Buy Now, Pay Later
Share This

Improve engine performance and longevity by adding a PERRIN Air Oil Separator to your Subaru WRX, STI, Legacy GT, or Outback XT.

Subarus have a reputation for high performance and being modification-friendly, making them a popular choice for tuners and enthusiasts. They also have a reputation for high oil consumption, which undercuts these qualities because it can lead to excess oil getting pulled into the intake and detonation that leads to ringland failure. These issues are also only exacerbated by the presence of power mods and imperfect calibrations. To help eliminate these problems and help fully realize the engine’s capability, we created an air oil separator (AOS) for the Subaru WRX, STI, Legacy GT, and Outback XT. 

What Is An Air Oil Separator?

An air oil separator is a device that separates oil from the air that flows through the engine crankcase ventilation system. In a typical engine, blow-by gases from the combustion process are forced past the piston rings and into the crankcase. These gases are then vented out of the engine through the positive crankcase ventilation (PCV) system. However, along with the blow-by gases, small amounts of oil can also be carried out of the engine and into the PCV system. This oil can accumulate in the intake system, reducing the effectiveness of the intercooler’s ability to cool and affecting the engine’s resistance to detonation/knock. Not only can this affect the engine’s HP output, but it can also lead to engine damage. An air oil separator traps the oil before it can enter the intake system and recirculates it through an oil drain.

Air Oil Separators vs. Catch Cans

Catch cans are like air oil separators, but they capture engine blow-by and store it in a container instead of recirculating it. This is a common but lower-performing design that requires constant attention and the removal of oil every few hundred miles.

Psp Eng 605 Split Graphicx800

How Do Air Oil Separators Work?

Air oil separators typically consist of a canister system mounted in the engine bay, connected to the crank case vent hoses coming off the engine. The canister contains a baffle or other mechanism that separates the oil from the air as it passes through. The separated oil then accumulates in the bottom of the canister, which is then drained back to the engine.

The PERRIN Air Oil Separator Vs. Standard AOS Systems

PERRIN Air Oil Separators feature an advanced design that improves the collection of oil from the crank case vents and reduces maintenance of the canister. The PERRIN AOS utilizes a multi-baffle design combined with swirl pot technology to increase the surface area capable of collecting oil, maximizing the unit’s efficiency despite its compact size. The base plate of the air oil separator is heated by coolant drawn from the engine, which decreases water condensation to allow oil, air, and water vapor to separate and pass through the body. This reduces the buildup of the milky sludge that happens in typical air oil separators.

As oil is separated and drains from the bottom of the PERRIN Air Oil Separator, clean, oil-free air vents out of the top to rejoin the intake tract. This means that the AOS does not have to be regularly removed, emptied, or cleaned.


Benefits Of Upgrading To A PERRIN Air Oil Separator

  • Reduce engine-damaging knock/detonation
    • By reducing the amount of oil from entering the intake system, an air oil separator can significantly reduce the potential for knock/detonation that occurs. This can significantly improve the engine’s lifespan and performance by controlling the engine’s damaging knock events. It can also help to prevent the buildup of carbon deposits, which can cause issues like knocking and decreased power output.
  • Reduced maintenance costs
    • Over time, the buildup of oil in the intake system can cause damage to components like the PCV valves, sensors, hoses, and intake valves. This can lead to costly repairs and decreased engine performance. By installing an air oil separator, you can reduce the amount of oil that enters the intake system and prolong the life of these components.
  • Improved fuel economy
    • A clean air-fuel mixture can also help to improve fuel economy by ensuring that the engine is operating at peak efficiency. This can result in savings at the pump over time, making an air oil separator pay for itself over time.
  • Easy DIY installation
    • Air oil separators are DIY-friendly to install on Subarus, with no special tools or modifications required. This makes it a convenient upgrade for those who want to improve their car's performance. 


Installing a PERRIN AOS takes about 3 hours and can be done with a basic set of tools. It is not difficult, but it does require patience as it involves rerouting a handful of hoses. The general process involves mounting the AOS, rerouting the oil drain hose to connect with the drain adapter, rerouting the coolant hoses into the AOS, rerouting the front and rear crankcase vent connections, and then setting up the AOS outlet and PCV connection.

Once the AOS is installed, it requires little maintenance. From time to time, it can be removed, and the oil residue can be wiped out, but that is about it.

Psp Eng 60Xrd Install Gallery 02


Air oil separators are a great upgrade to improve performance and longevity. By adding an AOS, you can eliminate the issues caused by high oil consumption and keep oil from getting pulled into your intake system. If you have already begun adding power modifications, this is a crucial upgrade to prevent detonation and related engine damage, as well as allow your tune to reach its fullest potential. If you have an intercooler on your car, the PERRIN AOS is also designed to complement it to further improve overall performance increases. Overall, adding an AOS to your Subaru is an easy decision that pays for itself.

Vehicle Compatibility

  • Subaru WRX
  • Subaru STI
  • Subaru Legacy GT
  • Subaru Outback XT
  • BRZ/FR-S/86/GR86
Share This

Thank you!

Thank you for signing up.

© 2024 PERRIN - All Rights Reserved
  • Visa
  • Mastercard
  • PayPal
  • Wire Transfer