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Old 04-15-2003, 06:00 PM   #1
geronimo66
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Default which rear diff?

I will be converting my L to a wrx drivetrain soon, and would like some rear diff. education. Which do I want, and do you know of any good sources?
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Old 04-16-2003, 05:32 PM   #2
Kha0S
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You'll need the 3.54 (or is it 3.45?) rear diff from a 5MT WRX, if you're going with a 5MT. It's sort of the bastard child of Subaru transmissions, so none of the other models use it.

/Andrew
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Old 04-16-2003, 07:25 PM   #3
geronimo66
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what does the trans have to do with the rear diff. ?
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Old 04-16-2003, 07:37 PM   #4
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The rear diff must have the same ratio as the front diff that is in the trans. If you swap the trans you must also swap the trans.
If you keep your current trans you can also keep the rear diff or swap to an LSD but in the same ratio. Those may not be easily found. The L's have a 3.90 ratio as do some XTs and the 1991 turbo legacy.
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Old 04-17-2003, 10:57 AM   #5
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Old 04-17-2003, 01:39 PM   #6
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the wrx does not have a 3.45 final drive. its 3.9
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Old 04-17-2003, 06:33 PM   #7
geronimo66
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you sure about the wrx having a 3.90?

Okay I was still in RWD car mode... wasn't thinking about the diffs having to match. thanks
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Old 04-17-2003, 07:24 PM   #8
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for sure 3.9

that RS has 4.1 and jdm stis have 4.444
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Old 04-17-2003, 08:18 PM   #9
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The final drive is 3.90 but the rear diff is 3.54 as there is a 1.1:1 reduction in the tranny. The front diff is 3.90 the rear is 3.54

This is all on a US spec WRX
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Old 04-17-2003, 08:23 PM   #10
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maybe so, but you need to get the rear diff designed for a 3.9 final drive.
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Old 04-17-2003, 09:43 PM   #11
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okay, so if I get a USDM motor and trans.. I just need usdm WRX Diff. right?
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Old 04-17-2003, 10:34 PM   #12
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I have to match my front diff right... sorry I think mom and dad were broth and sister...

My front diff should be 3.90, so I would need a 3.90 rear if I have anything other than a wrx trans.

Should I car what trans I get if I get only the motor... as far as best ratios, durability, and such... I know everything has to match 1.0 vs 1.1 and the diff thing.

Just wondering about power handeling.

Thanks everybody... getting ever closer.... oh baby,
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Old 04-18-2003, 12:17 AM   #13
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The USDM WRX drivetrain indeed has different ratios for the two diffs. As long as you get both the front and rear diff from a USDM WRX, you won't have to worry.

If you want to go nutty with crazy short gearing, the 4.44 ratios can be found in a bunch of the USDM automatic equipped Subarus. Since all of the R160 diffs can be fitted with these ring & pinion sets, you just have to find the parts and have a qualified mechanic install them. And while the diff is out, grab a Quaffe, KAZZ, STi, or USDM LSD.

If you have the choice of which tranny to buy, consider these points. The push style clutch trannys found in the N/A cars are cheaper than the pull sytle clutch trannys found in the turbo cars. However, once you upgrade the clutch, pull style trannys will require less clutch pedal effort because of their design. Ever drive in major city rush hour traffic with a big nasty clutch? It gets old after a while.

Assuming your Impreza L is already a 5spd, make sure the WRX tranny you are purchasing comes with the hydraulic clutch actuator (the plunger which pushes on the clutch fork). It's not an expensive piece, but it'll save you a trip to the junk yard if it's already there. If you're just upgrading the drivetrain, make sure to get the flywheel that came with the WRX tranny because the pull style clutches use a different bolt pattern on the flywheel for the pressure plate.
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Old 04-18-2003, 12:27 AM   #14
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Quote:
Ever drive in major city rush hour traffic with a big nasty clutch? It gets old after a while.
Yes, I've driven in Chicago rush hour traffic with my ACT clutch/lightened flywheel/cable tranny combo and it isn't fun at all. Good thing I don't get stuck in rush hour often...
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Old 04-18-2003, 01:57 AM   #15
geronimo66
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yea, I was planning on getting a new flywheel and clutch with the motor is out. Isn't the WRX front diff a 3.90 and the rear a 3.54 with a 1.1cntr.
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Old 04-18-2003, 02:34 PM   #16
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Quote:
Originally posted by geronimo66
Isn't the WRX front diff a 3.90 and the rear a 3.54 with a 1.1cntr.

Exactly
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Old 04-18-2003, 02:50 PM   #17
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agg

spririlsmurf .. sorry you are dead wrong

wrx manual front 3.9, rear 3.54, final drive ratio is 3.9
the wrx tranny has a 1 to 1.1 reduction

but for example a 2000 rs tranny has a 4.11 front and 4.11 rear with a 4.11 drive ratio (no reduction gear)

geronimo66
as mentioned above you will need a WRX clutch and fly wheel
your rs one won't work
also switching to a wrx tranny will reduce your final drive ratio .. (read loss of effective power but better gas milage)


why would you want to do this?

edit n/m thought ou werer dooing this to a RS .. an L comes with a 3.90 front and rear ..
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