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Old 07-17-2017, 02:51 AM   #1
huntercw2003
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Default 2002 Wrx r180 swap *please help*

Brief history - Yesterday I installed a JDM v7 R180 3.9 diff, STi rear axles, STi hubs, Rotors, brembos, Stoptech brake pads, braided brake lines. Rotors, pads and brake lines are brand new. Replaced wheel bearings in hubs as well. Shoes look ok. Fyi, front conversion was done about a month ago. This is all prep for a 6mt. After installing I took a spin, everything seems to be operating properly. Today I got on the freeway and about 7 minutes go by and the rear end starts vibrating like crazy. Pulled over smelled brakes so I went home took the wheels, calipers, rotors off and I find ebrake improperly adjusted. Readjusted ebrake, no more dragging ebrake. Got back on the freeway and almost at the same location as the time before the car starts shaking again. Jump out, no brake smell. Went home crawled under my car, checked everything to make sure everything was tightand made sure axles were properly seated in diff. What could possibly be wrong? Car drives fine all around town doing 30-40 mph but 7 minutes on the freeway the car starts shaking. Please help. Thank you in advance.
Aloha
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Last edited by huntercw2003; 07-17-2017 at 03:08 AM. Reason: More info to add
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Old 07-17-2017, 07:13 AM   #2
norcaltuna
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You don't mention what trans is in the car, but unless you swapped it, that's your center diff complaining.

It's looking for a 3.54 rear diff, not a 3.90. The overall ratio is 3.9 because they have 1.1:1 transfer gears, as 3.54 x 1.1 = 3.9.
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Old 07-17-2017, 11:06 AM   #3
huntercw2003
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Quote:
Originally Posted by norcaltuna View Post
You don't mention what trans is in the car, but unless you swapped it, that's your center diff complaining.

It's looking for a 3.54 rear diff, not a 3.90. The overall ratio is 3.9 because they have 1.1:1 transfer gears, as 3.54 x 1.1 = 3.9.
Sorry, the front conversion I was referring to was front brembos. The trans is the stock factory trans. So I definitely have the wrong rear diff? Could this have caused damage to my transmission? Should I not drive the car even if its short distance? Thank you sir👍👍
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Old 07-17-2017, 10:53 PM   #4
norcaltuna
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Default 2002 Wrx r180 swap *please help*

If you put a 3.9 rear diff in an 02 wrx without changing the transmission, you likely destroyed the center diff.


In general, JDM, and 04-06 usdm sti, have 3.90 front ratios, 1:1 transfer gears, and 3.90 rear ratios.

Usdm wrx and 07- sti also have 3.90 front ratio, but 1:1.1 transfer gears, and 3.54 rear ratios. 3.54 x 1.1 is 3.90; they have the same overall ratio but turn the drive shaft slower to get the same speed.

Your car presently has a 3.90 front ratio and, to the (now agonized) center diff, a 4.29 rear ratio. The roughness you're feeling is probably your diff overheating, locking itself, and dragging the rear end of your car like a dog with a turd stuck to its butt. Time to speed up that swap.


Ps read the FAQs and stickies, eh?
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Old 07-18-2017, 04:10 AM   #5
huntercw2003
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Quote:
Originally Posted by norcaltuna View Post
If you put a 3.9 rear diff in an 02 wrx without changing the transmission, you likely destroyed the center diff.


In general, JDM, and 04-06 usdm sti, have 3.90 front ratios, 1:1 transfer gears, and 3.90 rear ratios.

Usdm wrx and 07- sti also have 3.90 front ratio, but 1:1.1 transfer gears, and 3.54 rear ratios. 3.54 x 1.1 is 3.90; they have the same overall ratio but turn the drive shaft slower to get the same speed.

Your car presently has a 3.90 front ratio and, to the (now agonized) center diff, a 4.29 rear ratio. The roughness you're feeling is probably your diff overheating, locking itself, and dragging the rear end of your car like a dog with a turd stuck to its butt. Time to speed up that swap.


Ps read the FAQs and stickies, eh?
So I found a 3.54 that someone was nice enough to loan me until I can exchange the 3.9. I really hope I didn't damage the front diff. I'm thinking and hoping at the same time that the front diff is ok since the car still drives perfectly fine at low speeds. I could be completely wrong though as I am a newb. Thank you so much for clarifying norcaltuna👍👍. I did do some research before i pulled the trigger on this but must have got some false info from somewhere dangit! FAQs and stickies, gotcha👌
Aloha
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Old 07-18-2017, 07:39 AM   #6
Titter
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EJ207 Spec C 4EAT

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i think the front diff will be fine. its the center diff that gets toasted with mix matched gears.
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