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Discussion Starter · #1 ·
Okay so long story short my rear differential blew up and a 3.54 R180 then made it into my 2005 STi due to error at a supposedly reputable performance shop. After figuring out what exactly they had done I now know I needed the 3.90 R180 rear diff. Wish I had done the research and taken care of it myself in the first place but you live and you learn. Anyhow as you could imagine the center diff wasn’t very happy and now is damaged and in need of a rebuild. My question is what is my best option here? My front diff is 3.90 and is okay center diff 1:1 ruined pretty much operating open now, and I have a good 3.54 R180. Do I rebuild the center diff and try to find a 3.90 r180? Or do I rebuild the center diff and change the ratio to work with the 3.54 r180 since the front diff will work with either configuration?
Thank you for any input!
 

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Whe may trans was rebuilt, i was told something about center diffs for 05s not available and you end up finding a new trans . . . Luckily I didn't need one and perhaps I misunderstood. If you're not short on funds, then your cars power and power-band should inform your decision. In any case it's not going to be inexpensive. I'd stay with the short gearing if my car were under 400/400 with a wider than stock power band.. 3.4 to 3.9 is a pretty big change. I'd do the math and decide.
 

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Okay so long story short my rear differential blew up and a 3.54 R180 then made it into my 2005 STi due to error at a supposedly reputable performance shop. After figuring out what exactly they had done I now know I needed the 3.90 R180 rear diff. Wish I had done the research and taken care of it myself in the first place but you live and you learn. Anyhow as you could imagine the center diff wasn’t very happy and now is damaged and in need of a rebuild. My question is what is my best option here? My front diff is 3.90 and is okay center diff 1:1 ruined pretty much operating open now, and I have a good 3.54 R180. Do I rebuild the center diff and try to find a 3.90 r180? Or do I rebuild the center diff and change the ratio to work with the 3.54 r180 since the front diff will work with either configuration?
Thank you for any input!
Talk in terms of model, year and options and mods you have made! We are not mind readers. There have been a lot of variables over the years and if you get the combination wrong it wont work well! I know because I put in an early model transmission into my 2008 GR, STI 6MT and it was not the year that the seller told me it was. I had horrible problems, mostly in turns. One was a 1.0 and the other was 1.1. Same can happen with the rear diff.
 

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Discussion Starter · #5 ·
Okay then more info on the car, 2005 WRX STi all stock drivetrain minus my act clutch and lightweight flywheel. I have a built short block forged parts newer STi crank whatnot as well as beehive valves and springs GSC stage 1 cams reverse intake manifold custom front mount GTX3076R setup. Looking at 600whp on the E-85 tune. As far as 05 center diffs not being available I would go aftermarket the car is not a daily and noise is not a concern however durability is. Seeing as my transmission is in perfect shape with only 63k original miles on the whole car at this point, I would like to simply rebuild and change the center differential to work with the 2006+ STi rear differential since the front is the same through the years. Is that possible? Go from 1:1 to 1:1:1 center diff while keeping the same transmission front and rear diff?
 
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