Every turbocharged Subaru WRX owner hits the same crossroads eventually. Crankcase pressure is pushing oil mist into the intake, your intercooler is getting coated, and the forums are split down the middle: fit a catch can, or fit an air-oil separator? Both do the same basic job - keep oil vapor out of the intake tract - but they go about it in very different ways, and the one you pick changes your maintenance routine, your emissions compliance, and how tidy the install looks under the hood.
We sell both setups to UK builders every week, so here is the honest comparison. No marketing spin, just what each one does on a 2015+ WRX and where the Radium AOS-R kit earns its place.
The Problem Both Solutions Solve
The FA20DIT in the 2015+ WRX runs positive crankcase pressure and routes blow-by gases back into the intake through the PCV system. Those gases are not clean air - they carry oil vapor, fuel residue, and water vapor. Left alone, that mist condenses in the intercooler, intake manifold, and throttle body. The result is carbon buildup on the valves, reduced octane in the charge, and a slow drop in performance that most owners feel long before they understand it.
A catch can or an air-oil separator intercepts that vapor before it reaches the intake. The difference is what happens to the oil after it is caught.
How a Catch Can Works
A catch can is a sealed canister plumbed inline in the PCV or crankcase vent circuit. Blow-by gas enters, oil droplets and heavier vapor fall out of suspension and collect at the bottom of the can, and the cleaner gas continues on its way.
Key characteristics:
- Simple, mechanical, and cheap to buy
- Oil collects and stays trapped until you drain it manually
- Vent-to-atmosphere versions can be illegal for road use under UK emissions rules
- Sealed recirculating versions keep everything closed but still need regular draining
- No active separation media means finer oil mist can pass through
A catch can works. On a hard-driven track car it works very well. The catch is the word "drain" - it is a maintenance item, and a forgotten can will fill up and eventually pass everything through anyway.
How an Air-Oil Separator Works
An air-oil separator does the same interception job but adds an active separation stage. Instead of relying on gravity alone, the AOS uses internal baffling and a condensing surface to pull oil out of the vapor stream, then returns that captured oil back to the sump through a drain line.
That last part is the whole argument. A recirculating AOS does not need draining, because the oil it captures goes back where it belongs - the engine.
On the Radium AOS-R kit for the 2015+ WRX, the design goes a step further. The unit is heated by engine coolant, which stops the water vapor in the blow-by from condensing and emulsifying with the oil. That keeps the separator working in UK winter commuting conditions, not just on a warm track day. It is a bolt-in kit with proper brackets and plumbing, so the install looks factory rather than improvised.
Head-to-Head: The Four Things That Matter
1. Oil Control
A baffled catch can captures the bulk of liquid oil. An air-oil separator captures a finer fraction of suspended mist because the separation media gives the vapor more surface and time to condense. On a daily-driven WRX doing short trips, that finer fraction is exactly the one that causes valve and intercooler fouling over time. Advantage: air-oil separator.
2. Maintenance
A catch can is a consumable you have to remember. Check it, drain it, dispose of the waste oil - forever. A recirculating AOS returns captured oil to the sump and needs no routine draining. For a daily driver that sees 10,000 miles a year, that difference adds up fast. Advantage: air-oil separator.
3. Emissions Compliance
This one catches UK owners out. A vent-to-atmosphere catch can dumps crankcase vapor into the air, which fails emissions requirements for a road car. A sealed recirculating can is compliant, and a recirculating AOS keeps the whole system closed by design - vapors are separated, oil is returned, and nothing is vented. Advantage: air-oil separator.
4. Fitment and Install
A universal catch can needs brackets, hose, and a bit of fabrication to sit properly. A vehicle-specific kit like the AOS-R bolts to factory mounting points with the correct plumbing included. If you want a factory-looking engine bay and a weekend install, specific wins. Advantage: air-oil separator, when you buy a vehicle-specific kit.
Which One Should You Fit?
Fit a catch can if you are building a dedicated track car, you want the cheapest possible oil control, and you are happy to drain the can after every event. It is a valid choice and plenty of fast cars run them.
Fit a recirculating air-oil separator if the car is your daily, your weekend car, or anything that has to pass UK emissions. The lack of draining and the closed loop make it the sensible choice for a road-driven WRX - and the coolant-heated design means it keeps working in real British weather, not just in a lab.
For most 2015+ WRX owners, the honest answer is the AOS-R. The catch can is the compromise; the separator is the fix.
What We Stock
We supply the Radium AOS-R Kit for the 2015+ Subaru WRX as an authorised UK dealer, along with the Radium Catch Can Kit PCV for the Ford Fiesta ST, the Radium Dual Catch Can Kit for the Fiesta ST, and a full range of Radium fuel system hardware - fuel hangers for the Mitsubishi EVO X and EVO 7-9, fuel pulse damper kits for the MK4 Supra, fuel pressure gauges, and AN fittings.
Every part is genuine Radium Engineering, held in the UK for fast delivery, with the fitment detail that hands-on builders actually need. If you are still deciding which crankcase setup suits your build, get in touch - we will point you at the right kit rather than the most expensive one.
