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3.5 to 3.9 auto Conversion


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Hi all, back again :unsure:

So we pulled a nice running 3.9 V8 Auto box out of a range rover classic a few weekends ago and just looking for some advice that i havent been able to find with a search or on other forums.

First off is the Cats. Obviously we dont want the cats so with the removal of these ive read that the tune settings resistor needs changed to non cat mode.

Now i read that this setting does not require lambda sensors at all is this true? It would save the time installing ports into our tubular manifolds.

Also what size of resistor sets the ecu to the correct tune?

Next is the fuel, reading that near to 30Psi is required i found this pump on gbay but think its too good a price? am i missing something :unsure: ?


Okay, thanks for reading to this point, its much appreciated. Last thing i need answers for is the oil coolers. Do we strictly need an engine oil cooler for a challenge spec truck? I thought for long motorway driving in the range rover the oil would get hot but not just puddling around an off road site?

And for the auto oil cooler I was thinking along the lines of something like This But will it need a fan on it? We have the radiator on top of the current 3.5 and wish to keep it there so could it just be placed next to our winch? <_<

Finally id just like to say thanks in advance :D

Much appreciated

William Arthur

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There are a few different tune resistors used, cat, non-cat, different country emission spec etc.

Very easy to change once you have found it on the loom (it's normally all taped up). Mine was close to the ECU (Disco 3.9). It's on a nice 2-pin plug. As I understand it the ECU will switch to the relevant software map according to which resistor is fitted. A bit like having dip switches I suppose.

Now I think you want the green one. It's either that or the white one, I can't remember which one I removed and fitted, but I definitely had a green and a white one about. The one I removed is in my garage, I can have a look what colour it is later on.

Alternatively, Microcat will confirm which one you need, I don't have immediate access.

As for oil coolers, remember that off-road work can be very hard work for an engine, and you have very little air flow through the engine bay compared with m/way driving. An engine oil cooler is certainly a good idea, a auto box definitely needs one. I am using the RRC bog brush cooler for my transmission, I have seen two of these plumbed in series for extra cooling. I have no experience of the type you linked to, but as long as it has enough capacity/flow it will work. The one in your link looks like it's for small cars though :unsure:

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That fuel pump will only work in-tank, but apart from being suspiciously cheap it should be right. You can always buy a used one, Subaru owners are forever upgrading to ones with more numbers and selling their old ones on eBay for nowt. Anything EFI of about the same HP upward should work.

Ref coolers - more cooling is better, that's about it really.

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Thanks for the fast reply, (do you guys work :P )

i have microcat but have only recently tried to get my head around the system so dont think i could find the resistor values.

For the oil cooler will the standard engine oil cooler inside the radiator assembly suffice enough? as space is very limited now i see. And the Auto cooler, i think the largest panel radiator that i could fit beside the winch would have to do. (Its going to be awful tight for space :ph34r: )

Do you think the radiator for the box will need a fan on it aswell?

Thanks for the replies :D

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