IAG Street Series Air / Oil Seperator (AOS) - 2015+ STI

IAG Street Series Air / Oil Seperator (AOS) - 2015+ STI





Ships for free in the 48 states
The IAG Performance Street Series billet aluminum Air / Oil separator (AOS) vents crankcase gases back to the turbo inlet. The design is based on years of service on racing and high horsepower Subaru's. The AOS's approximate 1 liter capacity, fully heated housing, superior breathing and unique patent pending baffle design sets it apart from the rest.
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The IAG Performance Street Series billet aluminum Air / Oil separator (AOS) vents crankcase gases back to the turbo inlet. The design is based on years of service on racing and high horsepower Subaru's. The AOS's approximate 1 liter capacity, fully heated housing, superior breathing and unique patent pending baffle design sets it apart from the rest.

NOTE: Blue is close to World Rally Blue and done with automotive paint.


  • Breather Port Rotation For Install Ease
  • Dual Breather Port
  • Greater Pressure Stream Oil Separation

Notice the dual breather port? At first glance you might think this is excessive, however our reasoning is grounded in simple physics. As air and particles of oil enter into the AOS, the largest particles of oil will collect on the inner surface of the can and drain to the bottom, but the smallest particles will remain suspended in the pressurized stream.

As the air makes its way through the multi-stage baffle system, gravity pulls oil from the air. By increasing the overall volume of the breather opening on the top of the AOS, we have drastically decreased the exit velocity of the crankcase vent (CCV) stream. The slower velocity leads to greater oil separation from the pressure stream. A total of 9 mounting holes give the breathing port a full 180° of rotation. On the bottom, a thick Viton o-ring seals the can. Every o-ring in our AOS is formed from Viton which offers superior temperature and chemical resistance.


  • Large Billet Aluminum Design
  • Approximate 1 Liter Holding Capacity
  • Integrated 3 Port Billet Hose Barbs
  • Aluminum Body is Powder Coated In Red, Black, Gold, Dark Silver. Also available in World Rally Blue (automotive paint).

The side of the AOS features 3 port billet hose barbs. There are NO plastic fittings that require thread tape to seal.

The Subaru FA engine has 2 ports that vent crank pressure: two at the top of the block. Each engine vent port receives its own connection to the AOS. By giving each engine port its own entrance to the AOS we have allowed for more effective and faster excavation of crankcase pressure. Additionally this reduces overall system pressure. With a 4" OD and 5.75" height, the AOS has an approximate 1 liter of holding capacity. The larger the AOS, the more power you can safely make without having to worry about the AOS overfilling.


  • Unique Baffling Controls Crankcase Pressure
  • Returns Oil To The Engine
  • No Maintenance or Manual Draining Required

The bottom of the AOS is shaped like a snail, redirecting positive crank pressure into the same swirl pattern produced by the 3 side ports. Allowing maximum oil drainage while separating oil from the air in all the venting ports.

Why not use a catch can? Unmodified Subaru's may require catch can draining as often as every few weeks. Heavily modified race cars may be as often as every race.

Our AOS collects oil and actively drains the oil to the crankcase. The drain port is under the same crankcase pressure as the other ports that are on the side of the AOS. Under normal, off boost driving the weight of the collected oil will easily overcome the pressure in the drain line. However, under heavy boost (especially on higher powered engines) the crank pressure will push back up the drain with the same air/oil mixture that is seen on the side ports. The unique snail design allows the system to breath without disturbing the swirl pattern.


The AOS drain port is able to be rotated for ease of installation.


All fittings supplied with the AOS are anodized aluminum and feature Viton sealed ORB fittings.


The supplied coolant hose is built to exceed SAE J20R3 standards. It will withstand coolant temperatures up to 300°F. The oil vapor hose is designed to withstand oil and fuel vapors, exceeding SAE J30R7 standards.


  • Heated By Engine Coolant
  • Reduces Moisture & Oil Mixing

The back plate of the AOS features a unique coolant routing system that heats the entire AOS to the same temperature of the engine. This maximizes the evaporation of any water collecting inside of the can. The hotter the AOS, the less water will collect inside and mix with the oil.


  • Multi-point Adjustable Mounting Bracket

The center bracket allows for adjustable mounting. If your engine bay changes, you can easily change how and where you mount the AOS. A single laser cut mounting bracket is included that fits nearly all applications and configurations.

A laser cut harness relocation bracket is also included. The bracket allows the engine harness connector to be moved forward for additional clearance.


  • Ready To Install
  • Hardware, Lines & Fittings Included

Your AOS will arrive to you completely assembled and ready for immediate installation. However, the entire unit can be broken down for cleaning. With a single allen wrench, you can remove 4 bolts and the AOS easily separates into individual pieces.


A: In stock configuration, the un-baffled crank case pressure is re-circulated back into the intake stream. This can be harmful for two reasons:

1) Oil vapor mixed with the intake air effectively lowers the overall octane rating of your fuel. The more oil vapor that is sucked back into the engine, the more likely you are to experience engine knock (also referred to as detonation) that can destroy an engine.

2) Oil vapors cause oil & carbon build up over time. The intercooler, intake manifold & intake valves suffer the worst. This oil vapor can create air turbulence that leads to various running issues.

There are a variety of factors that determine how much oil will be in the CCVS. The most significant of these factors are: boost pressure, piston to cylinder wall clearance and oil viscosity. Smaller factors include: ambient temperature, engine temperature, piston ring condition and valve guide condition.

If your AOS is properly installed, there are not any risks.

Catch cans are a air/oil separator that does not actively drain the collected oil back into the engine. Most catch cans require you to drain the contents out manually. While acceptable on some engines that produce little oil in the CCVS, the Subaru engine is not this type of engine. By nature, a turbocharged boxer engine will produce significantly more oil in the CCVS than other engines of similar displacement.

NOTE: IAG recommends the STREET SERIES AOS for ALL cars running E85 or high-ethanol fuel blends. When tuning E85, ethanol vapors tend to stay in the crankcase, which ultimately results in the oil becoming diluted and requiring extremely frequent oil changes. Pulling vacuum via the Street Series recirculating design to the inlet is ideal for ethanol tuned vehicles, and will allow the ethanol to be removed from the crankcase and burnt off as part of the combustion process.

This Part Fits

  • 2015-2020 STI IAG Street Series Air / Oil Seperator (AOS)


Additional Information

Installation Instructions Download here.
Additional Warranty Information None

3 customer reviews

Average rating
(based on 3 reviews)
Ease of Installation
Fit / Quality
Overall Satisfaction
  1. Took 7 hours, instructions are out of date

    By Chris Bergstrom October 20, 2019

    I have a 2019 STi and thankfully I have some mechanical skills. The clamps on at least 6-8 hoses must be cut off with a dremel using a cutting disk. Before installing, order a new gasket for your throttlebody and recirc valve (I have a BOV installed though). Nowhere in the instructions does it tell you what to do with the electrical connections on the drivers/passengers side breather hose inlet. I'm sure not installing these would throw a CEL. So, install the hoses as they specify, then cut the hoses from both sides and install the blow by sensors or whatever they are called. The absolute most difficult part was getting the intercooler back onto the turbo and throttle body. Once we got it installed we tightened everything up and started the car. No leaks, everything worked well. I did order a bottle of Subaru Super Coolant to top off the cup or so we spilled. Lastly, I recommend ordering a 1/2" 90" coolant hose fitting and putting it between the passenger's side coolant line at the top of the thermostat housing and zip tying it to avoid having the hose start to kink against the intercooler. This was an oversight from IAG IMO. Otherwise, it works great. If you are in the Austin, TX area, hit me up via facebook and I can help walk you through the process. The second time would be much faster, maybe 4 hours since a lot of the "figuring stuff out" will already be done. Good luck, take your time and expect and entire day to install it unless you have someone to help you who has done it before. Overall, great product, just some issues with the instructions being outdated, but if you have a clue, you should be able to figure it out.

  2. Difficult install with tmic

    By Ken August 22, 2018

    Just finished installing, and all went pretty smoothly until reinstalling the factory intercooler. The place IAG recommends running the drain hose makes it incredibly difficult to reinstall the turbo to intercooler hose. Also, IAG seems to have instructions to install on 08-14 and 17-18 STI's, but forgot about the 15-16. I use the 17-18 instructions but there seem to be some differences. Hope this helps somebody.

  3. Amazing Piece of Equipment

    By Alex September 21, 2017

    My local tuner says an AOS is a must for the EJ and the IAG kit is quite the piece of equipment. Takes a lot of the guesswork out of the equation vs a catch can and some of the other solutions on the market. Haven't seen a drop of oil in my intercooler since install which was a bit of a pain but take your time, measure your hoses right, and you won't have any problems.

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