A different challenge...Not an Xm...but of course, a non-runner.

[Dieselman]

I've started on a new project car, not an Xm this time.

A sneak piccie of the rats nest that awaits. If anyone is interested I'll put more detail up.

WP_20260103_031.jpg

WP_20260106_002.jpg

[Degens]

Now, that looks like an Italian Spaghetti bowl. What sort of car is this?

[Dieselman]

It's a Jaaag XJS with a complicated history.

The issue is it had an engine swap
The car is designed for the later Magneti Marelli ignition system, the replacement engine is designed for the earlier Lucas constant energy system.
The options are to use the Lucas system and rewire the car and fit an associated Efi ecu, or, as is my plan, make the engine work with the more advanced, Marelli ignition system and the rest of the car stay as built.

The second image above shows the crank pulley for the Mareli system, with one of three timing trigger fingers shown.
I have fabricated a bracket to mount a crank sensor and drilled the engine block to be able to fit a flywheel sensor.
The sensors have to fit very close to the trigger fingers and flywheel ring gear (0.48-1mm clearance).

[Degens]

That is a nice car as well, but please stick to mainly XM

[xantia_v6]

An interesting project, I have quite a lot of history with Jaguar V12s and have rebuilt a couple of engines, but I never had one with Marelli ignition.

What is the compression ratio of the engine? Designated by the 2nd to last letter of the engine number. Marelli engines were all 'S' = 11.5:1, but most UK spec Lucas HE engines were 'H' = 12.5:1 and require a different ignition curve and fuelling.

I have all of the components of a Lucas ignition system and a matching ECU.

[Dieselman]

No idea of the compression ratio, but I should be able to find the engine numbers as paperwork for both cars should be available. Where is the engine number located on the block?
I note the Marelli loom is missing the "strategy link", which apparently sets the fuel octane rating between 91Ron, or 95Ron, so presumably the timing is less advanced.

The distributor and associated ignition components for the Lucas system are in the boot of the car and I understand there is also the Efi Ecu, but only the wiring loom fitted directly to the engine was saved. The swap was made 7-8 years ago, but the engine has never been able to run since.

My understanding is the Efi Ecu are different between Lucas and Marelli as the earlier one uses an analogue sine wave from the distributor as the speed trigger, the later one receives a digital pulse from the ignition Ecu, but I'm not sure that matters given afaik, both use the same 6CU Efi Ecu and the wiring diagram shows both systems to be wired the same, not withstanding the Lucas system uses a shielded co-ax cable as the speed pulse wire to Efi pin 18.

89 XJS EFi Electrical.pdf

I've just read a link https://www.jag-lovers.org/xk-lovers/li ... umbers.htm
which has a note
"Note that for V12s 1971 through 1979, LA and LB suffix was 7.8:1 CR and SA and SB suffix was 9:1 CR. For 1980, suffix HA was 10:1 CR. From 1981 ?? suffix was 11.5:1 CR and ?? suffix was 12.5:1 CR."

The car is My 1989 and iirc, the engine was from a 1985 car.

[xantia_v6]

The engine number is stamped on top of the block just in front of the bell housing, behind the oil pressure sensors.

A 1985 UK-market engine is sure to be 12.5:1, they are marginal for pre-ignition running with any fuel less than 98 RON. Be aware that they can suffer from 'silent' pre-ignition which erodes the cylinder heads without any audible pinking. I have run one (a 1983 XJ-S) on 95 RON with original Lucas ignition, but it was not driven hard, and I probably retarded the ignition by a couple of degrees.

The ECU model was updated from 6CU to 16CU before 1989, but the ECUs are specific to emission control version (with or without O2 sensors), and compression ratio. I believe that 6CU and 16CU are interchangeable if engine specification is observed. I had a NOS Lucas injection harness (the one that runs the length of the car), but sold it last year.

I think that maybe the Marelli ignition gives a different number of pulses per rev than the Lucas system, but I don't remember now.

Kirby's book is the best source of information, I contributed a few things to it back in the day...