IAG Performance Competition Series Air / Oil Separator (AOS) For 2008-14 Subaru Impreza WRX & 2008-19 STI

IAG Performance Competition Series AOS Air / Oil Separator For 2008-14 Subaru Impreza WRX and 2008-19 STI

Alternative Views:

MSRP: $398.99
Our Price: $379.99
Savings: $19.00
Product Info: 17-18 STI
Product Info: 15-16 STI
Product Info: 08-14 STI
Product Info: 08-14 WRX / 05-09 LGT

Availability: Usually Ships in 24 to 72 Hours
Part#: IAG-ENG-7251

AOS Color*:
Black Part# IAG-ENG-7251BK
Red Part# IAG-ENG-7251RD

Description Applications
The IAG Performance Competition Series billet aluminum Air / Oil separator (AOS) vents crankcase vapors to the atmosphere. 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 set it apart from the rest.

*May require tune recalibration.


  • 180° Breather Port Rotation For Install Ease
  • Large Breather Port
  • Greater Pressure Stream Oil Separation

Notice the large breathing 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 diameter 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 enables 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 Powder Coated Black, Red, Gold and Dark Silver. Also available in World Rally Blue (automotive paint).

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

The Subaru EJ engine has 3 ports that vent crank pressure: one on each cylinder head and another on 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 breathe without disturbing the swirl pattern.


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


The supplied coolant hose is built to SAE 20R3 standards. It will withstand coolant temperatures of +257°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 as 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
The AOS arrives 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.


Q: Why do I need an AOS?
A: In stock configuration, the un-baffled crankcase 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.

Q: What are the factors that cause oil in the crankcase vent system (CCVS)?
A: 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.

Q: Are there any risks to using the IAG billet AOS?
A: If your AOS is properly installed, there are not any risks.

Q: What's the difference between a catch can & the IAG AOS?
A: Catch cans are an 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 EJ 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.

Q: Which AOS is right for my vehicle?
A: It is IAG's opinion (which may differ from some builders and tuners) that you should use the following parameters to decide which AOS is correct for your vehicle:

  • Less than 600 WHP on pump gas – IAG recommends the Competition Series AOS. The Competition series provides the best venting of the PCV system and also prevents any oil suspended in the PCV gases from re-entering the turbo inlet. If you prefer not to retune the vehicle for installation of the AOS, the Street Series may be used below 600 WHP.
  • Less than 600 WHP on E85 – IAG recommends the Street Series AOS
  • Greater than 600 WHP on E85 / PUMP GAS / RACE GAS – IAG recommends the Competition Series AOS

*Please note that IAG recommends the STREET SERIES AOS for ALL cars under 600 WHP 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. When running greater than 600 WHP on E85 IAG recommends the COMPETITION SERIES AOS and 1500-2000 mile, or every race event, oil change intervals. Please note that the COMPETITION SERIES AOS at any power level will require a custom tune.

PDF Instructions for 2017-19 STI IAG Performance Air Oil Separator

PDF Instructions for 2015-16 STI IAG Performance Air Oil Separator

PDF Instructions for 2008-14 STI IAG Performance Air Oil Separator

PDF Instructions for 2008-14 WRX IAG Performance Air Oil Separator

PDF Instructions for 2005-09 Legacy GT

* The IAG AOS is not compatible with most aftermarket strut tower bars.

WARNING: This product can expose you to chemicals which are known to the State of California to cause cancer and reproductive harm. For more information, visit www.P65Warnings.ca.gov.

Average Rating: Average Rating: 5 of 5 5 of 5 Total Reviews: 18 Write a review »

  0 of 0 people found the following review helpful:
5 of 5 Great product April 29, 2019
Reviewer: John Rodriguez from Hobart, IN United States  
I really like the iag aos. I'm glad I purchased this over other aos brands. I highly recommend this aos to anyone that is looking for one.

Was this review helpful to you?

  0 of 0 people found the following review helpful:
5 of 5 Air oil separator March 5, 2019
Reviewer: Alyssa K. Wolf from Syracuse , NY United States  
Quick shipping.  Not on the car yet

Was this review helpful to you?

  0 of 0 people found the following review helpful:
5 of 5 competition series AOS January 15, 2019
Reviewer: chris Fogg from lake oswego, OR United States  
Best quality AOS I have seen in the past 10+yrs of working on Subaru's. quality parts from the zip ties to hoses and coolant fittings. only thing I would add is some fire sleeve for the hoses close to the turbochager.
keep up the kick ass products!

Was this review helpful to you?

  0 of 0 people found the following review helpful:
5 of 5 Great kit and product! December 17, 2018
Reviewer: Peter Modjeski from Hartford, WI United States  
I have a heavily modified 2009 WRX STi.  I have a much larger turbo, in the stock location, with upgraded fuels rails, lines, injectors, different mounting brackets for the intercooler, etc... I had removed the previous AOS from another vendor as part of this work, and I pulled my intake manifold (not required by the instructions for the IAG Competition AOS, but nice to do based on my other modifications).

My fuel pressure regulator was relocated to the side where the IAG AOS is recommended to be mounted, and my wiring harness was already relocated.  I had to make some adjustments for the size and placement of my other non-stock parts, but the IAG kit and the instructions, with great pictures, provided everything I needed to safely route everything and remove the PCV system.  The fitment, though tight, is perfect for my car.  With this kit and the instructions provided, I was able to revert my engine bay to be more like stock than it had been plus I upgraded to a much better AOS.

Was this review helpful to you?

  0 of 0 people found the following review helpful:
5 of 5 5 Stars August 22, 2018
Reviewer: Jarrett Mansfield from Charlotte, NC United States  
5 Stars

Was this review helpful to you?

View All Customer Reviews