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2007 Forester SportsXT AT
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Discussion Starter · #1 ·
I was just wondering if the time came that I wanted to install a beefier rear diff such as a r160 or r180 what exactly else would need to be done so that it would work with a 4eat? Would I have to change the gears in the front diff as well?

thanks.
 

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2005 Forester XT 4eat
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The front and rear diff ratios would need to match, so you'd most likely need to change the front ratio. Do you know the final drive ratios of the rear diffs you are considering?
 

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Discussion Starter · #5 · (Edited)
Thanks for your response. Somebody said they were putting in a r180 with a 3.9 final drive ratio, so I guess I'd need to find a front diff from a 4eat with a 3.9 front diff. Maybe some wrx 4eats came with this?
 

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Discussion Starter · #6 ·
taken from this thread: Gear Ratio Reference Thread - NASIOC

2006/2007 WRX 2.5l models:

Automatic transmission gear ratios:
1st - 2.785
2nd - 1.545
3rd - 1.000
4th - 0.684
reverse - 2.272
final drive ratio - 3.90

2008 WRX 2.5l models:

Automatic transmission gear ratios:
1st - 2.785
2nd - 1.545
3rd - 1.000
4th - 0.684
reverse - 2.272
final drive ratio - 3.90
 

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Discussion Starter · #7 · (Edited)
So if i was to get my hands on an r180 diff with a 3.9 ratio, then I'd also have to get a front diff from a 2006-2008 wrx 4eat? Would the new diff just swap right in to my 4eat?
 

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2008 Forester XT Sport 4EAT
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Interesting to find out, probably not a lot of experience out there. Very few people have swapped a front diff in a manual transmission, even less even consider it in an auto.

I'm pretty sure for the 5MT transmission the 4.11 diff took up the most space, if that holds true for the 4EAT then you shouldn't have to clearance the case with a grinder.

If course you need to swap the pinion and ring gear to do the swap, which, at least in a manual transmission, means that you need to swap out the main shaft. This is why most people just do the whole transmission swap, much easier.

Sent from my phone, sorry for any typos or brevity
 

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Discussion Starter · #10 · (Edited)
Interesting info, thanks!

I'm sure if you have a manual then it's better to just swap out transmissions but autos from different years/models aren't always plug and play plus I'd like to keep my vtd centre diff so that's why I'm asking if I can install a different front diff.

Thanks again for the response.
 

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98 GM67 + 03 SG
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357 Posts
What differential do you currently have? You should have a R160 in there, which should hold up just fine. A 3.9FDR with the auto will be pretty slow, since I think most 4EATs come with a 4.444 to offset the torque converter's sluggishness.

The bigger issue is the transmission conversion. Is it 1:1 or 1:1.1? That makes a difference on what ratio the front differential will have. I can't remember which transmissions came with the 1:1, as I think most Subaru transmissions are 1:1.1.

It is far easier to swap the style of differential (open vs. VLS, etc.) than changing the drive ratio of the differentials. If you are going to put a R180 back there, you might as well swap the 6MT it comes with into place. I don't think any 4EAT or 5EAT came with a differential that big. Don't quote me on that, as I gave up trying to remember that information a long time ago.
 

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Discussion Starter · #12 · (Edited)
What differential do you currently have? You should have a R160 in there, which should hold up just fine. A 3.9FDR with the auto will be pretty slow, since I think most 4EATs come with a 4.444 to offset the torque converter's sluggishness.

The bigger issue is the transmission conversion. Is it 1:1 or 1:1.1? That makes a difference on what ratio the front differential will have. I can't remember which transmissions came with the 1:1, as I think most Subaru transmissions are 1:1.1.

It is far easier to swap the style of differential (open vs. VLS, etc.) than changing the drive ratio of the differentials. If you are going to put a R180 back there, you might as well swap the 6MT it comes with into place. I don't think any 4EAT or 5EAT came with a differential that big. Don't quote me on that, as I gave up trying to remember that information a long time ago.
Well as you can see in post #6, 06-08 wrx 4eats came with a 3.9 ratio so I don't think it would be that much slower, plus you'd probably get a little better gas mileage. Anyways I was just wondering in case I ever decided to mod the engine then I'd also want to improve the power train and the stock rear diff seems to be the weak link.
 

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98 GM67 + 03 SG
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357 Posts
As long as you have the bolt on side caps, the rear differentials are quite strong, especially when you pay attention to them and swap the fluid out. Biggest thing you can do is upgrade from an open to VLSD by swapping the ring gear from the open to the VLSD and shoving it back in the case.

Biggest thing you can do with an automatic is put a secondary cooler after the stock heat exchanger. Heat is the automatic's worst enemy, so helping to keep the ATF from overheating and breaking down is the best protection. If you plan on auto-x'ing, tracking, or taking the car to the drag strip, having this extra cooler is a must.

Otherwise, I'm not sure if the front differentials from a transmission utilizing a R180 rear differential are able to swap into the 4EAT case. Taking a look at Opposed Forces might shed some light on that.

I've never handled anything on the front end of the transmission. I've swapped open to VLSD in the rear differential a number of times with zero issues. Everything on the front end is black magic to me. Andrewtech Automotive might be able to provide incite on front differential compatibility, since they handle a TON of transmissions.
 
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