3.90 & 4.11 rear differentials

[ashino]

Hello

I have almost succeeded in turning my 98' AT wagon into a 5mt, now only one problem remains.

My rear diff was originally a 4.11 ratio, with axles that had no split and just popped out if you pulled hard, it had long bolts extending from the back of it, and a rather large "wheel" that connects to the drive shaft.

Now i have a rear diff from an impreza, 3.90, but with axles that are stuck on with a split. It has too short bolts extending from the back of the diff and the "wheel" where the drive shaft connects to the diff, so i had to bore out the holes in the diff plate, and therefore, it is not completely centered, so the car shakes at high speed.

Now i want to buy a diff, but i am not sure if that has the 3.90 ratio or the 4.11. The differential looks the same as the original 4.11 that came with the AT, but it is from a manual car, (that i know because thats where i got the slightly longer drive shaft I needed) so I am asking, to your knowledge, was there a 2.0 98' legacy with manual gearbox with 4.11 ratios, and if the different shape, and axle mounts mean that it is 4.11.

I would really appreciate your help, regards bragi

(i apologize if this is difficult to understand, i´m icelandic)

[chuckthefuk]

In North America there was never a 2.0 liter non-turbo Only 2002+ WRX's come with 2.0 liter turbos.

The only manual legacy in 98 that had a 4.11 FD is the GT model. I am not sure about the automatic of the same year though..

I am having a hard time understanding 100% what your issue is but from what I can make of it you swapped a 3.90 Final Drive rear differential into your car that came with a 4.11 Final drive.

This is a bad move since your transmission is geared for a 4.11 Final Drive. So your causing binding since the front and rear are rotating at different speeds. You MUST match you Final Drive.

It does not make a difference if the rear diff came from manual or automatic. As long as its the same Final Drive as your Transmission.

Cheers
-Chuck

[ashino]

In North America there was never a 2.0 liter non-turbo Only 2002+ WRX's come with 2.0 liter turbos.

The only manual legacy in 98 that had a 4.11 FD is the GT model. I am not sure about the automatic of the same year though..

I am having a hard time understanding 100% what your issue is but from what I can make of it you swapped a 3.90 Final Drive rear differential into your car that came with a 4.11 Final drive.

This is a bad move since your transmission is geared for a 4.11 Final Drive. So your causing binding since the front and rear are rotating at different speeds. You MUST match you Final Drive.

It does not make a difference if the rear diff came from manual or automatic. As long as its the same Final Drive as your Transmission.

Cheers
-Chuck

Ah sorry this didnt come exactly right out, i swapped my 4.11 auto trans. to a 3.90 5mt, and therefore need to replace the rear diff, the one i have now is not exactly right (although 3.90) the rear bolts are too short and it does not connect to the drive shaft correctly. I bought today another rear diff, but when i tried to count the diff rotations to one full rotation of one of the rear tires, i got that the driveshaft would move just over 2 rotations while the tire would do one, so that would be a 2.x ratio, which i thought was not possible.

In the legacy 98, were there other diffs, than the 3.90 and the 4.11, and is there anyway to find that out if the sticker has worn off?

regards bragi

[chuckthefuk]

*EDIT*
Legacy:
4.11 GT/Outback/SUS
3.90 5MT L/Brighton

-Chuck

[mike-tracy]

Legacy:
4.44 5MT Outback/SUS


I believe you meant 4EAT, FSM has all the 96-99 5mt Outback/SUS' with 4.11 FD. Even the 2.2 model.

Only USDM car I'm aware of with both 5mt and 4.44 gears is the Forester XT,
although I would speculate the 5mt Outback XT did too. That's the tranny I want. ;)

[chuckthefuk]

Whooops got my wires crossed.
-chuck

[rougeben83]

In North America there was never a 2.0 liter non-turbo Only 2002+ WRX's come with 2.0 liter turbos.

The only manual legacy in 98 that had a 4.11 FD is the GT model. I am not sure about the automatic of the same year though..

I am having a hard time understanding 100% what your issue is but from what I can make of it you swapped a 3.90 Final Drive rear differential into your car that came with a 4.11 Final drive.

This is a bad move since your transmission is geared for a 4.11 Final Drive. So your causing binding since the front and rear are rotating at different speeds. You MUST match you Final Drive.

It does not make a difference if the rear diff came from manual or automatic. As long as its the same Final Drive as your Transmission.

Cheers
-Chuck

Ah sorry this didnt come exactly right out, i swapped my 4.11 auto trans. to a 3.90 5mt, and therefore need to replace the rear diff, the one i have now is not exactly right (although 3.90) the rear bolts are too short and it does not connect to the drive shaft correctly. I bought today another rear diff, but when i tried to count the diff rotations to one full rotation of one of the rear tires, i got that the driveshaft would move just over 2 rotations while the tire would do one, so that would be a 2.x ratio, which i thought was not possible.

In the legacy 98, were there other diffs, than the 3.90 and the 4.11, and is there anyway to find that out if the sticker has worn off?

regards bragi

open up the back plate and start counting teeth. If you know the code of the transmission that your diff is bolted to, you can look up the final drive too.

also, http://spda-online.ca/modules/tinyconte ... tc_28.html (http://spda-online.ca/modules/tinycontent/rewrite/tc_28.html)

[ashino]

open up the back plate and start counting teeth. If you know the code of the transmission that your diff is bolted to, you can look up the final drive too.

also, http://spda-online.ca/modules/tinyconte ... tc_28.html (http://spda-online.ca/modules/tinycontent/rewrite/tc_28.html)

Marvelous, thank you, how many teeth should i count :), i.e. how many teeth are there in the 4.11 and the 3.90?

thanks for the enlightenment, bragi

[mike-tracy]

Basically you count the teeth on the larger wheel, and divide that number by the number of teeth on the smaller wheel.

[ashino]

Basically you count the teeth on the larger wheel, and divide that number by the number of teeth on the smaller wheel.

which smaller wheel?

the ones in the middle?

i counted 39 teeth on the big wheel btw..., so i better hope to have 10 teeth on the smaller one..

brag

Otherwise if i had 9 teeth on the smaller, i would have the ratio 4.3333 and if i have 11, 3.545

now that seems really unlikely to me.... , so i will now congratulate myself for having the right rear diff for the first time in 2 months!!

I would really like to thank every one who commented and helped me with this, i´m fairly new at this :)

[chuckthefuk]

Basically you count the teeth on the larger wheel, and divide that number by the number of teeth on the smaller wheel.

which smaller wheel?

the ones in the middle?

i counted 39 teeth on the big wheel btw..., so i better hope to have 10 teeth on the smaller one..

brag

Otherwise if i had 9 teeth on the smaller, i would have the ratio 4.3333 and if i have 11, 3.545

now that seems really unlikely to me.... , so i will now congratulate myself for having the right rear diff for the first time in 2 months!!

I would really like to thank every one who commented and helped me with this, i´m fairly new at this :)

39 / 10 = 3.9 TADA!

Your welcome and good luck with the assembly process.
-Chuck