3CU ‘upgrade’to what?

Hi all. Mid restoration of a 1981 XJ-S preHE D jetronic. Recently found that my ECU is a 3CU which l am guessing is the first batch, to be followed by 6CU, 16CU, 32CU. If it is recommended to change a 6CU to the 16CU what should l be looking for? Can l jump straight to a 16 or does bore size, compression, sensors etc come into it? TIA Richard. Melb. Australia.

I think you’re stuck with the 3CU. The 6CU and later ECUs are for the Lucas “Digital P” system; I can’t see how they’d work at all on the D-Jetronic system.


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What Doug said. The D Jetronic and the Digital P are completely different EFI systems. You’d have to pretty much convert the whole system over, and then you’d have to concern yourself with trying to use the later fuel maps on the earlier pre-H.E. engine.

A better idea would probably be to convert to Megasquirt or one of the other aftermarket EFI systems. With any luck at all, it’d be feasible to use it with your existing injectors, fuel rail, pressure regulators, etc., and the system would be tunable to work with your specific engine. Still not an easy task, though.

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As mentioned, it is 3CU or nothing or aftermarket EFI system.
I ditched the OEM system in my 1979 pre HE car back in the 80s.
Then I fitted a Melbourne made Motec system, which at that time was pretty much the best system you could buy for a street use car.
Later I changed the engine to HE and kept the Motec system.

Changing to any aftermarket system does need a fair bit of work. It is not rocket science but if you plan to do most of the work yourself it helps to be a very handy DIY character happy to do wiring. Lots of systems can be fitted to the V12, they do not need to have 12 cylinder sequential injection

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In addition to the above systems, you might also look at Speeduino, and RusEFI, both of which have proven to run V12s (Speeduino being the most basic but full open source, and RusEFI being the most varied and capable but 100% diy right down to the board and takes more to implement).

~Paul K.