Cats (or Lack of)

[BIG DUNC]

I am fitting a 4.2V8 out of a 94 RR LSE into my 92 RR Vogue in place of its 3.9.
The prime reason for this is that the cost of crank grinding and parts for making the 3.9 good again is going to be almost four figures and I have this 4.2 sitting in the back of the garage with no home....

I was going to use the wiring harness off the 4.2 as that has the tune resister in it that tells the ECU what engine its driving.

However, the 4.2 would have had cats when in the LSE (and presumably lambda sensors), and the 3.9 does not. My LSE has been decatted (by the previous owner) so this cannot be a big issue. What is the best way round this? I assume the lambdas are wired into the engine loom. Can I just not connect them (nothing to connect to) or will the engine be in permanent limp home mode? Or should I use the 3.9 loom, and change the tune resister (but then it will think it has lamdbas).

[lewis s]

Try posting this in the TVR section, where decatting is rife and there are plenty of knowledgable people about

[blitzracing]

Both looms have the cat wiring in them and on the 92 or before looms, the lambda probes connectors are left dangling. Before you decide how you want to run the engine (with or without cats) you need to have the relevent chip or ECU to go with the bigger engine, the 3.9 chipping will not be ideal. You can swap the tune resistor to change maps, as the ECU contains 5 maps in all, and the only thing you need to make the cat map work is a pair of probes plugged into the exhaust, and a "white" tune resistor. The cat' map has the big advantage that it can keep the fueling under much tighter control up to 3400 rpm. If I where in your situation, Id run the catalyst fuel map with lambda probes, but get say a TVR chip for the ECU that puts in more fuel, and removes the low rev limiter. Some of the OEM chipping does run somewhat lean. If you are feeling rich, the Mark Adams Tornado chips are very good, but around £450. (You will find them on Ebay if you search for 14cux). The ECU and fueling is not affected by whether the physical catalysts are fitted or not.

http://www.g33.co.uk/fuel_injection.htm

You could run the 4.2 on the non catalyst map (green tune) and forget about the probes, but chipping is more important as the ecu cant compensate for fuel mapping that is poor without the lambda feedback.

Edited by blitzracing on Wednesday 10th February 17:08

[BIG DUNC]

Thanks Mark.
To summerise, I can iether :

drill a couple of holes in the down pipe and buy a pair of lambda sensors, and use the 4.2 loom as is.

Or, I can not bother with the lambda sensors and change the tune resistor.

You would recomend the former as giving a better running engine.

Is that correct?