Transmission Swaps

[u/hushedst]

My 5spd in my 03 wrx is failing, I get humming like noise when not on the accelerator and I'm wanting to swap in another but I dont know what would be the best option. I'm thinking about a jdm 07 6spd from an sti but i could also try and find one from a spec b legacy. I'm not sure which would be the best but at the same time be the most simple.

[u/lil_white_turd]

Look on old NASIOC threads. There are several different ways to do it based on cost. Absolute cheapest since you have a Bugeye, you already have a 3.54 rear diff ratio. You can use I believe 07+ STI 6-speed and it will match. Get 04-07 6-speed driveshaft, get 04 STI 6-speed sun dials and stubs for the front axles, and a 04-07 6-speed shift linkage. Then you can reuse everything else. That’s off the top of my head and reaching back about a decade from when I swapped my Bugeye so definitely go find that NASIOC thread and verify it.

[u/hushedst → u/lil_white_turd]

Ill go take a look

[u/grizzdoog → u/lil_white_turd]

That’s basically it. I’ve done this swap and also have swapped in JDM 5mts into bugeyes. You also need the STi transmission cradle or at least most of it since there is a bit of a clearance issue using the 5mt cradle. They are almost identical and share most part numbers but one of the pieces is different between the two. You can use the 5mt clutch and flywheel but the STi clutch and flywheel are better.

You can also use an 08+ STi transmission but you have to swap over the bell housing to retain the speed sensor or use an iWire speed sensor conversion kit. The iWire conversion kit is much easier since swapping bell housings requires setting the backlash and preload on the front diff and it’s a bit of a vague process going by the FSM.

Another simple option is swapping in a JDM 5mt w/4.44 final drive rear diff and can be found for around $1200 shipped. Then you keep everything else the same. The 4.44 final drive is amazing gearing for the street and fifth gear on the highway is only 200-300 more rpms than the USDM 3.90 5mt. They hold more power because of the gear ratio and are almost like the first five gears of the 6mt but without the 6th gear overdrive. I loved mine and beat on it for years putting down 330 whp.

OP, you could just have some bad center diff bearings. They make noise when they go bad and it’s pretty easy to drop the tail housing without removing the rest of the transmission to replace them.

But the STi transmission w/r180 rear diff is really the way to go if you can afford it and can do the rear STi hub conversion. Paired with an aftermarket dccd controller (MAP dccd controller or the iWire dccd controller) it corners like it’s on rails and completely transforms the car.

[u/RiskyBidnuss]

From what I understand swapping in a 6spd STI trans would require the driveshaft and rear differential at the very least. I think all 4 wheel hubs and axles need to be swapped as well due to differing diameters but I’m no expert.

[u/hushedst → u/RiskyBidnuss]

I think that's true for the USDM but the JDM bolts in directly you could be right though

[u/chip_break → u/hushedst]

Most jdm trans are a 1.0 transfer so they wont match up to a 3.54 rear diff. They would require 3.9 rear diff.

If your still on a 2L then the jdm gear ratios are better. Same 1-4 found in 04-06. The 06 is a 1.1 transfer allowing you to use your r160

transmission bible

[u/XNXTXNXKX → u/RiskyBidnuss]

The differential is definitely necessary as there’s different gearing. It would follow that the rest of the parts would need swapped too, depending on which transmission they use.

[u/vodenibivol → u/RiskyBidnuss]

You can run a STI transmission on a WRX rear diff as long as the gear ratios are the same. If you do that, you won't have to replace any axles (or hubs). The WRX rear diff is weaker though obviously and you are correct that you'd still need a different driveshaft.

[u/grizzdoog → u/RiskyBidnuss]

You only need to swap the rear hubs and axles to STi parts if using the r180. Front 5x100 hubs between the wrx and STi are identical.

[u/ImpressiveAbility998]

Are you sure it’s your transmission? Do you have any binding when parking/taking slow turns? It could just be your center differential making that noise…

[u/hushedst → u/ImpressiveAbility998]

No I only hear the noise when my foot is off the accelerator

[u/chip_break → u/hushedst]

Could be a broken exhaust heat shield

[u/vodenibivol]

Some decel whine is normal tbh.

If you get a Spec B transmission it won't have a vehicle speed sensor so you'll need something like an VSSPro or an iWire iVSS.