Thread: 4.6 vs 3.9 Fuel economy

I would say at least 10-15%.
Thats what I gained when I last did that but remember, with the more power and torque you will be driving faster and be putting the boot in because it will be much quicker, like chalk and cheese.

best of luck.
Also try and get the 9.3 to1 compression motor, goes better once again.

Originally Posted by Rambo

I am looking at replacing my 3.9 D1 engine and have been told a cheaper option would be to fit a 4.6. Rambo

In would have thought a 4.6 would be far more expensive than a 3.9. 3.9s can be got quite cheaply where good 4.6s are normally attract big dollars.

Garry

i get very similiar economy out of my 4.6 that I had with my 3.9. in my series 1 disco.

A 4.6 is likely to cost you a fair bit more than a 3.9 and you will have some work to do to get the conversion to work. Either use a short motor and use all your ancilliaries and have your ecu remapped for the 4.6 or take a 4.6 from a RR and change over the fuel injection system. Someone will have a more technical answer as to the change in fuel injection system that is required.

If you use the 14CUX EFI on it, you probably wont see much difference if any at all. The 14CUX will just adjust itself based on the slightly increased intake volumes but in doing this you wont really gain the benefits of the 4.6.
It then becomes a matter of doing the full engine and ECU change, preferably to the Thor (MOTRONIC) system if you can spare the $$ or go for a late GEMS unit, but it is a big job wiring wise.

You can rechip the 14CUX, but drive a Thor powered P38 and a GEMS Disco and you will see the difference after having a 3.9 with a 14CUX!

FFS.. Now I want a P38 again..... AAARRRGGHHHHHH!!!

Thanks for the info, I was quoted about $7,500 for the conversion. Does that sound about right?

Rambo

Rambo
I was quoted $5k last week for a change over at TRS is South Australia.

Thats with my aux and a used 4.6 low comp motor.

Originally Posted by big guy

Rambo
I was quoted $5k last week for a change over at TRS is South Australia.

That's with my aux and a used 4.6 low comp motor.

Low compression...... Stick a blower on it
I'd be very cautious with a S/H 4.6 with out T sleeves fitted.

Tony