Okay,
I've read the excellent posts by AVOS and Adam699, lots of good info there, but they are far beyond my modest ambition. I have a line on a 2005 4.2 XJR engine. In a perfect world, I'd drop it in and bolt it up, but I know it isn't that easy.
I have made a series of questions I need to answer before I spend the money on the engine.
First, the overall concept is to use everything possible from the 4.2, and leave it stock. I have champagne tastes on a beer budget, so there will be no upgrades here.

Please feel free to chime in and add any questions to the list if I have forgotten anything.

First, what is stopping me from using the whole 4.2, rather than doing all the swapping and mixing and matching of parts?

1. Will the transmission bolt up, using the original torque converter?
2. Are the engine mounts the same design?
3. Will the 4.0 exhaust manifolds fit the 4.2 heads?
4. Will the original fuel pumps supply enough pressure and flow?
5. Can I wire the new injectors into the stock 4.0 wiring, and have them work?
6. What about the knock sensors?
7. Same for cam position sensors.
8. I understand the intercoolers sit too high to clear the hodd. Is this true?
9. If so, and I use the old ones with a spacer, am I restricting the engine, causing a noticeable power loss?
10. How about the accessories, starter, alternator, AC, power steering Any words of wisdom?

Thanks to all that can help.

 

To answer all of your questions - "no" to all...

It would be hard call to plant a 4.2 Non SC engine into a pre-2003 S Type let alone a 4.2 SC motor into a XJR vehicle line...

 

While I appreciate your response, it isn't very helpful. From my research, many of these issues appear possible, but I am looking for technical details as to why one thing or another wouldn't work.
Thanks

 

Rob, I believe my response sufficiently answers your questions - as far as technical details - as already mentioned - forget about dropping a 4.2 SC power plant into the 1998 XJR vehicle line.

I would strongly encourage you to search the forum to seek technical details of what you are asking...

 

1 Yes
2 Yes
3 Yes
4 Yes (for stock setup and 55psi base pressure)
5 Yes
6 Yes (use your original ones)
7 Yes (use your original ones)
8 Obvioulsy you didn't take my word for it, try it I would say
9 No
10 only thing you can use from the 4.2 is the starter

 

Thanks very much AVOS. I asked the inter cooler question because it seems your experience is with XKs rather than XJs.
Your response encourages me to try this. It seems the bigger issues would be the intercoolers and the ancillaries like the alternator.
I have made some measurements under the hood to see how tight it is.

 

Well I went and looked at the car. The measurements I made make me think it just might fit as is. I would be tempted to try it, but I didn't get a good feeling about the car and the whole deal. (The price suddenly went up, for one thing).
Anyway, I have a a question, because I saw something odd on the air intake. Look for what looks like a homemade fix on the bottom, it looks like someone glued a plastic bottle cap on the bottom.

 

At this point I am probably going back to plan A and fix the 4.0, because the finances don't make sense to spend twice the money on this 4.2 from a private party with no recourse if there are problems.
The tipping point is the bottom end of the 4.0. There was some water in the oil, and I am looking for feedback on whether that is likely to have damaged the crank and rod bearings.
If I have to spend another thousand on bearings, then the 4.2 idea resurfaces!
Oh Lord, decisions, decisions.