How To: Center Diffs

[Burnera]

Has anybody done it with usdm WRX stuff and failed?

If you're trying to find someone who has done it, maybe look at newer cars that came with both phase 1&2 transmissions in the same body. Such as the 1st generation impreza, and the 2nd gen legacy.

Even if you could rewire your whole car just to run the WRX tranny, would you really want to lose the functions tied into the phase 1 transmission ecu? Such as, ABS, Cruise Control, and other functions? Or were you thinking about swapping and rewiring those in as well, from the WRX?

ABS? Cruise control? I have no such thing.

Current thought is...
If my final drive is 4.11....swap over the center diff and rear LSD from an 02-07 AT WRX, fuse jump the TCU.
if my final drive is 3.90....swap over center diff from AT WRX and rear LSD from a MT WRX, fuse jump the TCU.


(If i could find a full swap, engine included, at a reasonable price, i'd do it, but i'm not seeing many donor cars around and i don't have the money saved up right now either. )
Seems like at some point, you just have to try and document your efforts, since there is a lot of conflicting information.

[mike-tracy]

Sounds like you have less obstacles than some/most for the swap. One thing to consider is splitting an auto tranny, and successfully reassembling it is something of a black art, compared to a manual. It would be far cheaper and less stressful to find a matching rear diff.

[86BRATMAN]

Manual wrxs are 3.54 final drive in the rear.

[James614]

One thing I'll add, I'd suggest a switch instead of a jumper for the duty C solenoid. Even with an LSD rear, getting one or both front wheels mired will still render you stuck without the clutch packs active on a VTD trans. (Really easy to do if you're used to all the traction of the factory systems).

[czei]

Does anyone know where to buy an assembled center differential? I just called the local Subaru dealer's parts department, and he said they don't sell a full differential for a '93 Legacy, just parts. This thread makes it seem like you can just swap out the entire diff, which doesn't seem to be the case.

[James614]

You need to find one off of another car. For the 1st gen Legacy specifically, you will find very little drivetrain parts still available. But stuff up to '98 should be be a direct interchange from any model that uses the same type of trans. Some newer stuff fits, but im not sure what does and doesnt.

[czei]

Given that there doesn't seem to be a test for wether the center diff is working or open my best bet would seem to be to take apart the diff I have (or by another used one) and replace any worn parts, especially the viscous thingie. I'm just guessing that after 150K miles and 30 rally races the center diff is shot, but that seems a fairly good bet. The problem is it seems difficult to tell if a replacement used diff is working.

[James614]

If its out of a car thats just been daily driven, its probably fine so long as it wasnt run for long periods of time on mismathed tires. It takes some abuse to wear them out (like autocross or rally racing). But how much power do you make? If youre turbo, wheelspin is probably unavoidable without upgraded differentials, or better suspension/tires to maintain better traction.

[czei]

This is my rally car that has aggressive competition rally tires and a 2.5L swap and lots of races.

[mike-tracy]

What about a 20kg center diff and a phase II tail housing? The 12kg center diff has been out of production for a decade or more.

[beatersubi]

What about a 20kg center diff and a phase II tail housing? The 12kg center diff has been out of production for a decade or more.

More info, please. Where might one find a 20kg diff?

[mike-tracy]

I was mistaken, rally spec carries both phase I/II center diffs:

http://www.rallispec.com/dif_gls_rst1215.html

Nuwan on here was/is selling a brand new 20kg center diff for significantly less.

[czei]

Thanks for the link, holy crap those are expensive! For the phase I wouldn't it be possible to take apart an existing diff and just replace the viscous section with the sti part?

[mike-tracy]

Like the 1st post in this thread says, any of the 5-speed center diffs can be used with a phase I tranny. If you grab a phase II center section, you have to install it with a 5mt phase II tail section on your tranny.

[czei]

Now, you can take it apart, but you can't buy internal replacement parts. If you were to upgrade to a STI center diff in phase 2, you get the entire assembly. For phase 1, you can purchase upgraded STI viscous coupling units without the entire assembly. A part number for example:
ST3850055010 = Centre Viscous diff Ver.5/6 STi 20kgf m/100rpm

FYI, I can't find that part number anywhere on the internets... The only mention of it is in copies of this very fine tutorial, but otherwise seems to be nonexistent.

[czei]

To follow up, I was just at the Subaru dealer today, and they have the 12kg viscous unit from a phase 1 transmission, part 38434AA021. Unfortunately the parts guys could not find the 20kg viscous unit, as it doesn't seem to be listed as a separate part for an STi. They could have just been trying to get me out of their hair.

Would you believe the new price on the 12kg viscous unit is $583! Rallispec has a phase 1 diff WITH the 20kg viscous already installed for $952, part RST-1212:http://www.rallispec.com/dif_gls_rst1215.html. There's no telling what a 20kg viscous would cost even if I could find one, which makes the prospect of getting the entire 20kg diff correctly put together more attractive.

[James614]

$583 is actually much cheaper than I thought it would run for the 12kg. It's tempting to pick one up for the hell of it.

[czei]

$583 is actually much cheaper than I thought it would run for the 12kg. It's tempting to pick one up for the hell of it.

The big problem here is its impossible to measure the difference between the performance of the center diff I've got with 150K miles and a new viscous unit. Hey, I'm an engineer, and specialize in before/after testing for performance tuning of websites, and if you can't measure it, it doesn't exist

[Legacy777]

....and if you can't measure it, it doesn't exist

Said like a true engineer

I only say that because I'm one too

[James614]

You managed to quote me instead of czei, lol.

[Legacy777]

You managed to quote me instead of czei, lol.

d'oh! fixed it

[czei]

http://www.rhdjapan.com/sti-center-diff ... gc-gf.html

OK, I managed to find the part ST3850055010 online. You can't find it by searching on that number, but rather I searched on the phase 1 center diff number to find a place that carried it, and then searched using similar product descriptions but targeting WRX STi.

The only problem here is the picture doesn't look anything like a center diff, but instead looks like a front diff. Under the picture it says "mages used may be generic. Actual product may vary", which isn't very encouraging. It does give a huge compatibility list going back to '92, so its got to be a 5MT compatible but doesn't actually say it. It also doesn't explicitly say its 20kgf.



Brand: STI
Product: Center Differential Viscous Locking Impreza GC/GF
Part Num: ST3850055010

SPECIFICATIONS:
Type: Viscous Locking Differential

COMPATIBILITY NOTES:
-Cannot be installed on DCCD, or ABS equipped vehicles
-Impreza (GC/GF) A B C1 C2 D E

If that is a phase 2 20kgf, its only $597 compared to the Rallispec's $1,190. Even with shipping this would have to come out ahead.

[James614]

This is a phase 1 differential. I'm not aware of a front viscous LSD Subaru ever made, so I'd assume it is indeed a center diff. No idea how you'd tell if it's 20kg or not though.

[czei]

This is a phase 1 differential. I'm not aware of a front viscous LSD Subaru ever made, so I'd assume it is indeed a center diff. No idea how you'd tell if it's 20kg or not though.

How can you tell? I've got a phase one center differential sitting on my desk and it looks nothing like that picture. Plus there's a picture of a phase 1 center diff in this thread and it looks nothing like that.

[James614]

The phase 2 diff looks nothing like that either. The photo is a generic differential stock photo. It fits pre-98 models, which means phase 1.