Jag Diff Ratios?

[craggle]

Hello all.

I am fairly new to this illness called cobra obsession and have now managed to save enough to order a Dax chassis and body after many years of wanting one.

My main question concerns the different Jag diff ratios and which one to use. I have already managed to obtain and strip all the required parts from a 1990 Jag XJR-S. The diff used on this car is a 2.88:1 ratio power lock unit. Most people I see said they have used a 3.54:1 diff with power lock if possible.
Does anyone out there use a 2.88 diff and if so how is it for acceleration and top speed.
The second question is I am about to order an engine and have been very impessed with the spec of a fulll Edelbrock equiped 420BHP 350 chevy supplied by Repower.
Has anyone had good or bad experiences with repower? I would like to use them as they supply a good specification turn key engine and are able to supply the gearbox fitted (Tremec) plus they are local to me allowing me to collect.
I Know I could order the engine from the states but like to use local suppliers whenever possible for conveience and any possible waranty claims.

Thanks in advance.
Craig.

[kdavies3]

Hi Craig,
Firstly I hope you've ordered a De Dion chassis as your IRS will be from an XJ40 as it's 1990.
There are loads of folks using 2.88 diffs. They do make the car very long legged in that your potential top speed will be very high (200+), but the aerodynamics of the the car make it impossible to achieve in reality. As a consequence you sacrifice acceleration too.
I'm currently running a 3.07 PL diff and with the TKO ratios I still have a top end that's not practical to use.
I'm changing to a 3.54 PL to improve the overall driveability of my car. As it is the accels are stunning and careful application of the throttle is necessary unless in a straight line. The 3.54 will make that worse but I'll get better use of my gearbox overall.
Think carefully about fitting the 2.88 because it's a git of a job to change later.
JMHO :tu

http://www.woodoouk.eclipse.co.uk/Wales.gif

Kev Davies
Dax, 383ci Chevy Stroker, Tremec TKO
South Wales

[craggle]

Hi Kev

Thanks for the prompt reply.
My chassis is the standard ladder frame as my jag rear suspension is, I believe the same as the older jags. It has inboard brakes and discs and was fitted with ABS.
I thought all XJ40 cars were identifiable by outboard discs. All the shortened wishbones and drive shafts supplied by Dax fit. Correct me if I am wrong (usually am!)
The other thing my diff has that I have not seen before is a cast aluminium rear cover plate instead of the pressed steel cover.
I do have in the garage a non powerlock 3.54:1 ratio diff. which one would you use out of choice? Is it possible to Remove or buy a new crown wheel and pinnion gear and fit them to the powerlock diff to change the ratio but maintain the limited slip unit?
The car is going to be used mainly for road use with the occasional track day and it will be fitted with the 420bhp 350 chevy and tremec box.
I would like to keep the diff as it is in very good condition and only done about 40000 miles but on the other hand I dont want to end up with a car that is undrivable at lower speeds.

Craig.



Craig.


:7

[kdavies3]

I know that the XJ40 diff usually had the ally cover, but it will fit on the earlier diffs too, 'cos I've got one.
Where are the main mounting points for the diff?
Post a picture and I can tell you if it's XJ40 or earlier.
As I say I'm pretty sure if it's 1990 it'll be XJ40, which as far as I know won't fit in the ladder frame chassis (unless Dax has changed something).

http://www.woodoouk.eclipse.co.uk/Wales.gif

Kev Davies
Dax, 383ci Chevy Stroker, Tremec TKO
South Wales

[kdavies3]

Unfortunately the 2.88 diff is different to all the other ratios and you can't transfer the PL to the 3.54.
Given the choice I think I'd go with the 3.54 open diff in preference to the 2.88 PL. Mainly because you'll never know the PL is there unless you drive on the edge a lot on trackdays etc.

http://www.woodoouk.eclipse.co.uk/Wales.gif

Kev Davies
Dax, 383ci Chevy Stroker, Tremec TKO
South Wales

[DON50]

hi the diff of a xjsr is the same as a xjs with an alloy cover,jag fitted the alloy finned cover stop the oil geting to hot,an xjs r has a 6lt v12 not 5.3lt, i used a 1992 model for my ak 427 the 2.88 power lok diff is still in my shed, it is only good for an auto,i used a 3.31 powerlok cheers don ak 427 351cleveland zf s5 box getting there

[craggle]

Hi Kev

hopefully this post will include a couple of pictures of the diff and brake assemblies.
I did make a mistake though. the other diff i have is a 3.31:1 ratio non PL unit but on close inspection there is quite a lot of play in the crown and pinion and the in small gears in the centre. It also feels a bit notchy when turned by hand so I will only use this as a last resort.
I did speak to Ward engineering about it the other day and they said that 3.54 PL units are getting hard to find and a good one is going to be expensive but he did point out that the V12 engine of the Jag had less power than my Chevy engine and it was only going to push a car that weighed a fraction of the Jag.
Maybe I could devise a removable cover in the boot of the Dax to allow access to the 4 top mounting bolts if I wish to change later?

Craig.

[russell_ram]

Craig,

thats a standard Dana 4HU unit (not XJ40). 3.54 PL are actual NOT rare, thay are fitted as standard to all 6 cylinder XJS. Most common as Wards say is 2.88 PL as they are in all V12 Jags. All the specialsit Jag breakers overcharge for diffs (especially PL units), have a trawl round your local 'dirty' breakers yards, if you can find one you should be able to pick up any of the many varieties/ratios for under £100.

However, I would agree with Kev that PL is 'overkill' for the road and may actually be detrimental to 'normal' handling characteristics.

Russ

I can re-build your 3.31 to better than new standard for a small fee (plus parts).

[nbracken]

Hi Craig,
Firstly regarding Repower, I bought my Tremec and clutch kit from them to attach to my Chevy ZZ4 and Brian was really helpful with good advice and service.

Re the diff ratio it is horses for courses as they say. I have the 2.88 PL in my car and I am very happy with it. I get excellent accelleration (my engine is rated at 355 ps) and when I do long haul trips on the autobahn (and I do use my car quite extensively each year) I can cruise at 80 mph at a little over 2000 rpm with no stress and reasonable economy.

At Elvington last year I could still do 1/4 miles in 13.6 which, while nothing special is ceratinly not slow.

If I were to build again I would probably seek out a 3..07 or maybe 3.31.

HTH

[skid]

So how the hell do you tell the different ratios when you are crawling around under a dirty (or maybe even clean) car?????????

Skid