Bill de Creeft wrote:
I’m just putting in a word for the transmission originally installed.
Considering that it cruises about 1800 to 2,000 at about seventy mph, and that 3,000 is a good healthy rpm for the v12, and that the redline is at least 6,000, and that 4,000 and above is really fun (and not legal anywhere, it seems like),…then we are only talking about economy!
I think someone is choosing the wrong tool for the job.
The biggest problem is that putt-ing around below 2,000 is where the problems in the HE head engine come from…causing the need for high speed runs to clean it out…and a jag V12 run between 3 and 4 thousand rpm, mostly, will need only to get onto the road more often…Drive it ‘Con Brio’ and don’t bother with Italian Tune-Ups, I suggest.
Money spent on fuel beats money spent on cleaning out carbon from choked up engines.
One cannot really disagree with that, Bill…but…but…
We are of course talking about ‘old’ technology on the fuel management
side - in principle the engines, fuellling well managed, should not soot
up, but they do. But also; the rpms/speed is not necessarily related to
the number of gears. That this is the case with ‘our’ readily available
options, the 4th gear being on overgear, is in your context
‘unfortunate’ - aiming specifically at economy and disregarding engine
Modern 6-speeds are of course paired off with advance fuel control
systems, more able to keep the engine clean. And also; they do not
necessarily go to top gear at the drop of a hat like ‘ours’. The gears
are more closely spaced to allow a better match between the power
required from the engine, rpms, for the driving demands - which
undoubtedly is ‘good’. 6-speed will outdrag a three speed with the smae
The V12 has of course the torque required for the three gears
available, it would make out even with two :), but the shift points
also tends towards lazy rpms at moderate speeds - or even a bit lower.
The fourth gear on the boxes available isn’t all that much higher - the
‘problem’ is more that the shift points are so designed for excessive
use of it. Which of course is eliminated by a manual box - some swear by
the 5 gear manual, giving the driver a wider choice…and at will…
One can certainly counter this by simply select ‘3’ or whatever - it
will keep the engine clean and happy. The ‘italian tune-up’ is not
really about high road speed, but high rpms - which is readily availabel
by manual gear selection. Cruising at 60 mph locked in ‘2’ will
certainly have the desired effect - but will play havoc with your
I notice on list that there are people preferring diff ratios for better
acceleration, and others who go for cruise economy. the latter accepting
the drawbacks you outline - or the fines involved. There is really no
‘right’ or ‘wrong’ in all this - they are personal preferences, with the
consequences to be accepted with the choices…
With options open I would go for more gears, but will certainly not use
money to convert from my well working (touch wood) BW66. But dawdling
remains dawdling - with associated sooting, unless the driver changes
xj6 85 Sov Europe (UK/NZ)>In my opinion, of course.
It’s a great car in snow, slush, crawling on rain-slick ice, and at 100 mph plus, on graded snow way below Zero in any language, and 125 - 145 mph on good pavement and good tires…(if there is anywhere you can find to enjoy that in this crowded & over-regulated world off the race track)
And I have never wished for a higher gear or for a lower one…but for me it’s for grand touring; long distances through open country at high speeds…and no slouch zinging down the Interstate…I wish for higher speed limits, not lower engine speeds.
GM 400 is cheaper to overhaul, “bullet proof” and as adapted for use by Rolls -Royce, Ferrari, Land Rover, etc., I think., as well as the V12 Jag, is a good tranny, and can be tuned up to be more responsive if desired, in the ECU as I recall…‘AJ’ comes to mind.
Marko should be sure, if he sticks with the GM 400 to be sure it meets the Jag 12 specs…and same for the torque converter; it’s
Don’t mean you are wrong, Frank…but I’m just focusing on the intent of the Builder and the Owner…
When ‘they’ built the car ‘they’ were aiming at 27 mpg at 55mph…I suppose they got that; I just wouldn’t know, didn’t buy it for that !!
(grin…yellow teeth and sparse white hair but still grinning!)
On Dec 21, 2010, at 10:29 PM, Frank Andersen wrote:
In reply to a message from Frank Andersen sent Mon 20 Dec 2010:
Thanks for the advice. It sounds like it is indeed time for a rebuild. I was thinking about picking up a second hand box, but I think best to rebuild the one I’ve got as a second hand one probably isn’t going to be any better.
It’s the devil you know, Marko…
I’m not at all sure that ‘shift - kits’ has any longevity relation - I rather doubt it. Altering shifting generally means tinkering with the valve body for some specific effect - so you better be clear as to what you want. The Jaguar did all this for us - arriving at some result considered suitable for the engine/body combination…and perceived Jaguar driver preferences. They likely also added longevity as a factor, though that is within the driver’s box treatment domain as well…
I doubt if they will guarantee that the kit fitted will make the box last longer - and if they do I’d like their rationale for so stating…
The real drawback on the GM 400, and indeed the 6’s BWs, is too few gears…
I would like to look into firming up the shifts to help the transmission last longer. Do yo know a shift kit that would do the job ? I just fitted a Transgo full manual shift kit to my XJS with the 4speed auto box so I guess it would be a similar kit to that.
The archives and FAQ will answer many queries on the XJ series…
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