I have exchanged the 3.9 for a 4.6 in my 90 classic. I want to rechip the ecu. I've been told to use the 4.2 chip. Is there a 4.6 chip or is the 4.2 chip appropriate for this exchange?
I have been around Rovers for a long time, but the only people who are re mapping the fuel software are guys like RPI engineering. They are also only doing it in conjunction with things like hotter cams, and increased air flow ( porting polishing, cold air induction though an oil air filter, and free flow exhaust). Even with all this stuff the HP output is minimal compared to a turbo or supercharged engine chip. These engines are built for low end grunt, you can probably get the best bang for buck improvement with a new air filter, good plugs, wires, clean injectors, and a free flow exhaust than you could with a simple ECU swap.. And to be perfectly honest, I dont think that would work anyway.. Lucas stuff is fickle enough, if you plug in a new map chip it may screw up more than you bargained for... Ohh and remember.. "someone said" does not carry any gurantee or warantee.. Including my 2 cents!
I am assuming ( dangerous) that you used a 4.6 bottom end and put your 3.9 frontcover on, with pump & distributor. What cam did you use, as the 4.6 cam won't drive a distributor gear. I am not sure why you want to use a stock 4.2 chip, as opposed to one writen for 4.6. Are you doing this just because you are still using the 14CUX Lucas box?