Disco Motronic ECU differences
There are quite a few differences between the NAS (North American Specification) and ROW (Rest Of World) 4.0 and 4.6 Discoveries, and P38 Range Rovers.
I have studied them all in great detail for hundreds of hours in order to be able to reprogram them (which I have been doing successfully for over a year now), so the differences will be of interest. I don't want to be accused of promoting my business here, so I'm just going to stick to providing system information or responding to specific non-sales questions here.
The 4.6 Disco II was only sold in the NAS market, and it was fitted with the high-compression version of the engine. All of these vehicles had SAI (Secondary Air Injection) without exception. They were also fitted with a larger AFM (Air Flow Meter) akin to the P38, so they also had a different air filter housing and AFM to plenum trunk to accommodate the physical difference. Therefore putting a 4.6 ECU onto a 4.0 vehicle will not work properly, unless you use the larger AFM and associated parts also.
Using an SAI ECU in a non-SAI vehicle will cause it to put the CEL (Check Engine Lamp) on, although it doesn't always seem to cause them to go into limp-home mode.
There are three part numbers for the hardware versions of the the Disco Motronic ECU, but dozens of different tunes that are loaded into them. The ECU contains only one tune, specifically for the vehicle it was originally programmed for. It also contains the history, programming, and security information for that specific vehicle. Every ECU carries a label from the factory with the VIN number and software number that it was loaded with, unless it is a replacement part.
Note that it is not possible to implant a non-SAI tune into an SAI ECU or vice-versa, since there are enough hardware differences to stop it working. It runs, but really badly with lots of faults flagged. That is quite a pity, although I am continuing my studies to see if that can be overcome. Obviously it will never be possible to put an SAI tune into a non-SAI ECU (and who would want to?) since there are quite a few components not installed.
However it is quite straightforward to reprogram a 4.0 Litre non-SAI ECU with a 4.6 tune for the standard AFM, or even larger engines of the type we see over here (4.8 and 5.0 Litre are well known).
It seems that when BMW owned LR their marketing weasels deliberately nobbled the Disco against the P38 when tuning it, to prevent it being as quick. To that end they knocked a massive hole in the mid-range ignition timing, and it runs Lambda 1.0 across nearly the entire operating range. It does run open-loop at high load - it's just that the target is still Lambda 1.0 apart from the highest load and rev range. The P38 does feature a richer mixture under high load, but again the timing is not optimal.
These engines often pink or detonate because they have too lean a mixture, rather than just being an ignition timing issue. A colder grade of spark plug is good too.
