the type that I've got a lead on, which is some of the inspiration:
viewtopic.php?f=21&t=5288

It's still quite a big job, but not as big a job as fitting the XJS unit, outlined in the thread that Richard linked to.

i think I'd better go and have a look at this thing!
Click to expand...

I think I heard there was a company that was selling uprated internals for the standard final drive unit, input shaft and output shafts, and I am fairly certain I've seen an LSD customised to fit inside the original casing, so nothing is impossible.

If you do want to do it, first question is can you weld, second question is can you fabricate? As you probably appreciate it's a big undertaking, so a good sized workshop space is essential. You will need to pay for the driveshafts to be shortened or buy new, also the propshaft will need replacing.

I like how this one is being done:

1-515.jpg


More info here http://www.classic-rover.nl/home/4
 
sowen said:
From what I've seen, there are very early Dunlop brakes, and regular Girling brakes, no others, so XJ6, XJS etc as far as I can tell are all the same.
Click to expand...
That's interesting to know, thanks. I'd assumed that the brakes on the XJS would somehow be uprated over those of the P6 - it was the rear bias bit that put that idea in my mind.

sowen said:
Having messed about with both rwd and 4wd variants of the LT77 and R380, there is a reason why people generally don't do what you are contemplating, the main difference is the mainshaft is entirely different so would be an extremely involved job. I don't think it would be impossible if you were to find the same suffix boxes, but maybe with the taller final drive the lower 1st, 2nd and 3rd may not be as much of an issue?
Click to expand...
I never thought it would be easy, but then I tend to do things out of curiosity and appreciate a challenge :mrgreen: Plugging what I think are the van ratios into the calculator, (3.985 / 2.519 / 1.507 / 1 / 0.831) with the 2.88 final drive ratio, things don't look too bad, so represents a much easier option. I think I'll keep an eye out for a cheap one to have a play with.

sowen said:
If you do want to do it, first question is can you weld, second question is can you fabricate? As you probably appreciate it's a big undertaking, so a good sized workshop space is essential. You will need to pay for the driveshafts to be shortened or buy new, also the propshaft will need replacing.
Click to expand...
I can weld, although it has been a while. Can I fabricate? Again, yes, but it has been a while, I'm under no illusion that this is quite a big undertaking. I was going to get an engineering company to shorten the driveshafts - I don't have the facilities to do that myself. I'd been wondering whether a Jag propshaft might do the job - I recall reading that the Jag output yoke for the ZF 4HP22 bolted straight to the Rover propshaft, so a Jag one may bolt straight to the gearbox output and the diff unit, in theory.
 
Baron von Marlon said:
That's interesting to know, thanks. I'd assumed that the brakes on the XJS would somehow be uprated over those of the P6 - it was the rear bias bit that put that idea in my mind.
Click to expand...

The size of the calipers, pistons and pad surface area I believe contributed to the rear bias, the Irish rally P6 team also reported rear brake bias on theirs. However, I don't think 302Rover has reported any brake issues?

Baron von Marlon said:
I can weld, although it has been a while. Can I fabricate? Again, yes, but it has been a while, I'm under no illusion that this is quite a big undertaking. I was going to get an engineering company to shorten the driveshafts - I don't have the facilities to do that myself. I'd been wondering whether a Jag propshaft might do the job - I recall reading that the Jag output yoke for the ZF 4HP22 bolted straight to the Rover propshaft, so a Jag one may bolt straight to the gearbox output and the diff unit, in theory.
Click to expand...

If you are happy that you can make the installation, then good luck with it, be interesting to see how someone else tackles the conversion :D . The Rover and Jaguar use a standard propshaft flange pattern which is shared with a number of other British cars. You maybe lucky with a Jaguar XJ6 propshaft being close to the right size to fit, I have one on one of my projects I'm working on now, but the axle and engine are in different positions. You may be better off just measuring and having a new propshaft made to suit. They are expensive, but as long as you measure accurately, you should only spend once.

Going back to your first question of the gearing, I'm planning on fitting a 2.85 ratio axle to my Rover SD1 V8, which with 205/60/15 tyres, LT77 gearbox and 3.08 final drive should be comparable to the P6B with LT77. I find 5th to be too short on cruise, as brisk motorway overtaking happens with ease without changing gear.
 
sowen said:
The size of the calipers, pistons and pad surface area I believe contributed to the rear bias, the Irish rally P6 team also reported rear brake bias on theirs. However, I don't think 302Rover has reported any brake issues?
Click to expand...

Hi,

When we first did this conversion the rear bias was excessive, dangerously so. We cured it using a Tilton brake proportioning valve:

http://tiltonracing.com/wp-content/uplo ... -Valve.pdf

Not a particularly cheap solution but a very effective one, we use the one with the knob rather than the leaver.

Tim
 
I think I heard there was a company that was selling uprated internals for the standard final drive unit, input shaft and output shafts, and I am fairly certain I've seen an LSD customised to fit inside the original casing, so nothing is impossible.

If you do want to do it, first question is can you weld, second question is can you fabricate? As you probably appreciate it's a big undertaking, so a good sized workshop space is essential. You will need to pay for the driveshafts to be shortened or buy new, also the propshaft will need replacing.
Click to expand...

Alan at classeparts in Leighton Buzzard iirc....
 
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