Dears, I'm hoping you can shed some light on this. With the 4.2 XK engine, can you mix the 9.1 compression ratio US block with the euro head? My understanding is that only the pistons are different.. will the car run just fine? How about the ecu? Need to change that?

 

Since you mentioned the ECU I presume we're talking about the 4.2 as used in the Series III XJ6s, right?

As far as I know the high vs low compression cam only from having different pistons, as you say.

There were several part numbers used for the cylinder heads as used on the Series III cars. I think the only significant difference is "with" or "without" air injection. I am not aware of any differences that would prevent swapping the heads.

Let's hear what other have to say.

Cheers
DD

 

To clarify, the USA (and a few other?) cars had 8:1 compression; the Euro and others had 9:1

What are you trying to do? Go from a high compression engine to a low compression engine? Or other way 'round?

Cheers
DD

 

Thanks DD! So to clarify:
I have a Euro S3 4.2 with a worn block. Found a US doner car with decent block, and am thinking to swap them. The head on mine was freshly redone recently.

Paul

 

The blocks are the same, you just have to swap your higher compression pistons over. Is the US block a "slotted block" or not ? If not, check the block face for cracking between the bores before you shell out good money for it. Cracking between the bores on 7L and 8L blocks is commonplace and only permanently cured by fitting lipped cylinder liners.
Here is an article on block cracking
https://www.jagtas.org.au/torque/tec...-engine-block/
and another
https://forums.jag-lovers.com/t/crac...ssue/354111/19
The "slotted block" was an attempt to cure this cracking when the replacement engine to the XK, the all-aluminium AJ6 engine, was getting ready for sale. It is basically what is called a "spatchcock", a quick fix that will hopefully stop the problem. Well, it did stop the cracking problem, but head gaskets still fail on these later blocks but at much greater mileages.