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Old 04-30-2014, 01:23 PM   #1
Purpose_Built
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Default Is the 3.9 95-99 diff a direct replacement for the 02-07WRX?

I'm doing a sti swap with the 1:1 center and need a 3.9 r160 to save money. So far i havent found any information that suggests the 95-99 3.9fd impreza diff is a direct bolt in. I'm really only concerned with the axles because i know the wrx diff fits gc's. Any info would be great
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Old 04-30-2014, 02:03 PM   #2
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Yes...
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Old 04-30-2014, 02:46 PM   #3
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Yes...
You may just be the single greatest person on NASIOC, no, on the planet. Thanks for the quick response.
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Old 05-02-2014, 06:47 AM   #4
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Originally Posted by Purpose_Built View Post
I'm doing a sti swap with the 1:1 center and need a 3.9 r160 to save money. So far i havent found any information that suggests the 95-99 3.9fd impreza diff is a direct bolt in. I'm really only concerned with the axles because i know the wrx diff fits gc's. Any info would be great
the ej205 gd wrx rear end is NOT the same ratio as the ej255 gd wrx rear end

so they are NOT straight swap....but it gets complicated when you add at trans and wagon or not into the miz

you must find the right rear end with the right ratio....i dont think any of the 06-07 wrx's has the 3.90, regardless
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Old 05-02-2014, 12:47 PM   #5
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Quote:
Originally Posted by Uncle Scotty View Post

the ej205 gd wrx rear end is NOT the same ratio as the ej255 gd wrx rear end

so they are NOT straight swap....but it gets complicated when you add at trans and wagon or not into the miz

you must find the right rear end with the right ratio....i dont think any of the 06-07 wrx's has the 3.90, regardless
This^^

The 02-05 are 3.90
The 06-07 are 3.70
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Old 05-02-2014, 01:01 PM   #6
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Yes...
This is the right answer and only answer you need to be concerned about.
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Old 05-02-2014, 06:36 PM   #7
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Originally Posted by Purpose_Built View Post
I'm doing a sti swap with the 1:1 center and need a 3.9 r160 to save money. So far i havent found any information that suggests the 95-99 3.9fd impreza diff is a direct bolt in. I'm really only concerned with the axles because i know the wrx diff fits gc's. Any info would be great
Depends. There are two different R160 diffs in the early impreza, the small one with the finned cover is not a bolt in the axles are different. The one with the smooth cover bolts in and the axles fit.
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Old 05-02-2014, 06:59 PM   #8
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Wrx's have a 3.54 rear diff and I needed a 3.9 for the 05 trans. Found a 3.9 out of a 99 Impreza outback 5 speed. This should work from what I gathered. The 95-99 5 speeds all have 3.9 rear diff. There are more types of diffs that will work but I didn't want to take any risk. I'm going to look into what cover mine has based on your recommendation.
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Old 05-02-2014, 07:05 PM   #9
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Originally Posted by 2Fast4U1DAY View Post
This^^

The 02-05 are 3.90
The 06-07 are 3.70
The 02-05 front diff is 3.9 rear is 3.54. I don't know about the 06-07. Many of the transmissions have a 1.1 center
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Old 05-02-2014, 07:18 PM   #10
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Wrx's have a 3.54 rear diff and I needed a 3.9 for the 05 trans. Found a 3.9 out of a 99 Impreza outback 5 speed. This should work from what I gathered. The 95-99 5 speeds all have 3.9 rear diff. There are more types of diffs that will work but I didn't want to take any risk. I'm going to look into what cover mine has based on your recommendation.
Generally the 5 speeds had the "normal" R160 and the automatics got the smaller one with the fins on the cover that won't fit wrx axles. You really should swap your lsd into the new diff, it's easy to do.
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Old 12-02-2014, 02:49 PM   #11
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what about an 2005 Rs? I went through this same issue last year. I cant remember exactly what 3.9 diff i ended up buying but the transmission shop swap the 3.9 r&p from it into my wrx diff.

i trashed the wrx diff last night having some fun and the lsd in it def isnt reusable. i dont think the 3.9 r&p is either. I dont want to have to buy a wrx diff and another diff to make this work
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Old 12-02-2014, 11:17 PM   #12
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08+ WRX diffs are 3.9FD.

btw. I have one for sale from my 08 WRX (upgraded to R180 LSD)
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Old 12-03-2014, 11:34 PM   #13
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Originally Posted by Purpose_Built View Post
The 02-05 front diff is 3.9 rear is 3.54. I don't know about the 06-07. Many of the transmissions have a 1.1 center

the 06-07 are all 3.7 and the stepper is 1:1 so the front is also 3.7

all the mt 2.0's were 1.1:1 stepper and had 3.54 rear diffs and 3.9 fronts
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Old 12-04-2014, 03:17 PM   #14
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Originally Posted by stlnation View Post
what about an 2005 Rs? I went through this same issue last year. I cant remember exactly what 3.9 diff i ended up buying but the transmission shop swap the 3.9 r&p from it into my wrx diff.

i trashed the wrx diff last night having some fun and the lsd in it def isnt reusable. i dont think the 3.9 r&p is either. I dont want to have to buy a wrx diff and another diff to make this work
Get an open diff 3.900, next get a REAL LSD then go have fun again.
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