Click here to download installation instructions (PDF)
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-"R" 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. The AOS is constructed of three CNC machined sections that screw together and are sealed with O-rings.
Compatibility
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02-07 Subaru Impreza WRX
02-07 Subaru Impreza WRX STi
04-08 Subaru Forester XT
05-06 Saab 9-2X Aero
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09-13 Subaru Forester XT
08-14 Subaru Impreza WRX
08-14 Subaru Impreza WRX STi
15-21 Subaru WRX STi
NOTE: For 2015+ Subaru Forester XT/WRX (non STi) go HERE.
Dimensions
AOS Height: 6.3" (without fittings)
AOS Diameter: 3.45" Round
AOS Capacity: 20.3 oz (600mL)
Ports
Inlet: 7/8"-14 threaded for 10AN ORB fittings
Outlet: 7/8"-14 threaded for 10AN ORB fittings
Return: 7/8"-14 threaded for 10AN ORB fittings
Coolant: 7/16"-20 threaded for 4AN ORB fittings (x2)
Benefits
-Effectively raises the 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
Included
-Machined alumium Radium Engineering Air Oil Separator
-Subaru specifically designed mounting bracket
-All necessary SAE J30R7 rated PCV/Emissions hose
-Coolant hoses and fittings
-All necessary aluminum fittings and hose-ends
-Stainless steel mounting hardware
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*For 02-05 Subarus with a cable-actuated throttle body, our Cruise Control Relocation Bracket must be used.
*If operated in freezing environments, Radium Engineering recommends installing insulation sleeving (not included) over any hose exposed to cold air flow. This will help prevent the natural phenomenon of water condensation freezing and potentially clogging the hose.
*For RHD vehicles, our Universal AOS-R must be used for custom mounting and plumbing.
*The Radium AOS-R kit may not be compatible with all other aftermarket products.