High-powered modified Subaru engines tend to generate excessive blow-by oil and gasses. In these cases, an ordinary small catch can may quickly fill with oil and not be adequate. Like the Radium Engineering Subaru Dual Catch Can Kit, the AOS (Air Oil Separator) kit is responsible for keeping blow-by oil and other pollutants out of the intake pipe, manifold, turbocharger, and intercooler during all engine loads. The main difference is that an AOS system returns what is collected back into the oil pan so there is no servicing required. The built-in heater keeps water from condensing in the AOS. And because water is more dense than oil, there is an integrated water trap found around the bottom perimeter of the can. This prevents water from descending into the oil pan if ever-present. Furthermore, because the water is trapped against the heater, it will eventually boil and evaporate out of the system.

NOTE: This AOS uses the Radium Brake Master Cylinder Brace as a mount, and is necessary to install this AOS. If you already have one, please select “No, I already have 20-0255” in the above drop-down menu.

This kit is compatible with the 2015+ Subaru WRX and 2014+ Subaru Forester XT with the FA20F engine (left-hand drive models only).


Just like an AOS for the EJ engines, the same theories apply for this application. However, the direct-injected Subaru FA20F engine, in particular, has an inherent problem since there are no port injectors to clean the backside of the intake valves.

The AOS is constructed of CNC machined sections that screw together and are sealed with O-rings. The AOS can be completely disassembled for cleaning or servicing.

Benefits
-Effectively raises the fuel octane and increases engine performance with cleaner inlet air
-Prevents oil buildup in the intake or intercooler system walls, throttle body, intake manifold, etc.
-Lowers hydrocarbon emissions

2015+ Subaru WRX Installation
The Radium Engineering AOS kit installs on the LH side of the engine bay, near the brake master cylinder. The outlet (side port) of the AOS is plumbed back to the turbo inlet pipe, creating a completely sealed system.

The AOS mounts to the Radium Brake Master Cylinder Brace, as shown below. This brace is REQUIRED for installation.

The PCV valve is replaced by a custom machined straight-through adapter fitting, shown below. Oil collected in the AOS drains out the bottom fitting and back into the engine block through this fitting.

Kit Contents
-Machined aluminum Radium Engineering Air/Oil Separator
-Specially designed mounting bracket
-All necessary SAE J30R7 rated PCV/Emissions hose
-Coolant hoses and fittings
-PCV delete fitting
-All necessary aluminum fittings and hose-ends
-Stainless steel mounting hardware
-20-0255 Master Cylinder Brace (IF SELECTED)
Measurements
AOS Height: 6.94″ (top of dipstick to bottom of AOS)
AOS Diameter: 3.45″ Round
AOS Capacity: 20.3 oz (600mL)

NOTE: If operated in freezing environments, Radium Engineering recommends installing insulation sleeving (not included) over any hose exposed to cold airflow. This will help prevent the natural phenomenon of water condensation freezing and potentially clogging the hose.