3:90 to 4:11 swap

BajaBran · 04-30-2025, 02:23 PM

I've heard that people have swapped only the ring gear to get their rear diffs to 4:11, is that true? if not, could i take my diff out and swap it into the case with the 4:11 gears? the reason why i wanna swap the gears and not just the whole diff all in one is because the 4:11 diff has some pretty bad rust inside where the axle splines go, and i'd rather not keep "gambling" on different used diffs hoping to get a good one. just hoping i can get something usable while i have it

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Unabomber · 05-05-2025, 07:05 AM

I mean you can, but you'd blow up your center diff in short order. To do a proper swap, you have to ensure your front diff, center diff and rear diff all play nicely with proper ratios, so just doing the rear is not wise at all. Get a used 3.90 pumpkin and swap the whole ding dong thing out as they are cheap.

Charlie-III · 05-05-2025, 10:44 AM

I believe you can swap your 3:90 gears/diff (complete) into a 4:11 case. I believe the case is the same.
You should still check gear engagement since the machining of the case may be different and you may need to change shims.
Do NOT just "swap parts of the diff", rings and pinions are different (I believe) AND the ratio needs to match the other 2 diff's in the car.

As Unabomber stated, just get a "new to you" diff., YMMV.

BajaBran · 05-06-2025, 04:59 PM

Reason why I'm swapping to 4:11 is because my replacement tranny is 4:11. I was planning on making sure I've got good engagement on the ring gear after I swap my pumpkin over, just wanted to make sure that's something I can even do before I rip everything out. Thanks a ton for the insight, both of you

SubieDoucheDrew · 06-20-2025, 08:54 PM

The ring AND pinion are different from a 411 to a 390 diff, why not just get a 411 diff from a junk yard?

04-30-2025, 02:23 PM	#1
BajaBran Scooby Newbie Member#: 540470 Join Date: Mar 2025 Vehicle: 2003 wrx sonic yellow	3:90 to 4:11 swap I've heard that people have swapped only the ring gear to get their rear diffs to 4:11, is that true? if not, could i take my diff out and swap it into the case with the 4:11 gears? the reason why i wanna swap the gears and not just the whole diff all in one is because the 4:11 diff has some pretty bad rust inside where the axle splines go, and i'd rather not keep "gambling" on different used diffs hoping to get a good one. just hoping i can get something usable while i have it * Registered users of the site do not see these ads.

06-20-2025, 08:54 PM	#5
SubieDoucheDrew Scooby Newbie Member#: 531630 Join Date: Oct 2022 Chapter/Region: NWIC Location: Las Vegas, NV Vehicle: 2000 00Gc8, 03GD Silver/both	not so sure The ring AND pinion are different from a 411 to a 390 diff, why not just get a 411 diff from a junk yard?

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05-05-2025, 07:05 AM	#2
Unabomber Big Ron Super Moderator Member#: 18062 Join Date: Apr 2002 Chapter/Region: MWSOC Location: Muncie, IN Vehicle: 2004 WRX Sport Wagon Platinum Silver Metallic	I mean you can, but you'd blow up your center diff in short order. To do a proper swap, you have to ensure your front diff, center diff and rear diff all play nicely with proper ratios, so just doing the rear is not wise at all. Get a used 3.90 pumpkin and swap the whole ding dong thing out as they are cheap.

05-05-2025, 10:44 AM	#3
Charlie-III Scooby Guru Member#: 30669 Join Date: Dec 2002 Chapter/Region: Tri-State Location: USA, North NJ, 07456 Vehicle: 1998 Legacy 2.5GT Silver Sleeper BK, 5MT	I believe you can swap your 3:90 gears/diff (complete) into a 4:11 case. I believe the case is the same. You should still check gear engagement since the machining of the case may be different and you may need to change shims. Do NOT just "swap parts of the diff", rings and pinions are different (I believe) AND the ratio needs to match the other 2 diff's in the car. As Unabomber stated, just get a "new to you" diff., YMMV.

05-06-2025, 04:59 PM	#4
BajaBran Scooby Newbie Member#: 540470 Join Date: Mar 2025 Vehicle: 2003 wrx sonic yellow	Reason why I'm swapping to 4:11 is because my replacement tranny is 4:11. I was planning on making sure I've got good engagement on the ring gear after I swap my pumpkin over, just wanted to make sure that's something I can even do before I rip everything out. Thanks a ton for the insight, both of you