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Discussion Starter #1 (Edited)
There's not much love from the AOS manufacturers for TMIC equipped right-hand drive STI’s. Certainly not here in Australia, and surprisingly not even in Japan, as they seem to prefer catch cans. The only Japanese AOS I can find is from Prova, which utilises a design similar to Grimmspeed's AOS and is unsuitable for the track. Crawford says their V3 AOS is suitable for RHD vehicles, but it only has a port for the PCV but not the rocker covers. They don't recommend it for track use either.

I bit the bullet on an IAG V3 AOS and had it shipped from the states, which turns out to be an excellent choice because it is designed as a universal AOS, so you can rotate the top and bottom ports, with two brackets designed for the WRX STI. And the brackets fit almost perfectly.

TL;DR: You only need to drill a 6mm hole on the passenger's side strut tower, and have a vice and hammer to bend the ECU connector bracket. If you are running an FMIC, it might be easier to use IAG’s FMIC bracket.

End results:



Stuff you need:
  • IAG V3 AOS Street (obviously) IAG-ENG-7181
  • 5/8" / 15.9mm / -10 AN size PCV / fuel (non-injection) hose, one metre is enough
  • M6 bolt (20mm min), nuts, and washers
  • 2x -6 ORB plugs (if you are not running coolant lines), hex key that fits
  • 1x -8 ORB plug (if you have a TMIC), hex key that fits
  • 1/2 barb tee junction (if you have a TMIC)
  • nylon braid (make sure it's polyamide/PA and not PET, which can melt under engine heat) and heat shrink
  • high temp electrical tape (I used 3M Super 33+) or silicone tape
  • bench vice
  • hammer
  • drill and 6mm / 1/4" bit
  • 10mm hex key
Depending on your fueling system: I use a GFB FX-R fuel pressure regulator, which may have made this install slightly easier. If you run a stock FPR, you may or may not need to reroute some of your fuel hoses. The OEM fuel system uses 8mm fuel injection hoses. You can use Gates’ Barricade 5/16" fuel injection hose (#27348 for 15’, #27340 for 25’), and 14mm fuel injection T-bolt hose clamps.

Optional but highly recommended:
  • hands with fingers
  • functioning eyes
  • a brain
Note: Not all of the steps on the IAG manual apply to your car, especially ones regarding the PCV parts. US cars have more emissions equipment than Australian cars. Australian intake manifolds have a simple mechanical check valve for the PCV port instead of whatever wizardry they use over there.

Throughout the guide, I will make references to parts in the official IAG installation manual. You can download it here. Make sure the header of the document says IAG-ENG-7181- Street Series Air / Oil Separator (AOS) for 2015-20 STI rev. | 46783 v4 [Published] 2020-12-31 12:47 to ensure you are using the same version as I am. If you don’t know how to do something, IAG has probably explained it in their manual too.

  1. Remove your intake manifold.
  2. Remove your engine harness bracket.
  3. Remove your fuel pressure regulator for easier access.
  4. Pull out the OEM right angle PCV hose between your engine block and manifold. Remove the IAG 1/2” PCV hose (ref step 5/137) and stick it into the OEM PCV hose. The Australian PCV hose’s inner diameter should fit the IAG PCV hose’s outer diameter perfectly. You will be able to blow air from the IAG hose into the OEM hose if the hose's check valve orientation is correct. Tape the two hoses to ensure there won’t be any vacuum leak.
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  6. Install the bracket to the AOS using the bottom left holes. Torque the bolts to spec.
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  8. Undo the screws for the oil drain hose and rotate it to the angle shown.
  9. Connect the 1/2" oil drain hose to the AOS. Do not cut the hose yet.
  10. Remove the bottom coolant lines on the AOS if you don’t want to run them. I am currently experiencing 30+ Celsius summer days so I am not gonna bother. Cap the holes with your -6 ORB plugs.
  11. Undo the screws for the top breather block and rotate it to the angle shown.
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  13. If you have a TMIC, remove the top -8 ORB barb on the AOS with the 10mm hex key. This barb will interfere with your TMIC if not removed. Cap the hole with your -8 ORB plug.
  14. Locate the holes used to install the AOS on your passenger’s side strut tower, behind the FPR bracket. The silver zinc bolt uses a factory hole. Drill the hole used for the yellow zinc bolt and nut. You may want to mockup the AOS with only one bolt and mark the other hole when it’s level before drilling. Insert an M6 bolt from the wheel well side and secure it with a nut. Slide the AOS over the stud you created and bolt it down. You may need to slightly bend the bracket and/or enlarge the upper hole on the bracket.
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  16. Assemble the 3/4” hose to the PCV Y fitting (ref steps 58-61/137). Cut the 3” of 5/8” hose required to create the right-angle junction (ref steps 107-108/137) from the 5/8” hose supplied by IAG.
  17. Install the PCV Y fitting and connect the 5/8” and 1/2" PCV vent hoses and oil drain hose. Make sure the oil drain hose runs downhill all the way. Use braiding to protect the hoses that may contact the gearbox or other metal parts. Route as shown.
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  19. Do not cut the hoses before verifying fitment with your TMIC! If you have a TMIC, cut 3” of 1/2” breather hose, connect one end to the middle vent port of the AOS and the other end to the tee junction. Connect both rocker cover breather hoses. Make sure to use a heat sleeve for the driver’s side hose over the turbo. Connect the PCV vent to the AOS using the 1m 5/8” hose you purchased. Make sure to run a heat sleeve over the turbo. Zip tie the hoses right above the pitch stop to prevent the hose from contacting it.
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  21. Time to route your intake breather hoses. You can run only one and cap the other, but IAG recommends that you use both hoses to reduce the air velocity and ensure maximum separation.
  22. Route one of the breather hoses under your throttle body and connect it to the port on your turbo inlet near the turbo.
  23. Route the other one along your firewall. You can either connect it directly to your turbo inlet near the filter or connect it to the factory hose that’s already there using the supplied 1/2" barb fitting.
  24. Bend the IAG ECU harness bracket as shown.
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  26. Install the ECU connector on the second hole under the hole you drilled for the AOS bracket.
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  28. Tie the engine harness to the A/C pipe to ensure it doesn’t rub on the battery cable bracket.
  29. Put your TMIC back.
  30. Done! Grab a cold drink to reward yourself.

496 Posts
Wow very well done! Wish we didn't have half the emissions stuff here...I feel it's over rated but what do I know decatting all my cars...

5 Posts
Discussion Starter #3
Wow very well done! Wish we didn't have half the emissions stuff here...I feel it's over rated but what do I know decatting all my cars...
Australia's pretty lax on auto emissions laws in general. Our petrol is high in sulfur, which increases pollution and clogs particulate filters, which is why European carmakers remove the filters for most of the cars in the Australian market. I guess it doesn't matter much because Aus is 78% of the area of the US but has only 8% of the population, so there aren't many cars going around here. Subaru Australia just celebrated the "milestone" of selling 10,000 STI's, despite having been selling them since 1998, so you get the picture.

Pollution is a terrible business though, there should be good emissions equipment on new cars. If you visit China (Beijing, Shanghai etc.) during the smog season your throat literally starts hurting before you even get off the plane/train. Sometimes it gets so bad you can't see what's across the street. But the number of enthusiasts that remove said equipment is probably too small to matter, so I don't have a problem with some people running catless either.
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