I have a '94 XJ12 that is complete and runs fantastic but the body is done. Has anyone here swapped the later 6.0 motor and 4L80E transmission into an early 5.3 turbo 400 car? I am contemplating buying a 1985 XJS that is missing it's original motor and trans. I like the idea of having the overdrive but know I will have to use the ECM and wiring loom out of the 94 XJ12. Do the transmission crossmembers interchange between the two body types? I am assuming the motor mounts are the same on the 6.0 so fitment of the motor should not be a problem. Any advice? Would I be opening a real can of worms?

 

This type of swap is very doable, however why not just buy later 6.0 XJS and start from there. Perhaps one with a engine or transmission issue. I could understand the effort if... perhaps this combo was never made and intriguing to you. You will need many unique parts that are not easy to find and you probably wont be able to achieve an OE level of integration with out a complete donor car.

 

I do have a complete XJ12 and it runs well. I am not that experienced with these cars but have 30 plus years as a mechanic. The particular 1985 XJS is close by and cheap and looks to be in good condition. It comes with a suncoast conversion kit for the GM V8 so someone thought it was worthy of fixing up. I am sure I can make everything work but don't have the other car for comparison. I kniw I must swap out the shifter as the 4L80E is not a cable shifted trans like the TH400 is. I am on the hunt for a later body but curious if anyone has successfully done this swap before.

 

Baxtor has done this swap. I do know he had to massage the transmission and tunnel to get the transmission to fit.

The 6.0L V12 will slip right in with no problems. The 4L80 is cable shifted and could be matted to the XJS shifter. The additional torque of the 6.0L is well worth the effort of swapping.

There are a couple of differences to your 85 V12.

- Transmission bolt pattern is different, 6.0L has TH400 pads but they are not drilled so TH400 will not bolt to the 6.0L block without drilling and tapping these holes.

- Ignition is different the early 6.0L's used Marelli and the later ones dual Ford EDIS these are identifiable by having 2x 6 coil packs and a Denso ECU.

- The 6.0L also gets rid of the troublesome rope rear main seal in favor of a 1 piece lip seal. My 5.3L has the lip seal and it is about the only place my engine does not leak oil.

 

l have that combo in my 1989 XJRS. Works fine but gearbox casing or trans tunnel needs some slight mods to fit. Motor runs original marelli ignition and injection setup is also original with upgraded ecu.
Gearbox is controlled via aftermarket TCU.
Not a weekend job but very doable and rewarding.

 

Thanks for the feedback. I have the complete 94 XJ12 so I can retrofit any parts needed. Do the rear subframes interchange between the '94 XJ12 and a '85 XJS? My car has the outboard rear brakes that I think are cool. It is lot equipped with an LSD rear though so I would want to swap out the differential from the XJS.

 

No the rear subframes are NOT interchangeable as a unit. Some parts are. You could use the driveshafts to convert the IB to OB brakes with some machining of the XJS LCA's and park brake fabrication.

I fitted 3.58 LSD centre from an XJ40 into my XJS diff

 

I read that all XJS diffs were LSD is this not true? My XJ12 has a 3.54 open diff.

 

As far as I am aware yes all XJS diffs were LSD at least in our market. They had ratios - 3.31, 3.07 2.88 and 3.45 depending on year and engine.

 

Hey Baxtor did you have to change your driveshaft to the later style with the 4L80E?

 

Simply shortened the shaft. (about 74mm from memory)