Hi, I have an XJS V12 automatic convertible pre-facelift model. (Also have a facelift convertible V12 with a five speed manual conversion.) Are the any thoughts here the feasibility of putting a modern six speed automatic sport transmission or similar on a V12 XJS? I have a BMW with a six speed sport/automatic transmission and it is great. Many thanks. Tim

 

Tim, I have looked into this as well. It is very pricey, but a company called Gear Vendors makes a overdrive for the GM Turbo 400 which is in all XJSs pre facelift.
https://www.gearvendors.com/hrgm3s.html
I have found that ANYTHING is possible if you are willing to throw enough dollars and or time at it, but this overdrive unit appears to be the easiest/fastest way to at least go to a 4 speed automatic.

 

http://www.tciauto.com/tc/6x-six-spe...ion-850hphtml/

Another cost no object solution:
Later model facelift V12 (6.0l) were fitted with 4 speed electronically controlled auto transmission GM 4L80E .
There are American companies that will supply a 4L80E transmission modified to have six gears and capable of high power output (850 HP)

 

Tim,

Is your facelift convertible a 6.0ltr or 5.3ltr?

If it's a 6.0ltr car, do you know who did the conversion & which gearbox & bell housing were used?

Regards

A.

 

Hi Guys, Thanks for the replies. Yes, the formula; enough time and money and it can happen. As for the overdrive, I have looked at that but it does not provide the solution that I am looking for, which may in fact not be optimal. I would like to shift between the gears on the great British back roads with a sport type automatic shift mechanism. It puts me somewhere between my three speed auto and five speed manual. But I haven't researched things like final gear ratios and intermediate ratios, for example, on a six speed box, so it may be that putting a six speed sport shift auto box in the V12 might be a waste of time or sub optimal. In any case, always grateful for the thoughts and inputs.

My V12 is a 5.3 ltr. I will get the name of the company that did the conversion.

All the best, Tim

 

TM
The gear ratios and number of gears in modern cars' gearboxes are far more a function of the torque characteristics of the diesels they all have in their ranges than any other factor. As diesels have such a small usable torque range, particularly so when emissions are such a concern, it is essential from the performance and emissions viewpoints to keep the diesel engine in a very small rev-range.
I appreciate your other car may well be a petrol one, but I suspect the same considerations apply to a surprising extent, and also there is the "in the pub" marketing factor to consider with the number of ratios, not to mention box standardisation, cost etc.
With the V12 you have very decent close to peak torque across a wide enough rev range (at least 3,200 to 5,000 RPM) to make 6 speeds unnecessary, at least from the objective performance point of view. Four speed boxes, absolute maximum 5, given proper ratio spacing being plenty from the performance "keep it on the boil" point of view, for the V12.
In my view where the car would really benefit is from the more efficient nature of the modern autobox, compared with the lovely to drive, but old design, of the GM400. A modern box needs an electronic controller, and these are available aftermarket and work. Given that, and assuming you could do the needed machining on the block to fit one; a 4 speed from a facelift might be the most effective route, or a more modern design still based upon that used in the Jaguar facelift 4 speed, might be easiest.
Greg

 

Hi Greg, Thanks for that. Very interesting. This all came about last summer when I went to a wedding in France and hired a car from Hertz in Marseilles (airport) to drive up to Bonnieux. It was a BMW Z8 with an automatic (tiptronic?) transmission with both standard centre shift and paddle shifters, etc. We stayed for a long weekend and drove all around the area. It was so much fun to drive I wondered if the XJS with that configuration would be as much fun. I know I am talking apples to oranges here, very different cars, different suspensions, etc, and twenty years of R&D, but the thought stuck with me. Interestingly, the current XJ, XF, and F Types all offers an 8 speed auto with the petrol engines. Hmmm. Thanks, TM