Essential Insights for WRX & STI Owners
If you drive a Subaru WRX or STI, protecting engine health while preserving performance should always be a priority. One of the most commonly recommended reliability upgrades, especially for turbocharged Subaru boxer engines, is an Air Oil Separator (AOS).
But is an AOS actually worth it for your setup? Whether your car is stock, lightly modified, or fully built, this guide breaks down what an AOS does, why it matters, and when it becomes essential.
What Is an Air Oil Separator (AOS)?
An Air Oil Separator is designed to manage oil vapor created during normal engine operation.
In turbocharged boxer engines, blow-by occurs when combustion gases slip past the piston rings. These gases carry oil vapor into the crankcase, where the factory PCV (Positive Crankcase Ventilation) system routes them back into the intake to be burned again.
Over time, this oil vapor:
- Coats intercooler piping
- Reduces effective octane
- Causes carbon buildup
- Increases knock risk
An AOS intercepts the oily air, separates oil from vapor, and returns clean air to the intake while draining oil back to the engine, keeping everything cleaner and safer.
Key Benefits of an AOS on a WRX/STI
Engine Protection & Longevity
Oil entering the intake can lead to carbon buildup, restricted airflow, and detonation. An AOS significantly reduces this contamination, helping preserve pistons, valves, and turbo components over the long term.
Performance Preservation (Not Power Gains)
An AOS won’t add horsepower, but it protects the power you already have. Cleaner intake air means:
- More consistent combustion
- Improved throttle response
- Better long-term fuel efficiency
This is especially important for direct-injection Subaru engines, which are more prone to carbon buildup.
Essential for Modified Builds
Running higher boost, upgraded turbos, or aggressive tuning? The factory PCV system quickly becomes overwhelmed. An AOS is critical for:
- Tuned WRX/STI setups
- Track-driven vehicles
- High-RPM or high-load use
Without it, oil contamination increases dramatically under boost.
Smart Upgrade Even for Stock Cars
Even on a completely stock WRX/STI, oil vapor slowly accumulates over time. Installing an AOS early is a preventive upgrade, protecting factory performance and reliability for years to come.
Low Maintenance vs Catch Cans
Unlike traditional catch cans, a quality AOS:
- Operates in a closed-loop system
- Automatically drains oil back to the engine
- Requires little to no routine maintenance
Catch cans must be manually emptied and can overflow if neglected, sending oil right back into the intake.
How an Air Oil Separator Works
Inside the AOS, hot blow-by gases are slowed and cooled using:
- Internal baffles
- Expansion chambers
- Condensation surfaces
As the vapor cools, oil droplets separate and collect at the bottom of the unit.
- Clean air is routed back into the intake
- Oil drains safely back into the oil pan
This continuous cycle keeps the intake system clean without disrupting airflow.
Choosing the Right AOS for Your WRX/STI
When selecting an AOS, look for these key features:
- High-Quality Materials
Cast or billet aluminum for durability and effective heat transfer. - Integrated Heating or Coolant Routing
Prevents moisture buildup and sludge in cold climates. - Efficient Internal Baffle Design
Maximizes oil separation without restricting airflow. - Adequate Capacity
Especially important for tuned or track-driven vehicles with higher blow-by.
AOS Installation Overview
Most AOS kits are bolt-on and can be installed with basic tools:
- Ratchets and sockets
- Hose clamps and pliers
Installation involves routing hoses from:
- Valve covers
- Crankcase
- Turbo inlet
Always follow the manufacturer’s instructions carefully. If you’re not comfortable working in the engine bay, professional installation is recommended for proper routing and leak prevention.
Is an Air Oil Separator Worth It? Final Verdict
Yes, especially if you care about long-term reliability.
An Air Oil Separator is one of the smartest preventative upgrades you can make for a WRX or STI. While it won’t increase horsepower, it protects your investment, keeps performance consistent, and reduces the risk of costly engine issues down the road.
Whether stock or modified, daily-driven or track-focused, an AOS is a set-it-and-forget-it solution for engine health.
If you’re looking for quality Air Oil Separators and catch can solutions for your WRX or STI, explore our selection here: WRX & STI Air Oil Separators and Catch Cans.