If you have spent any time tuning a 2015+ Subaru WRX, you already know the problem: blow-by. Under boost, crankcase pressure pushes oil mist and combustion gases past the rings and back up through the PCV system into the intake. On a direct-injection FA20DIT that mist coats the intake valves, the intercooler and the charge piping, and over thousands of miles it turns into the kind of carbon coking that costs you power and eventually head work.
A Radium air oil separator is the fix. This guide walks through a workshop-credible install of the Radium Engineering AOS-R on a 2015+ WRX, covering mounting, the crankcase vent plumbing, the coolant-heated routing that keeps the separator working in a cold UK climate, and how to check the system before you drive it. It is a Saturday-afternoon job with basic hand tools - no fabrication, no welding.
AIR OIL SEPARATOR VS CATCH CAN: WHAT IS THE DIFFERENCE?
Before you buy, it is worth understanding why an air oil separator beats a plain catch can on a street-driven WRX.
A catch can traps oil mist in a can and holds it there until you manually drain it. That works on a track car where you are checking levels every session, but on a daily driver it is easy to forget a drain and end up with a can that is full and passing vapour straight through.
The Radium Engineering Universal Air Oil Separator - VTA works differently. Blow-by gases enter the separator, spin through an internal baffled chamber that drops oil out of suspension, and the collected oil returns to the oil pan. That means no servicing schedule to remember - the separator keeps doing its job for the life of the build while the intake stays clean.
If you want to run a dedicated can instead, the Radium 2022+ Subaru BRZ / Toyota GR86 Dual Catch Can Kit Fluid Lock shows the Fluid Lock design and 10AN port layout Radium uses across the platform range. Both approaches remove oil from the intake air - the AOS just does it without you having to think about it.
WHY THE 2015+ WRX NEEDS ONE
Three things make the FA20DIT a strong candidate for an air oil separator:
- Direct injection means fuel never washes the back of the intake valves, so any oil that arrives there bakes on and stays.
- Boost raises crankcase pressure, which pushes more oil mist through the vent system than a naturally aspirated engine sees.
- The intercooler and charge pipes collect that mist, gradually reducing their ability to transfer heat - exactly where you cannot afford a loss on a tuned car.
Clean inlet air protects octane, keeps the intercooler efficient, and lowers hydrocarbon emissions. For a WRX that sees regular street duty and the occasional track day, that is a straightforward reliability win.
PARTS AND TOOLS YOU NEED
- Radium Engineering AOS-R air oil separator (universal VTA specification)
- AOS-R mounting bracket and hardware
- Crankcase vent hose and fittings to suit your routing
- Coolant hose, clamps and a T-piece for the heated circuit
- Drain-back line and fittings to return oil to the sump
- Basic hand tools, hose cutters, thread sealant, cable ties and a drain tray
- Fresh coolant to top up after you break into the coolant circuit
If you are also upgrading the crankcase venting, the Radium PCV Valve 10AN ORB to 6AN Male is the correct part to meter flow by engine load rather than fitting a check valve that creates a vacuum leak.
STEP 1: PREP THE CAR AND PICK THE MOUNTING POINT
Let the engine cool fully, disconnect the battery negative terminal, and put a tray under the car. You will be breaking into both the crankcase vent system and the coolant circuit, so work cold and work clean.
The AOS-R mounts in the engine bay where there is clearance from exhaust heat and a straight run to the vent lines. On the FA20DIT the usual spot is on the driver's side of the bay near the strut tower, high enough that the drain-back line can run downhill to the sump with no low spots to trap oil. Test-fit the bracket before you commit to holes.
STEP 2: MOUNT THE SEPARATOR BODY
Bolt the bracket to the chosen point and mount the AOS-R body. Keep it upright so the internal separation chamber drains correctly - Radium's design relies on gravity to move collected oil down and out. Route the drain-back line from the base of the separator to a suitable return point on the sump, and keep that line as short and steep as you can.
STEP 3: PLUMB THE CRANKCASE VENT LINES
Connect the crankcase vent outlet to the separator inlet using the correct size hose for your fitting - the AOS line sizes are matched to the vent port diameter, so do not neck down. Use O-ring or clamped connections throughout; a pressurised induction system will find any weak joint quickly.
Orient the hoses so they fall gently toward the separator and avoid any section that dips below the connection point, which would pool oil and block the flow the system needs.
STEP 4: ROUTE THE COOLANT-HEATED CIRCUIT
This is the step people skip, and it is the one that matters most in a UK climate. The AOS-R uses engine coolant to keep the separator body warm. That heat stops the oil and water emulsion that forms inside the unit from thickening into sludge during short, damp, cold-start journeys - the exact duty cycle most WRX street cars live with.
Tee into a suitable coolant hose, feed the heater port on the separator and return it to the circuit, then refill and bleed the cooling system properly. Check for weeps at every new joint once the engine reaches temperature.
STEP 5: LEAK CHECK AND FIRST START
With everything connected, reconnect the battery and start the engine. Let it idle and watch for three things: coolant weeping at the new T-piece and heater ports, oil weeping at the drain-back fittings, and any change in idle quality that would suggest a vent leak.
Bring the car up to temperature and check the separator body actually warms up - if it stays cold, the coolant circuit is not flowing and you need to re-bleed it. Then take a short, gentle drive, recheck the joints hot, and confirm no warning lights appear.
MAINTENANCE AND TUNING NOTES
Once installed, an AOS-R is largely hands-off: the oil returns to the sump, so there is no can to drain. During routine servicing, check the hose condition, the clamp tightness and the drain-back line for blockage, and confirm the coolant circuit is still flowing cleanly.
If you are chasing fuel system stability at the same time, a Radium Fuel Pulse Damper pairs well with an AOS build - absorbing injector pressure pulses to smooth idle and stop the lean spikes that make a tune hard to finish.
FIT AN AOS-R THE RIGHT WAY
An air oil separator is one of the highest-value bolt-ons on a 2015+ WRX: it protects the intake valves, the intercooler and your tune without adding a servicing job to your list. Mount it upright, plumb the vent lines with no low spots, and do not skip the coolant-heated circuit - get those three right and the separator will quietly do its job for the life of the build.
Radium Shop UK is an authorised UK dealer for genuine Radium Engineering hardware, with fast UK delivery and correct fitment for your platform. Browse the full Universal Catch Cans and AOS range or get in touch and we will confirm the right part number for your WRX.
