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Old 07-20-2017, 11:02 PM   #1
Sparc
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Default 2015+ VA WRX to STI Transmission Swap Thread

This post covers specifically swapping a 2015-2017 STI driveline into a 2015-2017 WRX MT. It should apply to 2018 WRX without issue. 2018 STI donor is questionable due to a new DCCD (not enough info on it or any transmission changes). You can use older transmissions but not other parts (driveshaft must be 15+ for example). Why put old parts into a new car though?

Difficulty
If you can replace a clutch and do basic wire splicing you can do this swap. It's about as straight forward as a transmission swap can get.

Why?
STI transmission, diff, and axles handle much more power.
STI transmission shifts immensely smoother compared to the WRX.
STI has DCCD center, helical front, and clutch type rear LSD. WRX has none.

Parts required from 15+ STI:
These are the only parts required. You do not need anything else from the STI!
  • Transmission and bellhousing bolts (no transmission mount or bracket needed)
  • Transmission wiring harness
  • Starter and bolts (STI starter bolts are shorter than WRX)
  • Slave cylinder and banjo bolt (different banjo bolt, same size banjo). It's possible the WRX one will fit, I didn't check.
  • Clutch fork
  • Clutch fork return spring
  • Clutch fork dust boot
  • 9 total pressure plate bolts (WRX has only 6)
  • Flywheel - if not using new
  • 6 flywheel bolts - if not using new. From target WRX, or donor STI, they are the same.
  • Pressure plate - if not using new
  • Clutch - if not using new
  • Throwout bearing - replace with new always, it sucks to have this part fail
  • Clutch housing dust cover (pressure plate hits the WRX one)
  • Driveshaft
  • All 4 axles
  • Rear diff (no mount needed)
  • Shifter, shifter linkages, and shifter transmission bushings
  • Shifter mount (body bushing)
  • Shifter tunnel seal (steel plate and rubber boot to chassis hole)

Harness work:
Remove o2 sensor extension harness as it's not needed.

There is a premade harness here, you can make your own as well.

There are two ways to splice the harness that I can think of that are good:
Cut WRX transmission harness body connectors off and put those ends on the STI transmission harness, which is the way I went. You have to make your own DCCD connector if you are wiring it up. This leaves the WRX body harness intact.

Cut the STI body harness and put the STI end on the WRX body harness. This has the advantage of providing a connector and leads for the DCCD on the body end. Disadvantage of cutting the body harness on both donor STI and target WRX.

There are three circuits from the STI transmission: reverse, neutral, and DCCD control.

The reverse circuit is just a momentary switch that is engaged when the transmission is put into reverse. This is identical to the WRX, except the wire colors are different. The polarity don't matter because it's just a single pole switch, you can wire it "backwards" without issue.

The netural circuit is the same design.

The DCCD controller is a high current PWM signal and polarity doesn't matter here either.

The WRX body harness connectors are grey and brown. One is the reverse sensor, one is the neutral sensor. I forgot which is which, but if you pull the old transmission you can shift it around and find out which is which. One will be a closed circuit in neutral, the other while in reverse. If someone confirms, I'll update this, or next time I pull my intercooler.

Fab work:
Create I pipe hanger to replace the old one. Literally, that is it. I took the old one, hammered it almost flat, cut it to shape, drilled a new mount hole, and bolted it to a bolt that lined up.

DCCD controller:
You don't need one, but it is recommended. Wiring this up is the same as wiring it to a STI except you need to run the DCCD leads.

Notes:
I did this on four sturdy and tall jack stands, you don't need a lift. However, you absolutely need a transmission jack. Don't try using a floor jack, it won't work, you will just hurt yourself. I highly recommend a lift, since getting the transmissions under the car sucks on stands.

The wheel hubs and axle splines are identical, no swap needed there.

The bell-housing bolt pattern is identical, no surprises there.

Some have said there is nub on the engine block that should be trimmed as it could interfere with the flywheel. I've had no issues here but others have shaved it for good measure.

You will experience more NVH with this transmission, at a minimum through the shifter. I'm on full poly mounts, previously had a small vibration at 3100rpm to 3400rpm, it's now much more apparent and 3000rpm to 4000rpm. I'm wondering if others that have done this with poly mounts have the same vibration or not.

The gear ratios and final drive are different. You will lose highway MPG. I gained 1-2mpg on average so far, with how I drive and where I live. With the WRX transmission, I'm forced to constantly downshift to 4th to pass on the highway. No longer is that necessary. 6th gear at 60mph is 2400~rpm and 5th gear is 3100~rpm at 60mph.

I find the gear ratios of the STI transmission much more natural than the WRX.

If you don't mind some extra NVH, do every shifter bushing while it's out of the car, some of them require pulling the entire shifter and linkage, so don't do it twice.
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Last edited by Sparc; 08-25-2017 at 12:03 AM.
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Old 07-21-2017, 03:49 AM   #2
immortal
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Nice post man, very detailed. I'm about to do this swap on my 15 WRX.

I look forward to the improved gear ratios.
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Old 07-21-2017, 01:40 PM   #3
killadawg
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Hey guys, I'm currently doing this now as well on my 2016.. I have the full front and rear sub frame from my donor Sti as well.. (I found one with less than 1k miles on it) I will be putting in MapDCCD with Advanced VSS. I as well will be changing every bushing.. even the sub frame ones as well as other components (looking for a free weekend now, as I have limited time with 2 kids and full-time gig "time" is limited).

I'm also changing all the suspension components that I have not upgraded while I'm at it

VooDoo trailing arms, and Rear Toe Arms to go along with my current Velox LCA and Perrin SS end links. I will be adding all remaining Whiteline bushings etc.. nvs, prob. some.. I already have Perrin motor, trans, and pitch stop brace with whiteline pitchstop.

for the transmission, I will be replacing with a South Bend Stage 3 endurance clutch kit

upgrading the drive shaft to a PST carbon drive shaft

(I hope the sti 2015 carbon shaft works.. already bought it)...

Is is worth it to replace the front LSD with a Quaife? or just leave as is?

gonna run motul ester based gear oil.

BTW thanks for this post, I will chime in as I get mine done and take pics for others once I get there.

Any other info would be great as this will be first swap (any pictures of your fab work and or wiring?)

Thanks, Great post!
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Old 07-21-2017, 03:14 PM   #4
Sparc
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Quote:
Originally Posted by killadawg View Post
I have the full front and rear sub frame from my donor Sti as well..
The rear subframe is identical so no need to swap. The front I'm not sure will work with the WRX motor or rack.

Quote:
Originally Posted by killadawg View Post
for the transmission, I will be replacing with a South Bend Stage 3 endurance clutch kit
Thats the clutch I used, feels good so far.

Quote:
Originally Posted by killadawg View Post
upgrading the drive shaft to a PST carbon drive shaft

(I hope the sti 2015 carbon shaft works.. already bought it)...
Yes they will, the VA STI/WRX is identical body with only different components. If you replace both components (diff, transmission), anything between them will fit as well.

Quote:
Originally Posted by killadawg View Post
Any other info would be great as this will be first swap (any pictures of your fab work and or wiring?)

Thanks, Great post!
I typically forget to take pictures. I did the harness, wrapped it up, and forgot to take pics. There is only 3 circuits 2 wires each. I'll edit the top post with some more information here.
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Old 07-22-2017, 03:50 PM   #5
killadawg
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Awesome advice! You rock, now to make sense out of my mapDccd box that came with more wires than anything else.. :-) I might have a pro do that one..

Good to know about the hubs, I was dreading having to swap those. Looks like I need to purchase a trans jack for this, or rent one. And get some big blocks of wood to get higher off the ground.

Thanks again
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Old 07-24-2017, 09:19 PM   #6
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i heard something about the gear position sensor not working, what's up with that, is it true? there's ECU tables that need it and I'm not willing to downgrade/retune...

i got FF tuned and now with the torque i'm hearing the front inside tire light up on corner exit... i need an LSD at the front. i bought an old 4.1 R160 to put a Quaife ATB in but i'm not sure if i'm going to do that yet since there's no clear front LSD upgrade path for this trans and an sti swap is only a little more than f/r diffs+axles and whatever else you need for this...
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Old 07-29-2017, 01:53 AM   #7
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Is it 100% the case that the front axles are required for the swap? I have read on multiple threads that only the rears are required, and this is the first conflicting report I've seen so far
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Old 07-30-2017, 05:05 PM   #8
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I believe they are the same length, and the same spline. I don't see why you want to keep the weak WRX axles however. The STI ones are much beefier.
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Old 07-30-2017, 05:07 PM   #9
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Quote:
Originally Posted by _brian_ View Post
i heard something about the gear position sensor not working, what's up with that, is it true? there's ECU tables that need it and I'm not willing to downgrade/retune...

i got FF tuned and now with the torque i'm hearing the front inside tire light up on corner exit... i need an LSD at the front. i bought an old 4.1 R160 to put a Quaife ATB in but i'm not sure if i'm going to do that yet since there's no clear front LSD upgrade path for this trans and an sti swap is only a little more than f/r diffs+axles and whatever else you need for this...
The gear position sensor is a map between wheel speed and RPM in the ECU. We cannot adjust this map because cobb has not exposed it. The only gear I notice any issues in is 5th, where it always reads as 4th. It sometimes takes an extra half second (like it's hunting) to display the value.
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Old 07-30-2017, 05:25 PM   #10
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Quote:
Originally Posted by Sparc View Post
The gear position sensor is a map between wheel speed and RPM in the ECU. We cannot adjust this map because cobb has not exposed it. The only gear I notice any issues in is 5th, where it always reads as 4th. It sometimes takes an extra half second (like it's hunting) to display the value.
thanks
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Old 08-03-2017, 04:11 AM   #11
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i have cvt wrx 2016. my question is do i need different part like (wiring) or it's the same part and progress to do the swap?
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Old 08-03-2017, 01:56 PM   #12
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Quote:
Originally Posted by Boowrx View Post
i have cvt wrx 2016. my question is do i need different part like (wiring) or it's the same part and progress to do the swap?
Your wiring will be much more complicated. I'm not sure someone has done it yet. Basically, you have a transmission controller unit we don't have with the manual. The ECU and TCU talk to each other over the CAN bus. It could be, this is one way (ECU->TCU). If the ECU is expecting output from the TCU, it's unlikely this swap will work without a complete ECU/harness swap.
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Old 08-03-2017, 04:46 PM   #13
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Hey Sparc

Any pics of your wiring? I will be doing this on Sat. I will take as many pics as I can an if it isn't a problem, I will add those to this thread so others can follow if they choose to do so as a DIY.. looks like a couple buddies of mine will let me use their lift!! fingers crossed all goes well, I will let you guys know by Sun. night or Monday morning!! ;-)
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Old 08-04-2017, 12:19 AM   #14
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Good luck and be sure to take/share plenty of photos! I'm sourcing components right now and I'll be doing the swap myself soon.
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Old 08-04-2017, 10:16 AM   #15
killadawg
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Well I hope I can figure out my mapDCCD wirring.. lol here is a pic of the wire mass that they ship with.. ;-)
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Old 08-04-2017, 08:52 PM   #16
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Postponed my install as I decided to get a ready made harness from iwire subaru guys.. gonna be another week or two. ..
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Old 08-05-2017, 04:46 PM   #17
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Mapdccd is much worse than the transmission wiring. It has many taps to read signals like RPM, throttle position, etc.

Feel free to post pictures etc here, more info for others is a good thing.
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Old 08-07-2017, 02:26 PM   #18
killadawg
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BTW this will help with the trans swap even if you don't do a DCCD .. will require no splicing etc.. and power to DCCD module if you have one is already setup..


I will let you guys know how this goes.. I will do install, use the above pigtails to hook it up and have ESC Performace do the MapDCCD install.. a bit over my head.. lol and I wish to retain warranty on my MapDCCD unit anyway.. lol best of luck

Last edited by killadawg; 08-25-2017 at 09:30 AM.
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Old 08-07-2017, 09:50 PM   #19
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That harness pictured won't work at all. Those are not the connectors present on the body harness on the VA WRX.
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Old 08-08-2017, 12:49 PM   #20
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Quote:
Originally Posted by Sparc View Post
That harness pictured won't work at all. Those are not the connectors present on the body harness on the VA WRX.
Correct, but this one would - https://www.iwireservices.com/produc...-trans-adapter
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Old 08-08-2017, 02:23 PM   #21
killadawg
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looks like I choose the wrong one.. Iwire caught this thanks to Sparc!! they are sending me the correct wires now! It seems they had not done a VA one and the setup is similar to the NA setup. Thanks to both.. again I will update once I have all my parts in and installed.
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Old 08-08-2017, 04:02 PM   #22
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Looking at that NA harness, it appears to be correct but I don't have a side by side picture with the body connectors. It is in fact two, two pin connectors, of similar shape.
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Old 08-09-2017, 06:58 PM   #23
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Quote:
Originally Posted by Sparc View Post
Looking at that NA harness, it appears to be correct but I don't have a side by side picture with the body connectors. It is in fact two, two pin connectors, of similar shape.
I dug into a WRX close to us and verified that they went back to the older Subaru NA trans connector style, but he'll confirm once he gets it.
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Old 08-10-2017, 04:41 AM   #24
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Just curious on what all this would cost or what I would be looking at spending to have all of this installed. I recently got denied for a warranty on my tranny due to "agressive driving" and my pinion in the transmission was basically shaved down to nothing. I have a 2017 wrx with 8k miles on it so I am looking to do the same as you in swapping for a STI trans and everything on that list you just named off. Just looking into who can do it and how much it will cost to get done. The trans is out of my car right now so I would just be paying for parts and install of everything. Also are you in SoCal and if so who did you go through to do all your work or did you do everything yourself.
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Old 08-10-2017, 06:54 AM   #25
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Quote:
Originally Posted by TrapMickey View Post
Just curious on what all this would cost or what I would be looking at spending to have all of this installed. I recently got denied for a warranty on my tranny due to "agressive driving" and my pinion in the transmission was basically shaved down to nothing. I have a 2017 wrx with 8k miles on it so I am looking to do the same as you in swapping for a STI trans and everything on that list you just named off. Just looking into who can do it and how much it will cost to get done. The trans is out of my car right now so I would just be paying for parts and install of everything. Also are you in SoCal and if so who did you go through to do all your work or did you do everything yourself.


Interested to find out as well. I've seen STI transmission, rear diff, and axles for sale for $3500+. If you want to do a new clutch and flywheel add at least another $500+. And then the cost of install is ?
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