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Old 09-05-2016, 11:33 AM   #1
WanaHaveRex
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2015 2015

Default 04 Sti swap into 2006 WRX (mismatched rear Diff?)

Good afternoon everyone, I recently purchased a 2006 wrx from a friend of mine. The car has a JDM 2004 STI transmission in it, and I recently decided to check the rear diff to see if he actually matched the r160 final drive to 3.9 (as I read you have to for the swap). Anyway, I jacked up one corner of the rear, put the car in neutral with the E-brake OFF, and I spun the driveshaft input (propeller shaft) until I did 1 full revolution of the output axle.To my surprise, the driveshaft spun about 3.6ish turns.....I also verified the gears are 3.7 using borescope camera.

The transmission swap was done like a year and a half ago, and the car drives perfectly fine going forward in any condition: cornering, full lock tight circles at slow speeds etc.. not an issue... The only thing the car does do that is slightly abnormal (not unheard of) is bind up at full lock in REVERSE (which I try to avoid) but doesn't make any noises or anything. Could be any number of things from what I'm reading.

I'd really appreciate some help from you transmission gurus out there that know way more than I do about swaps, differentials etc.... Is it possible that the STI center diff is still working properly, and the car driving fine with a mismatched rear diff final drive?? This is so perplexing... I read that this would cause failure instantly to the center diff and the car would NOT drive correctly... Please chime in and let me know if there are any other tests I can do to to check if the transmission is still OK after this long using a mismatched rear diff.... Thank you all so much.

EDIT: I also did a full fluid change in the transmission and the rear diff. I also dropped the transmission pan to check the magnet and it was completely clean ... NO chunks of metal or anything, same for the rear diff magnet...
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Last edited by WanaHaveRex; 09-07-2016 at 06:28 PM.
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Old 09-05-2016, 06:17 PM   #2
WanaHaveRex
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nobody?
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Old 09-06-2016, 02:42 PM   #3
Celery GT-5
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2002 WRX SRP

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Lololol
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Old 09-06-2016, 03:04 PM   #4
WanaHaveRex
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Thanks for the bump ^^
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Old 09-07-2016, 08:19 AM   #5
WanaHaveRex
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Update: I'm throwing in the correct 3.9 today, let's see what happens. I'm still unsure how the car drives perfectly fine without binding or noise with the wrong final drive. I verified the rear diff as an 06 WRX r160 with 3.7 gears (using a borescope). My transmission is exactly an early 2003 production 04 STI transmission with a 3.9 final drive and 1:1 center diff.

Only symptom I notice is binding in reverse at full lock, but that could be because I added lightweight shockproof to my recent oil change and the friction modifiers aren't playing nicely with my plated center diff. I also read a few forum posts that suggest wheel speed sensors going south, so I downloaded FreeSSM and will be checking those when I get the proper cable.

Still looking for some help from you guys to figure out what the hell is going on here... How is the car running the wrong final drive for so long w/o issue? Thanks!
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Old 09-07-2016, 06:06 PM   #6
WanaHaveRex
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Waiting on new diff to arrive and i'll be throwing that in tomorrow... I'm very surprised that nobody has any input? Is there any missing information from my post? I just wanted to hear from someone knowledgeable about how the car essentially drives fine with the wrong final drive... It seems very bizarre.
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Old 09-08-2016, 01:12 PM   #7
mrsaturn7085
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2006 Impreza WRX STI
WR Blue

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Had a long chat yesterday via text message:

OP has a R160 rear diff with a 37:10 ring gear (3.7 FD).
Transmission is marked as 2004 USDM STi 6MT.

Front and center diff. are unknown, but assumed to be OEM parts.

DCCD wires are clipped externally and no DCCD controller is present. If DCCD *is* installed, it is operating only on mechanical LSD portion of diff. and the plates/discs are likely worn due to mismatch in front and rear FD gearing.

No OEM 3.7 front FD gearing exists for the 6MT (to the best of my knowledge), and with a 1:1 transfer (again, assumed based on OEM gearing of transmission), I have recommended to pull the tail of the 6MT and visually confirm what diff. and transfer gearing is present before doing anything else.

If 1:1 transfer is present and diff. is a worn pre-'06 DCCD, recommending fitting 4kg 6MT viscous c.diff. and 3.9 FD R160 rear diff. ring gear unless DCCD is desired. ~$250-750 fix based on eBay prices.

If DCCD is desired, recommending DCCD rebuild kit and 3.9 FD R160 rear diff. ring gear as well as required electronics for DCCD control. ~$1250+ fix based on eBay prices and new rebuild kit pricing. DCCD controller cost not factored in.
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Old 09-08-2016, 01:21 PM   #8
WanaHaveRex
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Mrsaturn7085 is truly one of the best people I've come across on this forum. I just want to say thank you for devoting so much time to my unique problem, and helping me outline the best course of action (not bleaching tires and launching). I will keep this updated in the next two days while I either take apart the rear, or throw in a 3.9 that I just got in the mail. I might opt to throw in the 3.9 because I need the car for work, and I am going out to a friends land next week where I can safely test the performance of the center diff... The right thing to do would be tear it down now and see whats going on/replace parts. Unfortunately I don't have the time/funds to do it right now. Anyway, a big thank you for all of the help.
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