I cant think of a reason to fit a T56, too much cutting/mods to make it fit. And 4-5 are both overdrives. In order to get the shifter through the existing openings, you need a shifter location of about 23- 24 inches from the block using standard GM input shaft. The T56 will be way past that, probably getting close to the window switches. TKX is a good choice.

 

tkx is an okay transmission. It can't hold a candle to a T56 though. The primary disadvantage to the TKX is that it's a more crude transmission and it's kind of noisy. That really wouldn't be an issue is most cases because the tkx is designed to kind of be a muscle car transmission where noise and lack of refinement wouldn't seem all that out of place. But in a Jaguar v12 it just would not work as well as a T56.

There is almost no reason not to use a T56 because when you factor in the cost of a hydraulic throwout bearing for the TKX it starts to approach the cost of a T56. For some reason the hydraulic throwout bearing for a T56 is 50 bucks but costs hundreds for a TKX. No doubt that the T56 is a larger transmission and does require some cutting. The TKX can be fit with no cutting. And would be a good fit if you were building a Chevy Conversion Resto mod Jaguar where you needed extra space for large exhaust pipes but refinement wasnt a consideration. Other than that I really can't think of a good reason to use a TKX.

I have personally built many Jaguar manual conversions. with both the T56 and the tkx. At the present time I have two in my possession with a tkx and one with a T56 so I'm talking from 100% experience. On one of my more refined projects where I use the TKX I had to spend hundreds of dollars and do countless hours of noise mitigation techniques such as a rubber bushed shifter and thick sound deadening insulation over the entire tunnel. Also the retro fit hydraulic throwout bearings choices for the TKX are not great designs.

I would think long and hard before I would use another TKX.

 

going to have to disagree with you, i just installed a tkx in mine. No cutting at all, lines up where it should, even works with the factory manual tunnel cover. Shifting is extremely precise, admittedly almost too race car like but I like it. 2nd gear is a little loud on deceleration but by no means is it noisy.

why would you want to hack up the tunnel? and double overdrive is useless on a stock output v12.
TKX is arguably the best aftermarket 5 speed tou can buy right now.

The TKX was only released in 2020, and supply was scarce out of the gate. How many have you installed? I dont know of anyone other than myself. Regarding cost, i dont consider this a deciding factor as transmission swaps in old jags is never a sound financial decision lol

 

Right... they make noise, that's what I was referring to so you're not disagreeing with me. The noise is especially noticeable on decel.

No need to "hack up the tunnel" Many people have good metal working skills. When I'm done, the tunnel is far from being hack'd up. Reshaped, yes, hacked...no. A car is either origional or it's not. Metal work is either well done or it's not. Both of these conditions are with in our control.

I have 2 TKX projects done and in my possession at the moment and they both make gear noise on decel in the first 3 gears and some gear chatter is transmitted through the shifter on hard acceleration. The T56 doesn't do either of those... I've done two T56's and two TKX's I've got two more projects on tap... One is an XJ12C... All supporting parts for the T56 are at hand already becasue there is ZERO chance I'd build another refined V12 and then have to contend with all the noise and noise mitigating after treatments, it's just easier for me to use a T56 from the jump and get the other benefit of a better Hydraulic TOB setup too. The other project is a far from original XKE in the planning stages and the XKE may have to get the TKX strictly based on the size of passenger compartment but I'm hoping an improved version of the TKX will be available at some point to make path forward more clear.

T56 is a much more refined transmission, quiet with very precise shifting. The fact that gears 5 and 6 are redundant is not the only consideration. If you had the opportunity to drive a T56 / TKX back to back like I can you may appreciate my position. Dont misunderstand me, TKX is still a 'very good' transmission and hits it's target use case, muscle cars, dead on and where gear chatter and decel whine might be considered charming or unnoticed over a loud exhaust. Compared back to back, the T56 is much better transmission over all.

If you're satisfied with the TKX, that's all that really matters and I could see that being the case. In my case, having a different set of experiences....all things considered.... I'd rather do 2 hours of metal work on the tunnel 1 time than have gear whine every drive. That said the rubber isolated shifter and 1 inch thick sound blanket installed on the underside of the tunnel and then sealed with seam sealer did go a long way to making the noise far less harsh and just barely noticeable. That noise management endeavor was costly and I'd rather not have needed to do that in the first place.

 

But why did you say TKX requires modifications to the tunnel? Mine didn’t, mates up to OEM manual console perfectly. What bell housing did you use. I was convinced mine was the first in an xjs, my thunder has been stolen lol.

 

No-one stole your thunder on the TKX and Any manual in an XJS is a fantastic upgraded and very unique driving experience. I didn't put an TKX in and XJS. I did put a T56 magnum in about 4 years ago. I put two TKX in to XJ coups which do share a very similar floor pan with an XJS. Like I mentioned the next XJC will have a V12 and a T56, all things considered.

As for the metal work, I have a welder and I like to use it! So everything gets metal work whether it needs it or not. LOL. Seriously I like to seal the shifter opening with a proper boot and that boot requires a 4 X 5 flat and sturdy flange that gets fabricated out of a nice 3/16" piece and worked in to the tunnel.

 

Ahhh, ok.

this is how i sealed mine. Have added metal bracket and 4 screws as glue alone was t holding. Also, used some foam around the base before leather boot.

 

If you want to DIY. and have access to a drill press and a way to cut aluminum plate. This is an easy morning way to adapt a Chevy Bellhousing to a Jaguar V12. Seriously the first time I did it it only took 4 hours. It’s perfectly aligned because the Jag and Chevy alignment holes are exactly in the right place. The rest of the bellhousing bolt holes are for clamping purposes and they can be a little off without a problem. Doubt me? Stick the right bolts in the bellhousing. loose huh?
Ask I’ll walk you through it. I use these for racing

 

This pict makes me happy in some ways. Because I know how GREAT the V12 manual experience is. Jag "should' have made the 5 speed an option. Considering the time that the XJS was on sale and the target market, I'm sure few would have check'd the manual box on their order sheet. But a select few would have and those cars would have been special and far more valuable today. That would've help'd maintain more interest in the brand. I have 6 manual cars in my possession at the moment and 2 of them are Jag v12's. I can honestly say there is nothing like a V12 manual. All the other manual car I have are more powerful and some are much more powerful and that's nice but the V12 manual experience is special and unique. The idea that a person could buy a decent XJS and convert it to a manual and be all in for 25k is something more auto enthusiasts could avail themselves of and I think they should. It's worth it. Myself and others have paid far more for less. That said, not every conversion is great by default, I personally have driven 1 that was awfully done and had to redo another to make it right.


The Jag XJS V12 manual is the greatest GT car that was never built.
So in that sence I salute you and the others who've endeavored in this direction.