Fitting 3.9 EFI to a 3.5 ?

Hi All,

Is it possible to fit a 3.9 Hotwire injection system to a 3.5 with a Flapper type system ? (straight swap)

Also would it be possible to convert a flapper system by using the ECU and air flow meter from a hotwire system and retain the injectors etc from the flapper system ?

Any advice would be helpfull and appreciated.

Thanks, Dave

I don't think a straight swap is possible - the 3.5 flapper has a cold start injector, the 3.9 Hot Wire doesn't. I also seem to think there was no speed transducer on the flapper systems. A conversion should be possible but I would be inclined to leave it if your engine runs O.K.

That said, the basics of the two are very similar, I believe the injectors are the same but the fuel map on the flapper system is different. Would suggest you wait and see if eightinavee or irish rover offer any insight, they both seem pretty clued up on injection systems.

Yes,do it.Then you will have the reliable 3.5 motor with a decent efi system.You will need all the 3.9 bits,ECU,loom,injectors/rail etc and plenum with the throttle body and stepper.The electrics come out of a 3.9 very easily as one lump.
Well worth doing as the old flapper ecu's are dying of old age,plus the hotwire can run closed loop,(Worth doing even without Cats.) and it has proper control of idle.
Early 3.5 injection Disco's had this setup and I've seen them tolerate allsorts of abuse from single point LPG,coolant loss and oil leaks.
Easily the best combo.

Thanks for the replies,

Eightinavee, thanks for your very positive reply, it was what I was hoping to hear! plus the info you gave was very welcome.
Just one question, will the ECU need rechipping or something for it to run the EFI system on the 3.5 correctly ?

Thanks, Dave.

I guess if you were lucky enough to find an ecu from a Disco 3.5 hotwire then no problems.If you use a 3.9 one then running it closed loop with the correct tune resistor,(So it will look for inputs from the O2 sensors.)and a pair of O2 sensors in the downpipes you have a good chance of it running right.Open loop,you may need to adjust the air flow meter to compensate for the smaller swept volume.
Dont let it put you off,any of the options is better than the flapper or carbs.