Differential Swap

[Tintin]

I am having the diff rebuilt in a couple of weeks (4.2 litre engine, model 12BW gearbox, 3.31:1 axle).

I have always considered the engine "busy" at 60+ mph and now would be the time to swap the crown wheel and pinion for a 2.88:1 ratio setup. At present, 70 mph in top gives engine revs of about 3,000rpm; with a 2.88 ratio this would drop to about 2,600rpm. Put another way, the swap would give +10mph for a given engine rpm. Acceleration from a standing start would suffer slightly, but the Daimler is a cruiser, not a racer and with the present setup the box changes from 1st to 2nd very shortly after initial take off anyway.

Has anyone experienced this modification or have any observations?

Robbie

[mike020150]

Jaguar did go as far as using the 3.07:1 diff in US market autos but not the 2.88:1 which may help to influence your decision. The 2.88:1 ratio was used in later V12s and it blunted their performance away from rest noticeably compared with earlier V12s using either the 3.07:1 or the 3.31:1 ratios, so that effect would be magnified on a 4.2 litre car. Mike.

[Tintin]

Thanks Mike - that's very helpful.

A 13% rpm drop (the minimum that makes the swap worthwhile) does dictate the 2.88:1 ratio final drive. I wouldn't mind sacrificing the standing start acceleration but would mind a significant loss of acceleration higher up the range. The engineering firm I am using for the differential work is also cautious about the performance consequences - they have had success using the 2.88:1 on E types (manual gearboxes) but can't help on the auto saloons. If no-one else has done the modification, it rather suggests that it may not be such a good idea after all!

Thanks for you help.

Robbie

[Meonstoke]

Hi Robbie,
I think Mike described it well. GWautos do gearbox conversions for XJ6s and XJ12s but they are not cheap solutions. Given what you are looking for I would be sorely tempted to keep the 3.31 diff, keep the original autobox, and replace it with a Jaguar Salisbury 4 speed with O/D or a Tremec / Getrag 5 speed unit. It will transform your car.

M.

[davidr]

Robbie - have you considered a 5 speed manual conversion? It would tick quite a few of your boxes?

[Tintin]

Thanks M and David.

I have now committed to keeping the 3.31:1 diff (the car goes in on Monday) and as you both suggest am now musing over a replacement g/box in due course (and when finances permit!). In theory this would indeed provide the best of both worlds and is very tempting; I just have to come to terms with the idea of losing originality!

Robbie

[Tintin]

Just had a very helpful conversation with GW autos. The ZF box from an XJ40 will fit (with some serious modification work) and give transforming ratios of 2.48:1 1.48:1 1.00:1 0.73:1. The first three are very close or identical to the existing BW12 box. At top (with lock up and effectively an overdrive) the engine rpm will be less by about 1,000 rpm at 70 mph. I am completely sold on the idea - just need to find the cash!!!!

Robbie

[stevewarbs]

Hi Robbie, who did you use for the diff rebuild. Any info on pricing would be welcome.

I am about to order the parts for mine planning to have a go myself.

[RichardSEL]

And Robbie: which GW Autos have you been in contact with? There's three that gargle comes up with all claiming to be gearbox "specialists" -- Ta

[Tintin]

Hi Steve and Richard

Graham Whitehouse of Halesowen for the gearbox work - www.gwautos.com - but their web site is down at the moment - 0121 559 9800. If you have it done Richard, I would be interested to hear the results - at the moment I still can't justify the cost (IRO £10,000 if memory serves).

www.hardyengineering.co.uk in Leatherhead for the diff work (ask for Bill). To remove and refit the cage, rebuild the diff and fit the "extras" I bought separately IRO £1,500. The "extras" were new calipers, H/B pads, springs, dampers etc - thought I might as well have a thorough job done whilst it was out. One very worthwhile upgrade is remote bleeders for the rear calipers - I bought mine from Fosseway Perfomance. I let Hardy source the diff parts - they know their stuff and I didn't want to interfere with their usual sources.

If you take out and refit the cage yourself and ship the diff to Hardy you will save significantly on the cost I suggested above.

Good luck.

Robbie

[RichardSEL]

Thanks for the quick reply Robbie...

For my XJ8 Sports diff please see my post in the X308 board:
viewtopic.php?f=89&t=12836