4EAT front differential changing (4.11 to 3.90)

[aaronllewellyn]

Is this possible? changing a 96 4EAT Differential (currently a 4.11) into a 3.9 (from the original 4EAT), I read somewhere it was possible but I have no Idea where to start... or the process involved. is it easier to just install the 4.11 rear differential...?

I'm more than capable of changing a transmission/ engine just not sure what's involved to make a 4.11 4EAT a 3.90 4EAT.


I'm getting desperate, I really need my car driveable again

[mike-tracy]

It would be far easier to replace your rear diff than swapping the front differential. That takes very specialized knowledge to successfully reassemble an auto.

I don't remember what year turbo you have, but if you're worried about keeping the LSD (if you have one), you can swap the ring and pinion from the 4.11 onto the LSD and reassemble everything in the 3.9's housing.

[James614]

+2 on it being far easier to swap the rear end. I had an ATSG service guide for the 4EAT, and believe me, you don't wanna crack that sucker open if you need your car back on the road, lol.

[aaronllewellyn]

It would be far easier to replace your rear diff than swapping the front differential. That takes very specialized knowledge to successfully reassemble an auto.

I don't remember what year turbo you have, but if you're worried about keeping the LSD (if you have one), you can swap the ring and pinion from the 4.11 onto the LSD and reassemble everything in the 3.9's housing.

The car's a 1990 legacy Ls with a transmission and engine from a 1991 SS (as well as a few panels, hood, trunk-lid hah.)

Sweet!, so that ultimately gives you a 4.11 that's a LSD?

Do we have a decent write up on changing the ring and pinion?

EDIT: found any FD ratio I wanted at opposedforces.com, under "train > Differential(transmission) > gear set-hypoid. This will be helpful in the PnP yard.

Thanks for the input guys!

[aaronllewellyn]

It would be far easier to replace your rear diff than swapping the front differential. That takes very specialized knowledge to successfully reassemble an auto.

I don't remember what year turbo you have, but if you're worried about keeping the LSD (if you have one), you can swap the ring and pinion from the 4.11 onto the LSD and reassemble everything in the 3.9's housing.

finally got down to it the last few weeks and learned the rear axles are NOT compatible with the NON-LSD diff. so woo here I am thinking I'll have a car again.. >.>

[mike-tracy]

The old LSD diff has different length stubs. Perhaps you left the stubs attached to the axles? They are required.