I happened across another Lucas ECU on Ebay. This one was being sold as an adjustable unit, having two externally-accessible potentiometers fitted. I'd seen it before on Ebay: the guy who bought it also got the airflow meter with it but evidently decided to split them for resale. Initially I contacted him and proposed that if I sent him a few quid, could he could open the unit and tell me which calibration resistors had been replaced by the pots; but although he promised to do so he never did, merely advising me that the ECU would be re-advertised.
In due course it was so I bought it, and someone else bought the AFM. The only shame is that the ECU and its (I presume) matching AFM were split, but I digress...
I duly whipped the covers off and found... an extra circuit board. One of the pots does indeed replace one of the fixed resistors whilst the other is connected the the extra PCB. I prised the PCB out and found it had the name 'Sprintex' etched on the track side. Sprintex of course make superchargers. The ECU case had 'JE 101 cam' and 'Range Rover 4.2' scrawled on it, along with a registration number. The interesting thing was that the production dates on the ICs on the 'piggyback PCB' were almost the same as those of the main Ferranti ICs used in the ECU, suggesting that the modification was carried out when the ECU was new or almost new.
I mentioned this on a well-known car forum and someone pointed out that the JE 101 cam is in fact a mild hydraulic camshaft, hardly the sort of thing to be found in a tuned engine. Then one enterprising chap dug out some old magazines and discovered that JE Engineering were involved with building supercharged Range Rovers for the Paris-Dakar rally around 1989, the dates on the ECU.... and the registration number checked out (on the RAC website, sadly the facility has been removed) as one of a batch of R-Rs that were registered at the same time. The mild cam would be more suitable on a supercharged engine, or so said internet experts.
So, it seems I'd bought something of a 'special' in the world of Lucas ECUs. I tested it on the 390SE and the car does actually run, but without a proper road test I can't comment further.
The inside of this ECU looks like this:
As to how the piggyback PCB works, well I have traced the circuit (of course!) and it appears to sample the engine rpm and then effectively parallels a couple of the sensor inputs to the ECU, so I guess there's a bit of jiggery-pokery going on... ;)
You recall I mentioned over-boring the throttle body... or more specifically, using a larger bore TB from another car...? Well I bought one! The standard TB bore is 65mm, most of the overbores I've heard of are 72mm with a couple of 74/75mm, but having looked at the amount of metal around the TB on the Rover plenum I can't quite see how they do it. As it happens there's a Jaguar TB with a 72mm bore and an identical throttle pot to the non-hotwire Rover setup ( later versions have a different design of pot but the resistance is the same):-
As you see, the Jag TB is a 4-bolt flange mounting unit. All I needed, then, was a matching flange on the Rover plenum in place of the existing TB and Robert would be your mother's sibling! I decided it was time to sacrifice a plenum to the god of swarf:
Next I cut a piece of 12mm alloy plate into a rectangle of approximately the right dimensions, marked it up using the Jag TB as a guide and set to work with a milling cutter (see, I knew that rotary table would come in useful!):
Eventually, once I'd sorted out the mains supply to the workshop to allow me to run the TIG set at full power, I was able to align the plenum and TB adapter and set to with Argon and rod...
Yes, I know it's not pretty, I never claimed to be a welder! It'll tidy up later: for now I just want it strong and airtight... however with a quick bead-blast and the TB fitted it looks rather like this (note I also tidied-up some of the casting imperfections with a die grinder):
Unfortunately, that's as far as I got. I'd rather made up my mind to sell the TVR as well as the Esprit so decided I'd have an Ebay clearout of some odds and ends, and the modified plenum/ throttle body was one casualty. It may be still out there somewhere, although I suspect the buyer only wanted the throttle body and probably threw the plenum away... and needless to say I didn't sell the TVR!