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Old 09-09-2021, 01:59 AM   #1
atbwrx
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Default Is there a 2015 STI Rear Diff with 3.90 gears?

Need a hand deciphering the rear diff that came with my transmission.

I bought the running gear that was advertised to be out of a 2015 STI. I'm doing a 6 speed swap into my 2014 WRX.

The tranny has the correct identifying tag number TY856UW9AA.



But the diff looks to be a clutch type LSD, and my understanding was that the 2015 STI is supposed to have the TorSen Diff?



Final mystery is that I bought the parts to totally rebuild this diff, and when I compared the pinions, this diff has a 10 tooth pinion. The one I ordered for the 2015 STI has an 11 tooth.



Never occurred to me to check this out before hand. They both have 39 tooth ring gears, so the diff that came with the tranny has a 3.90 ratio compared to the 3.54 that the 2015 STI is supposed to have.

So, did the seller just toss in a diff from an older STI, or was a 3.90 diff ratio an option for the 2015 STI, maybe a JDM ratio?

My problem is that I'm not sure how to resolve the issue of a mismatched ratio. If there is no such thing as a 3.90 rear ratio for a 2015 STI, and this was a diff from an earlier STI, then I'll just rebuild it as a 3.54 so it will match with the transmission.

But, if it is the correct rear diff (clutch plate diff w/3.90 ratio?) then I'll need to find a source for the 3.90 ring and pinion since I'm presuming I can't run the 3.54 with a tranny that is matched to a 3.90 rear gear ratio.

If anyone can help solve the mystery, please pipe in.

Adam B.
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Last edited by atbwrx; 09-10-2021 at 10:26 AM.
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Old 09-09-2021, 11:28 PM   #2
atbwrx
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Trying to research this online and came across this on the flatirons website which matched my diff exactly:

2004-2005 R-180 Specifications:
Part # 27011AA910 (DISCONTINUED)
3.90 Gear Ratio
Mechanical clutch type LSD
Will only work with 04-05 6 speeds that have the 1:1 reduction gear

If anyone could help answer these questions it would be appreciated:

1) Is there any way to verify by casting marks, part #, etc if I have an '04-'05 diff?

2) Is there a way to verify which reduction gear my tranny has without dropping it?

3) If my tranny has a 1:1.1 reduction, can I load the '04'05 R180 case with the 2015 internals, or do I need one specific to 2015.

Any comments appreciated.

Last edited by atbwrx; 09-10-2021 at 06:47 AM.
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Old 09-11-2021, 12:27 AM   #3
atbwrx
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Vendor swears that tranny and diff came from same USDM vehicle.

I'm stumped on this one.

Last edited by atbwrx; 09-11-2021 at 03:44 PM.
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Old 09-12-2021, 02:27 AM   #4
atbwrx
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If anyone can confirm that the 3.90 gear ratio only came on the 04-05 STI, that would be very helpful.

I bought the running gear from a salvage vendor. I'm beginning to think that maybe the owner of the 2015 STI that this running gear came out of maybe decided to swap his torsen rear diff with a clutch plate diff, and just swapped the whole rear end not knowing that there was a gear ratio difference.

At this point, the only thing I can think of to do is to pull the tail shaft housing off the tranny, and confirm that the reduction gears on the DCCD are a 1.1:1 ratio, which I'm presuming they are. If that's the case, then the center diff may be smoked from running the 3.90 rear gears. I'll rebuild the R180 with the new 3.54 internals and should be good to go for the rear diff. I'll just need to inspect the center diff to see if it got smoked or is still good.

This 6 speed swap is turning into a pretty deep rabbit hole. If anyone has any info to share that they may have on confirming whether the pic I posted is an 04-05 clutch plate diff, or if its the right diff for a 2015 STI it would really be helpful.
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Old 09-26-2021, 03:39 AM   #5
atbwrx
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I pulled the tail shaft housing off of the transmission to see WTF the deal was with this pallet of running gear I received that was supposed to be from a 2015 STI.

Tranny was marked the serial number from a 2015 STI. Vendor said everything came out of a USDM 2015 STI. The R180 is clearly from a 2004/2005 STI with a clutch plate diff and 3.90 gears, and not a 2015 STI with a torsen diff and 3.54 gears.
The 2015 STI has a 1.1 : 1 transfer gear reduction which allows the car to run a 3.90 front diff ratio and a 3.54 rear diff ratio. The only way the diff that came with this tranny could work is if someone put a 1:1 transfer gear set in the 2015 tranny. So I had to check to make sure.

Well it's the stock 1.1:1 transfer gear set.



That means that if this tranny and diff did come from the same car, the front diff was spinning with a 3.90 ratio (stock) and the 3.90 rear gears with the 1.1 transfer gear reduction were spinning at a 4.29 ratio in the rear. Which means the DCCD is probably smoked. Having trouble disconnecting the two plug terminal to extract the DCCD, but once that is done I'll pull it apart and have a look inside. Stay tuned.

Last edited by atbwrx; 09-26-2021 at 10:17 AM.
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Old 09-26-2021, 11:55 PM   #6
atbwrx
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Pulled apart the DCCD today. Actually the condition wasn't as bad as I expected.

The front clutch pack was in pretty good shape. I don't know what the disks look like when new, but there appears to be a fair amount of friction material left. It definitely is not down to the metal anywhere and the wear pattern is even.



The rear disks were not bad for the most part but there are some areas where the friction surface is worn down pretty badly at the bottom of the pic.



Since its out, I'm going to either rebuild or replace.
The 3.90 clutch pack dif is getting rebuilt with 3.54 gears and a Cusco clutch diff. Took awhile to figure out what I was working with, but now that I know I'm looking forward to getting my ride back on the road.
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Old 09-27-2021, 03:05 AM   #7
murrdogg24
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yeah looks like they just gave you a GD STI rear diff, looks just like the 04 STI rear diff I have
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Old 09-27-2021, 11:44 PM   #8
atbwrx
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It's really been a hassle figuring this out. This was supposed to be a weekend install. Lol.
Glad I caught it now rather than driving on it and trying to figure out what was going on.

Last edited by atbwrx; 10-08-2021 at 10:17 PM.
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Old 10-06-2021, 02:40 AM   #9
atbwrx
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Wrestled with getting the tail shaft housing back on. Took awhile, but figured it out. Here are some pointers:

1) There are two sets of split needle bearings on the DCCD shaft. Use axle grease to hold them in place. If they separate during the install the tail shaft housing will not go all the way on.

2) Apply sealant to the mating surface on the tranny. There is a two piece plastic oil rail that carries oil from the main compartment of the tranny to the tail shaft bearing. The tail shaft (transmission output shaft) has a drive gear on it that transfers power to the DCCD. This drive gear is captive in the rear most plastic oil rail when the tail shaft housing is on, meaning if you keep the tail shaft in the tail shaft housing (meshed with the DCCD drive gear) when you attempt to install the housing the oil rail will hit the face of the drive gear preventing the tail shaft housing from going all the way onto the tranny.
You need to pull the tail shaft out of the housing, and hold it in position with the front cone bearing in its race in the back of the tranny. There's enough room to come at this from the side and then you can position the gear in the space where the oil rail is designed to go around the gear. While holding the tail shaft in place like this, use your other hand to position the tail shaft housing. Shift rod goes through its hole in the tail housing first, then the DCCD shaft. Note that the first split needle bearing is on the end of the DCCD shaft so go easy, you don't want to knock the bearing off the shaft and have to start over. Once you have those two started in their respective holes, use needle nose pliers to reach in through the tail shaft housing opening to grab the output shaft (be careful not to damage the seal) and pull it back through the hole. You'll need to turn the shaft a bit to get the gear on the shaft to mesh with the DCCD drive gear. When that happens the shaft will pull back with a loud clunk and the housing will slide all the way onto the dowel pins with some jiggling. A light tap with the plastic mallet and it will go all the way on. Install bolts and snug until you see the sealant start to ooze. Wait an hour, then come back and torque it all down.
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Old 10-06-2021, 02:41 AM   #10
atbwrx
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Duplicate post deleted.

Last edited by atbwrx; 10-08-2021 at 10:16 PM. Reason: Duplicate post deleted
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Old 10-07-2021, 08:00 PM   #11
retrac
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I've got a 3.54 R180 from a VA, and need a 3.9. Trade?

Last edited by retrac; 10-07-2021 at 08:08 PM.
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Old 10-08-2021, 12:42 AM   #12
atbwrx
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Duplicate post deleted.

Last edited by atbwrx; 10-08-2021 at 10:13 PM. Reason: Duplicate post deleted
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Old 10-08-2021, 12:42 AM   #13
atbwrx
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Too late for a swap, I’m in the middle of putting 3.54 gears into the 3.90 housing. Would’ve been a perfect trade a few weeks ago. If you want the 3.90 ring and pinion with the clutch plate diff, those are available if you want to make an offer.
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Old 05-13-2022, 03:37 AM   #14
neil7771
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Do you still have the ring/pinion and diff? If so how much do you want it for it. Been trying to find a good 3.90 R180 and can’t so I’d like to swap these into mine
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