Vehicle & Technical > Range Rover

3.5 to 3.9 INLET SWAP

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Range Rover Blues:
The 14CU  EFi system was introduced on the 1988 RRC (along with concealed hinges) for the US market.  It was ANALOGUE and did not include lambda sensors but had the same plug and pinout as the 14CUX introduced on the 3.9 just over a year later.  Imprtantly, the 14CUx also existed on Disco 3.5 up untill about '95 IIRC.

Chipped properly the 14CUx works well on either engine, on the 4.2 and on the 4.6 (hell, I run it on a 5.0).

But it has to be chipped properly.  IN "standard" form it can re-tune to cope with perhaps a K&N filter and possibly a set of headers, no more.  THe EFi headers were far better than the carb exhaust and the 3.9 system better than the 3.5 (well bigger anyhow).

The 3.9 also had a better camshaft and this, rather than the capacity gave it the extra 15BHP.

The EFi needs rechipping to allow for the fuel map to cover the whole operating range of the engine, at above 80% tyhrottle the fuelling is different and the chip needs to know the engine is full-throttle and almost flat-out.  Sticking a 4.2 ECu into a 3.9 for example would lead to excessive fuel consumption and rich running at med throttle, wherreas running a 4.2 on a 3.9 ECU would cause limited power and lean-running at higher engine speeds.

So, exhaust-wise the 3.9 is best of the standard (it's the same pipe in the 4.2 but longer) unless you fit tubular headers, these are usually the same for both 3.5 and 3.9 engines so biggers gains can be had fitting them the 3.5s.  Note however that the late visible-hinge cars had a different gearbox crossmember and exhaust routing because of it, plus different hangers!

Inlet wise, you cannot just change the inlet. the injectors, sensors and plenum are all different, plus there are subtle changes in the cooling system.  If you retro fitted the whole hot-wire EFi, the 14CUX then you would get better engine control and possibly economy.

Fitting a better cam, or the 3.9 cam is worthwhile, as is replacing the timing chain on an otherwise standard engine.  Cam wear is often first symptomised by high fuel consumption, the retarding effect of cam wear being the culprit, stands to reason that a worn timing chain will have the same effect.

IN standard frorm the inlets are very similar, if you want to tune the inlet then better to replace the trumpets within the plenum with shorter, wider-brimmed parts and spend time on smoothing out the throttle and pipework outside the plenum.

So the exhaust is an easy upgrade, the rest of it I'd leave alone unless you want to run the hot-wire EFi rather than the flapper.  Hot wire will run in open loop (non-tuning) mode without lambdas and closed loop (self-tuning) with lambdas.  You change a resistor in the wiring loom to tell the ECU what to do.

Wiring a 14CUX Hot-Wire system into a 3.5 car is possible but it will be missing the road speed signal and gearbox signal on the auto that it needs to work properly in closed loop mode.  Without them you will get slightly higher fuel consumption, though mine runs smoothly enough.

bogie:
well that answered his question!

squaddie_fox:
I'm currently running a two door range rover with a 3.9 block and injection sytem, 14CUX ECU, 3.5 front end and tubular manifolds with no Cats. the rear silencer on the exhaust is looking likely to be taken out and turned into a side exit, leaving just the middle pipe for the back pressure. Once i have worked out the fuel consumption after fixing a major leak while going around left hand bends :roll: :oops: i will be changing the trumpets with slightly shortened ones and a K&N (not a cone type) filter, as i will be fitting a snorkel, but facing it backwards.


if you want to change the injection system from a 3.9 onto a 3.5 its not that difficult, but wont make much difference without also swapping over the later type air flow meter and ECU Etc.

i would just stick a cam from a 3.9 in and change the timing chain and go from there.

craig_midz:
me personally id put ya tubular manifolds on and a 3.9 cam u can use the 3.9 injectors but the increase aint worth the hassle but changin the cam u will defo notice the difference

UK CRAIG:
WOW...................................

Seems like a lot of info for my liitle brain to take in!!

With regard to the 3.9 cam, is it best to use a standard or a slightly modified one from Crower/Comp Cams etc?

Thanks to all for the posts.

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