What is Crankcase Blow-By and Why Does It Matter?
Every internal combustion engine produces blow-by gases that slip past the piston rings during combustion and pressurise the crankcase. These gases carry oil vapour, unburned fuel, and moisture. In a stock engine running at normal loads, the PCV (positive crankcase ventilation) system handles this by routing the contaminated air back into the intake to be burned.
The problem starts when you increase power. Turbocharging, higher boost levels, aggressive driving, and track use all generate significantly more blow-by than the factory PCV system was designed to handle. The result is oil mist coating the inside of your intercooler, throttle body, and intake manifold. On direct-injection engines like the Subaru FA20, FA24, Toyota G16E-GTS, and the Volkswagen EA888, there are no port injectors to wash the intake valves clean — so the oil bakes onto the valve backs over time, reducing airflow and performance.
This is where oil catch cans and air oil separators come in. Both are designed to intercept the contaminated blow-by before it reaches the intake. But they work differently, and choosing the wrong one for your build can mean either constant maintenance or an inadequate solution.
How an Oil Catch Can Works
An oil catch can is the simpler of the two options. It sits inline between the crankcase vent or PCV line and the intake system, and its job is to separate oil and moisture from the blow-by gases before they reach the intake tract.
Inside the can, incoming gases pass through internal baffling and filter media that cause the heavier oil droplets to condense and fall to the bottom of the reservoir. The cleaner air continues through to the intake or, in some configurations, is vented to atmosphere. The collected oil and moisture sit in the can until you manually drain it.
Quality catch cans like the Radium Engineering range use CNC-machined billet aluminium construction with two-stage internal baffling, stainless steel reusable filter media, and O-ring sealed designs. Radium's catch cans feature 10AN ORB main ports, a 4AN ORB drain port, and an integrated dipstick so you can check the fluid level without removing the can.
Most Radium vehicle-specific catch can kits are closed-loop systems — they do not vent to atmosphere. This means the filtered air is recirculated back to the intake, keeping the system emissions compliant and track legal. This is an important consideration for Australian vehicles that need to pass roadworthy inspections or comply with state EPA requirements.
When a Catch Can Is the Right Choice
- Street cars and mild builds — if your vehicle sees mostly road use with occasional spirited driving, a catch can provides excellent protection with minimal complexity.
- Direct-injection turbo engines — the FA20, FA24, G16E-GTS, and EA888 all benefit significantly from catch cans because there are no port injectors to clean the intake valves.
- Dual-bank engines — vehicles like the Nissan R35 GT-R (VR38DETT) benefit from multi-can setups that intercept both the PCV and crankcase vent lines. Radium's R35 triple catch can kit covers both the PCV line and the two head breathers.
- Vehicles without an oil return port — if your engine does not have a convenient gravity-fed return path to the crankcase, a catch can is the practical choice.
The Trade-Off
Catch cans require regular maintenance. You need to check and drain the can periodically — the frequency depends on how hard you drive and how much blow-by your engine produces. A stock WRX on the street might only need draining every few thousand kilometres, while a heavily modified track car could fill the can in a single session. Excessive oil collection can also indicate engine wear, making the catch can a useful diagnostic tool.
How an Air Oil Separator Works
An air oil separator (AOS) serves the same purpose as a catch can — intercepting oil and moisture from blow-by gases — but with one key difference: it returns the collected oil back to the engine's oil pan automatically. There is no reservoir to drain.
The Radium Engineering AOS-R achieves this by using gravity to drain separated oil from the bottom of the unit back into the crankcase through a dedicated return line connected to the engine block or oil pan. The unit must be mounted higher than the oil return port on the engine to allow gravity to do its work.
A critical feature of the Radium AOS-R is its integrated coolant heating system. Engine coolant is routed through the base of the unit to keep the separator warm during operation. This prevents water vapour from condensing inside the AOS and contaminating the engine oil. Because water is denser than oil, Radium also includes an integrated water trap around the bottom perimeter as a secondary safeguard — if any condensation does form, it is held against the heated surface until it evaporates rather than draining into the oil pan.
When an AOS Is the Right Choice
- High-output builds generating heavy blow-by — engines making significant power with increased crankcase pressure will overwhelm a standard catch can quickly. An AOS handles this volume continuously without filling up.
- Track cars and endurance racing — if you do not want to (or cannot) stop to drain a catch can during extended sessions, an AOS is the maintenance-free solution.
- Engines with a suitable oil return port — the AOS needs a gravity-fed path back to the crankcase. Subaru EJ and FA engines, for example, have well-documented return routing options that make AOS installation straightforward.
- Builders who want a fit-and-forget solution — once properly installed with coolant heating connected, the AOS requires essentially zero ongoing maintenance.
The Trade-Off
An AOS is more complex to install than a catch can. It requires coolant plumbing, a suitable oil return port on the engine, and careful mounting to maintain correct height relative to the return point. It also costs more upfront. If the coolant heating is not connected properly, water condensation can form inside the AOS and drain into the engine oil — the opposite of what you want. Correct installation is critical.
Catch Can vs Air Oil Separator — Side by Side
| Factor | Oil Catch Can | Air Oil Separator (AOS) |
|---|---|---|
| How it works | Traps oil in a sealed reservoir | Separates oil and returns it to the engine |
| Maintenance | Periodic draining required | Essentially maintenance-free |
| Installation complexity | Simple — hose routing and bracket mounting | More involved — requires coolant plumbing and oil return line |
| Best for | Street cars, mild to moderate builds, DI engines | High-output builds, track cars, endurance use |
| Emissions compliance | Closed-loop systems are compliant | VTE (vent-to-engine) versions are compliant |
| Diagnostic value | High — you can see how much oil is collecting | Lower — oil returns to the pan unseen |
| Cost | Lower | Higher |
| Risk if neglected | Can overfill and push oil back to intake | Condensation risk if coolant heating fails |
Which Engines Benefit Most?
Any turbocharged or high-revving engine benefits from blow-by management, but some platforms are particularly prone to issues and should be considered high priority for a catch can or AOS installation.
Subaru FA20 and FA24 (WRX VA, VB, BRZ, GR86) — These direct-injection turbo engines are notorious for blow-by related intake valve carbon buildup. The FA20 in particular has no port injectors, so oil vapour bakes directly onto the valve backs. Radium offers dual catch can kits and AOS kits specifically designed for these platforms with vehicle-specific mounting brackets and hose routing.
Nissan VR38DETT (R35 GT-R) — The twin-turbo V6 in the GT-R has two head breather lines and a PCV line, making it a candidate for Radium's triple catch can kit. This system intercepts all three sources of blow-by with dedicated cans for each circuit.
Toyota G16E-GTS (GR Yaris, GR Corolla) — Another direct-injection turbo three-cylinder that produces significant blow-by relative to its displacement, particularly under track conditions at high boost.
Subaru EJ20 and EJ25 (older WRX and STI models) — The boxer layout and turbo setup make these engines consistent blow-by producers. Radium has long offered both catch can and AOS solutions for the EJ platform going back to 2002 models.
Volkswagen and Audi EA888 (Golf R, S3, RS3) — The European DI turbo four-cylinder shares the same intake valve carbon buildup characteristics as the Subaru FA engines.
Radium Engineering's Approach
What sets Radium apart from generic catch cans is the engineering behind each product. Every Radium catch can and AOS is CNC machined from 6061 billet aluminium, media blasted, anodised, and laser engraved. But the differences go deeper than materials.
Radium's catch cans use a two-stage internal separator design. The first stage uses a diffuser to slow incoming air velocity, causing heavier oil droplets to fall out of suspension. The second stage uses stainless steel filter media to capture finer particles. The filter is reusable and cleanable — no replacement cartridges to buy. The two-piece or three-piece body unscrews for easy draining and cleaning, and every port is sealed with O-rings to prevent leaks.
In 2022, Radium introduced an updated catch can design that addressed a common problem across the industry: when a catch can is not drained regularly and the oil level rises above the inlet port, contaminated oil can be pushed back into the engine. Radium's redesigned standard catch can mitigates this with internal geometry changes that maintain separation effectiveness even at higher fill levels.
The AOS-R line takes things further with the integrated coolant heating chamber, water trap, and gravity-fed oil return. The closed-loop AOS-R recirculates filtered air back to the engine, while the vent-to-atmosphere (VTA) version features an oversized outlet that is over 183% larger than the 10AN inlets to minimise backpressure.
Recommended Radium Kits by Vehicle
Choosing the right product comes down to your platform, your power level, and how you use the car. Here is a general guide based on the Radium range we stock at Chicane Australia.
Street and spirited driving (mild to moderate power): A vehicle-specific dual catch can kit is the best balance of protection and simplicity. For the Subaru WRX VA (FA20), Radium's dual catch can kit intercepts both the CCV and PCV lines with dedicated cans and factory-hole mounting brackets. No cutting or drilling required.
Track days and regular high-load use: If you are tracking your car regularly and producing heavy blow-by, consider an AOS for maintenance-free operation during sessions. Radium's vehicle-specific AOS kits for Subaru WRX, STI, and Forester XT include all mounting hardware, hoses, and fittings for a complete bolt-in installation.
High-power and competition builds: For heavily modified engines pushing serious power, a catch can or AOS alone may not be sufficient. Consider pairing a catch can system with a Radium PCV valve (PN 20-0530), which features a high-temperature silicone seal specifically designed for managing boost pressure — unlike OEM PCV valves that rely on hard plastic-to-metal surfaces and often leak from the factory.
R35 GT-R owners: The triple catch can kit is the go-to solution, covering both head breather lines and the PCV line. Given the complexity of the VR38's crankcase ventilation system, a purpose-built kit with vehicle-specific brackets and hose routing is strongly recommended over universal solutions.
Installation Tips for Australian Conditions
Australia's climate adds a few considerations worth noting. In cooler months, particularly for vehicles in Victoria, Tasmania, or the ACT, condensation can be an issue. If running an AOS, ensure the coolant heating lines are properly connected and flowing — this is not optional in cold weather. For catch cans, shorter drain intervals during winter months are advisable as moisture accumulation will be higher.
For vehicles used on Australian circuits like Wakefield Park, Sydney Motorsport Park, The Bend, or Phillip Island, extended high-load sessions generate significantly more blow-by than street driving. If you are regularly running 20+ minute sessions at full throttle, size your solution accordingly — a dual catch can kit with frequent drain checks, or an AOS for uninterrupted running.
All Radium closed-loop catch can kits are emissions compliant and do not vent to atmosphere, making them compatible with Australian state roadworthy requirements. The closed-loop design also prevents oil vapour from entering the cabin — a comfort and safety consideration for daily-driven cars.
Frequently Asked Questions
Do I need a catch can on a stock turbocharged car?
Even a stock turbo engine produces blow-by that coats the intake tract with oil over time. On direct-injection engines like the Subaru FA20 and Toyota G16E-GTS, there are no port injectors to clean the intake valves, so oil vapour bakes onto the valve backs and reduces performance. A catch can is a worthwhile preventative measure on any turbocharged vehicle, stock or modified. It is one of the most cost-effective ways to protect your engine's long-term health.
How often should I drain my catch can?
It depends on your engine, power level, and driving style. A mildly modified street car might only need draining every 3,000 to 5,000 kilometres, while a heavily modified track car can fill a standard catch can in a single extended session. Check the dipstick regularly when you first install the can to establish a baseline for your specific setup, then set a drain schedule based on what you observe. If you are collecting large amounts of oil quickly, it may indicate excessive blow-by worth investigating.
Is an oil catch can legal in Australia?
Closed-loop catch can systems — where filtered air is recirculated back to the intake rather than vented to atmosphere — are generally compliant with Australian roadworthy and emissions requirements. Radium Engineering's vehicle-specific catch can kits are closed-loop by design, meaning they do not vent crankcase gases to the atmosphere. If you are concerned about compliance in your state, check with your local transport authority, but closed-loop systems are widely accepted.
What is the difference between a catch can and an air oil separator?
Both intercept oil-contaminated blow-by gases before they reach the intake. The key difference is what happens to the collected oil. A catch can stores it in a sealed reservoir that you drain manually at regular intervals. An air oil separator (AOS) returns the collected oil directly back to the engine's oil pan via a gravity-fed drain line, making it essentially maintenance-free. The AOS is more complex to install because it requires coolant plumbing and a suitable oil return port, but it eliminates the need for ongoing drain maintenance.
Which is better for track use — a catch can or an AOS?
For regular track use, an AOS has a practical advantage because it does not require draining between sessions. Extended high-load driving generates significantly more blow-by than street use, and a catch can may fill up during a long session. However, a catch can offers better diagnostic visibility — you can see exactly how much oil your engine is producing, which is valuable information for monitoring engine health. Many serious track builds use a dual catch can setup with frequent checks between sessions, while endurance and competition cars tend to favour the AOS for its hands-off operation.
Ready to Protect Your Engine?
Browse our full range of Radium Engineering catch cans and air oil separators at Chicane Australia. Every kit is genuine Radium, in stock and ready to ship with fast Australian delivery. If you need help choosing the right solution for your build, get in touch with our team — we are happy to help you spec the correct setup for your engine and driving style.
