5 speed auto suitable for V12s

[mike020150]

This is more of an idle thought, wet Sunday afternoon question, rather than the usual panic request for help to get a car back together. As Jaguar moves towards 8 speed autos and ZF announces a 9 speed version, I feel even more deeply the limitations of my coupe's 3 speed GM 400. The 4 speed mechanical ZF unit from the 3.2 X300 can be fitted but, although undoubtedly an improvement, it still seems inadequate today and it isn't a particularly cheap upgrade. Has anyone come across a suitable non electronic 5 (or even 6) speed box which would fit the bill? To cope with V12 torque I'd guess that it would probably need to come from a large engined US machine, preferably one with good spares support and an easily modified bell housing arrangement! A couple of extra gears and perhaps a tweak of the diff ratio (from 3.07 to 3.31) would transform the performance from rest as well as moving the mpg closer to a more acceptable 20. Or am I spending too much time dreaming?

[piman]

Hello Mike,

not being familiar with your model Jaguar, I can't answer your question, but this is a general comment on gearing in general.
There seems to be a trend with modern vehicles to have greater numbers of gears which either indicates a marketing ploy (as in my car's got more gears than yours) or the engines are putting out more specific horse power per given capacity therefore making the engine less tractable and means that to get the performance it's 'rowed along by the gearlever' which doesn't sound particularly useful in modern road conditions. (OK for an automatic I suppose but at what complexity and cost?)
As the V12 is large and has ample torque the current 3 speed automatic would be difficult to improve on, unless you do want to convert to manual and lower the final drive for acceleration while retaining a reasonable high geared fifth speed to improve fuel economy.

Alec

[mike020150]

Hi Alec,
Your final few words sum up nicely the problems of the old 3 speed unit. Despite being very torquey, even the V12 with only 3 gears has an insufficient spread of ratios to achieve snappy performance away from rest and reasonable economy at cruising speeds. Of course, changing the diff ratio would be an easy way of satisfying one but not both conditions. I'm not normally keen to appear to defend the marketeers but modern cars have a tough job satisfying the ever increasingly stringent emission regulations. More gears are almost obligatory in order to maintain engine revs within the narrow operating band required for satisfactory performances during emission test cycles.
I imagine the most common upgrade box for the V12 is the ZF 4HP 22 (often with some stronger 4HP 24 internals). It's mechanical rather than electronic and has a ratio spread of 3.74 compared with the GM's 2.48, a significant improvement. However, the conversion is far from cheap for the addition of a single overdrive gear, in addition to which, there remain large ratio gaps which detract from smooth progress. Never mind, perhaps it will stop raining soon and I will be able to get on with more pressings tasks for the Mot test! Mike.

[ptjs1]

Mike,

As you say, I suspect that the trend to 6,7 and now 8 speed gearboxes is about raising the ratios for optimum emissions and consumption. As you say, to do this and hold the engine within the desirable rev range requires more ratios.

Interestingly the 4-speed GM4L80-E electronic gearbox that was used in the late 6-litre XJSs locked out 1st gear and started from rest in 2nd gear, unless used in Sport mode. As the box can handle over 450 ftlb of torque I wonder if this was more to do with minimising the number of upshifts in normal driving conditions. Certainly, one couldn't accuse the car of being sluggish from rest even when starting in 2nd! I suspect most owners don't even know that they are starting in 2nd gear most of the time!

Would it be prohibitively expensive to fit this electronic 4-speed to your coupe? As with the XJS, I think its characteristics would suit the car well.

Paul

[mike020150]

Hi Paul,

Sadly, this seemingly obvious answer won't work for a couple of reasons. The GM4L80-E needs electronic input which my old V12 can't provide from its rudimentary engine management system and I believe the rear of the 6 litre block/drive plate was altered so what might appear to be a bolt-on option just isn't. For a long time, I did harbour envious thoughts about this box until I dug a little deeper. Mike.