Apart from the dial selector, what upgrades or improvements do the transmissions have that are in the 5.0 compare to the 4.2? Is it the same gearbox just beefed up? An entirely different trans? The reason I ask is Im hoping to buy a xk in the next 2 years and dont want to get another car that is prone to auto trans problems: leaks, clunks, slamming into gear. Ive had a lot of bad luck and would hope the xk trans are robust.

 

I had a 2008 4.2, a 2012 5.0 (both XFs, rotary selector), and a 2011 5.0 Supercharged (rotary selector) - as far as I can tell the transmissions are identical, though I think the XKR might sometimes start in first whereas the XFs always started in second. It is a lovely box, smooth and with great ratios.

 

IRIC the 4.2 has the ZF6HP26 transmission and the 5.0 has the ZF6HP28.


The 28 supposedly shifts faster.

 

Frizzle is correct on the model numbers for the transmissions. The 5.0 cars have a winter mode which affects gear selection and torque delivery to better suit low-grip conditions. Both have Sport modes which lift the torque limit, allowing the car to rev higher and will lock the torque converter more readily, making for a more sporty experience. Both have paddle shift.

I haven't driven a 5.0 XK(R) but the 'box in my 4.2 XKR is a joy to use. Silky smooth when you want to waft along and responsive and predictable when you want to play. I expect the item in the 5.0 cars will be an improvement but I have no complaints about the one in my own car.

They're not known to be unreliable either, stories of things going wrong with them are few and far between, and often attributable to poor battery condition; this can influence shift comfort and speed for a start. In terms of maintenance, it is worth changing the fluid and filter every 60k miles or so, for peace of mind.

 

A problem that I believe is underreported on 4.2 models is that when using paddle shift mode, some sort of computer fault can happen, disabling the revmatching feature for downshifts until the car is restarted.

It may be battery related, but it has happened to me a few times even since I started using a smart charger.

 

The 6HP28 'box in the 5.0 has a few significant advances over the 6HP26. Things I can recall off the top of my head:

• Faster gear changes (200ms vs. up to 800ms with the 26)
• That is partly because it can skip gears rather than having to step through them in sequence before releasing the relevant clutch - so it can go straight from 6th to 2nd if that's what the situation requires
• The torque converter is faster to lock up, so that more time is spent with the converter locked and therefore not draining power from the system

 

I'm not convinced; I use my paddle shift a lot and haven't found any link between that happening and the twice I found the downshifts stopped rev-matching. IIRC both times were on my old battery, haven't noticed it since I installed an AGM type last year. I've had other electrical things stop happening since changing the battery too so I think it's more commonly battery related, rather than through use of the paddles.

 

Having had an 08 xk vert and now having a 12 xk vert; I can attest to the differences in the 12 vs the 08. IMHO 2010 and later xks are far different than the 2009 and earlier xks.

First off the direct injected 5.0 engine with 385 bhp is very much like the earlier xkrs. The new transmission is tighter and more responsive than the older transmission and offers more driving modes as noted above. In addition the suspension seems to be tighter and more responsive.

Don't get me wrong the 07 - 09 xks are wonderful cars, but the 10+ seem to me to be that much better; particularly where the drive train is involved.