4 speed auto conversion for V12 saloons

[mike020150]

This weekend, I have been busy removing the 3 speed GM auto box from my V12 coupe and toying with the idea of replacing it with a 4 speed conversion kit (possibly a ZF). Needless to say, there would have to be a major domestic capital approval process hurdle to overcome first but at this stage, it's still on the option list - just. Has anyone any experience of the kits that would assist the assessment of pros and cons? Due to low annual mileages, an mpg improvement would be useful rather than critical to the case. I quite fancy improved acceleration as well as cruising at lower revs so I'll probably need to factor in the cost of refurbishing a 3.54:1 LSD I have spare. 3.54:1 should give improved acceleration compared with my current 3.07:1 and still permit an additional 3.64 mph/1000 revs in top (2383revs at 70mph instead of 2829 for the ZF HP 220 box). It's tempting. Mike.

[steve_m]

Let me know how you get on Mike as I would like to do something similar to my XJSC; it's a while since I've had a V12 and I can't believe it is so slow. Of course, it could be just knackered I suppose.
Steve

[mike020150]

I managed to crack the domestic 'capital approvals process' and I've now ordered a 4 speed ZF HP 22 conversion. There will be quite a delay before I get the box because it will be built to order incorporating many HP 24 bits to cope with the V12's torque. I'm really looking forward to the outcome even if I'll never be able to justify the expense by way of the car's future resale value. I'll post some comments when it's up and running. Mike.

[Meonstoke]

Hi Mike, I saw your post from a few years back regarding changing your 3 speed auto to a 4 speed auto. Did you actually go through with this idea, and if so can you advise on the end results and your conclusions. I have a similar dilemma. Thanks.
Rgds,
M.

[J44EAG]

Harry Metcalfe is former ex-EVO magazines Editor. Quite a glorious character. Hope to meet him one day.

Harry has an XJC 5.3 which he bought for "a hoot"! It has a manual gearbox. Here is the link to the Youtube video. This is Part 1 before he did further mods to the car and filmed a Part 2. It is also on Youtube but not so prominent. You might have to look further for that.https://www.google.co.uk/url?sa=t&rct=j ... DPOpzCo8sJ

Just thought I`d add this as V12 gearboxes are being discussed here.

Mike K

[mike020150]

Hi,
Yes, I did make the change and after nearly 6 years, I can confirm it was a good way to go to retain and improve on the original Jaguar effortless performance feeling whilst gaining a little fuel economy. To get it right, you need to make some decisions about gearing. The Jaguar X300 ZF gearboxes have almost identical 1st, 2nd and 3rd gear ratios as the original GM and BW gearboxes used behind the V12. Hence, if you simply fit the later box, 4th gear is an extremely long legged overdrive ratio. I tried that set-up first and didn't like it because the V12 doesn't have massive torque below 2000 revs where it's likely to spend a lot of main road cruising time. 70mph motorway cruising was more economical but the car had lost too much of its performance edge with the original 3.07:1 differential still fitted, making the car feel sluggish. A much better compromise can be achieved by using a 3.54:1 diff ratio in conjunction with the ZF gearbox. The resultant gearing gives a better spread of ratios (slightly lower overall gearing in the lower gears and around a 20% reduction in revs cruising in 4th gear), incidentally, bringing the gearing very close to that which Jaguar adopted for 4 speed auto V12s. In case it isn't immediately obvious why you can't just use the GM 4 speed unit which Jaguar used, it's because the early cars don't have all the necessary electronic inputs to allow the GM 4 speed units to work. The basic hydraulically operated ZF box (from the 3.2 X300) isn't strong enough for a V12's torque. the box needs to be rebuilt with the stronger internals from the 4.0 litre electronic box (which is what Graham Whitehouse does as well as providing the necessary adaptor plate in his kit). Alternatively, some US specialists offer a cheaper conversion using a 4 speed auto box not commonly available in the UK but which is widely available in the States. However, the lower gears have very low ratios, leaving an even bigger jump to 4th gear. Consequently, a V12 can feel as if it's alternating between lots of revs and practically no revs. You pay your money and take your choice! Mike.

[Meonstoke]

Hi Mike,
Thanks for the update, much appreciated. Good to know that the Graham Whitehouse solution worked out well. By coincidence, I did contact GW autos recently however the quote was approx 10K GBP which is close to what the car is currently worth. In that respect, probably not the most sensible/economic route. But good to know that this option exists and it works. (I agree with you ref: the US solutions).

M.