The V8 Owners Forum -

I've just acquired a 3.9 intermediate engine from a '94 Discovery with the complete efi system attached. My plan is to swap the hotwire efi onto my carb'd Rover SD1, I've found a couple of 3.5 ecu's from the early Discovery if the 3.9 ecu isn't compatable with the smaller capacity, but the question I have is how important are the lambda sensors?

I have the multiplugs on the end of the looms, but not the sensors as the exhaust wasn't included. I've read about changing a resistor to change the spec of the efi from lambda with cat to without. I'm not interested in outright power, but more economy so was thinking that plumbing the lambda's back in is the way to go. Would it work fine without in the meantime?

I think (but am not 100% sure), that if you omit the lambda sensors, you have to set the tune resistor accordingly. Then when you do fit the lambda sensors, all you have to do is switch the resistor over to the correct one.

I would go for the 3.5 ECU rather than the 3.9. Although the difference might be hard to detect!

Chris.

Thanks, that's pretty much what I've read. What I don't know is if there's any advantages/disadvantages to going either way, apart from the cost of buying two lambda sensors?

The main plan is to go lpg, so I'll be happy if the hotwire runs well, if not at 100%. From what I've read, leaving the lambda sensors off and not changing the tune resistor sets the ecu into limp mode, which I also don't know how that effects the running/efficiency of the system?

The purpose of a narrowband lambda sensor is only to control the mixture to within limits which won't damage a catalytic converter. If you're not using a cat there is nothing to be gained at sticking to stoichiometric - indeed for maximum power and economy you'd do best quite some way from this.

So what you need is an ECU etc from an engine not fitted with a cat. That assumes your engine (and injection bits) is exactly the same as that engine. Later engines after cats become the norm may have different ignition mapping or valve timing etc.

Thanks for that, seems to be what I was reading, lambda's are only to protect the cat and don't really offer any advantages apart from that. Presumably the 3.5 hotwire efi would be early enough to be pre-cat so I think that would be the route to take