Radium Engineering 20-0607 AOS-R Air Oil Separator for 08-21 Subaru WRX/STI

Vehicle Fitment

• 2009-2013 Subaru Forester XT (EJ255; LHD only)
• 2008-2014 Subaru WRX (EJ255; LHD only)
• 2008-2014 Subaru WRX STI (EJ257; LHD only)
• 2015-2021 Subaru WRX STI (EJ257; LHD only)

Fitment Notes

• This kit is for left-hand drive (LHD) vehicles only. For right-hand drive (RHD) applications - JDM, AUDM, and other markets - the Radium Universal AOS-R kit must be used with custom mounting and plumbing, as this bracket and hose configuration is specific to LHD engine bay geometry.
• This kit covers the EJ-platform Subaru WRX and STI (EJ255/EJ257 turbocharged flat-four). It is not compatible with the 2015+ Subaru Forester XT or the 2015+ WRX (non-STI), which use the FA20DIT turbocharged engine. A separate AOS-R part number is required for those models.
• For 2002-2007 Subaru WRX, WRX STI, and Forester XT, as well as the 2005-2006 Saab 9-2X Aero, the earlier 20-0207-02 kit applies. The 20-0607 replaces the 20-0207-08 for 2008+ EJ-platform vehicles.
• For 2002-2005 Subaru models equipped with a cable-actuated throttle body, a Radium Cruise Control Relocation Bracket must be used in conjunction with this kit. Verify your throttle body actuation type before ordering.
• In freezing climates, Radium Engineering recommends installing insulation sleeving (not included) over any hose sections exposed to cold airflow to prevent condensation in the hose from freezing and potentially causing a blockage.
• The AOS must be mounted with its oil return port positioned higher than the engine's oil return port to allow gravity-fed return of collected oil to the oil pan. Verify mounting location clearance before installation.
• Engine coolant must be available for routing to the AOS. The coolant circuit heats the separator canister to prevent water condensation buildup. A coolant tap in the vicinity of the AOS mounting location is required; verify coolant line routing in your specific engine bay configuration.
• The Radium AOS-R kit may not be compatible with all aftermarket intake, intercooler piping, or engine bay modifications. Review the installation instructions on Radium's website for your specific vehicle configuration before ordering.

Features

• AOS-R (Air Oil Separator - Return) system - unlike a catch can that collects oil and requires periodic draining, the AOS-R returns collected oil directly back to the engine oil pan through a dedicated return port; this makes the system zero-maintenance - there is no service interval, no canister to drain, and no risk of the separator overflowing under sustained high-load driving conditions common on turbocharged Subaru builds
• Three-piece CNC machined aluminum canister construction, O-ring sealed - the AOS body is machined in three sections that screw together and seal with O-rings, making the unit fully serviceable; the stainless steel condensing media inside can be cleaned and reinstalled without replacing the entire unit
• Integrated engine coolant heater circuit - two 4AN ORB coolant ports route engine coolant through a heating chamber inside the AOS body; this keeps the separator canister warm at all times when the engine is at operating temperature, preventing water condensation from forming inside the canister - a critical failure mode in catch cans and unheated separators in cold climates where condensation accumulation in the oil pan degrades engine oil lubrication quality
• Integrated water trap at the bottom perimeter - even with the coolant heater active, small amounts of water condensation can form during cold starts or short trips before the engine reaches full operating temperature; the integrated water trap channel around the bottom perimeter of the canister captures water (which is denser than oil) and holds it against the heated walls of the canister until it boils off and evaporates, preventing water from passing through the return port and entering the oil pan
• Stainless steel condensing media - the inlet side of the AOS contains stainless steel filter material that accelerates oil droplet condensation out of the incoming crankcase vapor stream; larger oil particles coalesce on the stainless media and drain to the bottom for return to the oil pan while the cleaned vapor exits through the outlet port to the intake system
• Prevents oil accumulation throughout the induction system - by intercepting crankcase blow-by vapors before they reach the turbocharger inlet, intake piping, intercooler, and intake manifold, the AOS-R eliminates the progressive oil fouling of these components that reduces intercooler efficiency, increases the risk of detonation from oil-contaminated intake charge, and reduces the longevity of intake gaskets, throttle body shaft seals, and manifold coating
• Subaru-specific mounting bracket and complete hardware included - the 20-0607 includes a dedicated mounting bracket designed for the EJ-platform engine bay, all SAE J30R7 rated PCV/emissions hose, coolant hoses and fittings, all aluminum hose-end fittings, and stainless steel mounting hardware; no additional parts must be sourced to complete the installation
• Uses factory mounting points - installation requires no drilling, welding, or permanent engine bay modification; the bracket mounts to existing Subaru factory threaded locations
• Effectively raises intake charge octane quality - by removing oil vapor from the crankcase air before it re-enters the intake, the AOS-R eliminates the octane-degrading effect of oil-contaminated intake charge, which is a meaningful factor on high-compression or high-boost EJ257 builds running aggressive ignition timing

Specifications

• Part Number: 20-0607
• System Type: AOS-R (Air Oil Separator with Oil Return - zero maintenance)
• Compatible Engines: EJ255, EJ257 (turbocharged Subaru flat-four)
• Drive Configuration: Left-hand drive (LHD) only
• AOS Height: 6.3 in. (without fittings); 6.94 in. (top of dipstick tube to bottom)
• AOS Diameter: 3.45 in. (round)
• AOS Capacity: 20.3 oz (600mL)
• Inlet Port: 7/8"-14 (10AN ORB)
• Outlet Port: 7/8"-14 (10AN ORB)
• Oil Return Port: 7/8"-14 (10AN ORB)
• Coolant Ports: 7/16"-20 (4AN ORB) x 2
• Canister Construction: 3-piece CNC machined aluminum, O-ring sealed, fully serviceable
• Internal Media: Stainless steel condensing material (cleanable and reusable)
• Hose Standard: SAE J30R7 (PCV/Emissions rated)

Included Components

• 1x Radium Engineering CNC Machined Aluminum AOS Canister (3-piece, O-ring sealed)
• 1x Subaru-specific 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

Product Description

The Radium Engineering 20-0607 is the AOS-R (Air Oil Separator - Return) kit for EJ-platform Subaru WRX and STI models, covering the 2009-2013 Forester XT, 2008-2014 WRX, and 2008-2021 WRX STI (LHD only). The AOS-R is the definitive crankcase vapor management solution for modified Subaru applications - it goes fundamentally beyond a catch can by eliminating servicing entirely: collected oil returns directly to the engine oil pan through a dedicated return port, meaning the system can never overflow, never requires draining, and remains fully functional even during sustained high-load track use where a standard catch can would fill and stop collecting within a session. The 600mL aluminum canister uses stainless steel condensing media to separate oil from crankcase vapors before they can re-enter the turbocharger inlet, intercooler, and intake manifold - protecting these components from progressive oil fouling and maintaining intake charge purity.

Two 4AN ORB coolant ports route engine coolant through an internal heating chamber in the canister body. This keeps the AOS warm enough to prevent water condensation from forming inside at operating temperature - the fundamental weakness of unheated catch cans in cold climates, where condensation accumulation can contaminate engine oil and cause bearing wear. An integrated water trap at the bottom perimeter of the canister captures any residual condensation that does form, holding it against the heated walls where it eventually boils off without entering the oil return port. The canister is machined in three sections that screw together and seal with O-rings, making it fully serviceable: the stainless steel media can be cleaned and reinstalled without replacing the entire unit. All mounting hardware, SAE J30R7 rated PCV/emissions hose, coolant hoses and fittings, and aluminum hose ends are included for a complete installation using factory engine bay mounting points - no drilling, welding, or permanent modification required. For 2015+ WRX non-STI and Forester XT on the FA20DIT platform, a separate AOS-R part number is required. RHD vehicles require the Radium Universal AOS-R with custom plumbing.