IAG Air / Oil Separator (AOS) Fits 2002-07 Subaru Impreza WRX, 2004-07 STI, 2004-08 FXT

Part Details:

The IAG Air / Oil Separator (AOS) is designed to improve crankcase ventilation and reduce oil vapor entering the intake system on early turbocharged Subaru engines. By separating oil from blow-by gases before they are recirculated, this system helps keep intercooler piping, intake components, and the combustion process cleaner—supporting long-term engine reliability.

Unlike traditional catch cans that require periodic draining, the IAG AOS features a continuous oil return design that routes separated oil back into the engine. This closed-loop operation reduces maintenance while maintaining proper oil levels during daily driving, spirited use, and high-load conditions.

How the IAG AOS Works

Under boost, turbocharged Subaru engines generate crankcase pressure that carries oil vapor through the PCV and crankcase ventilation system. Without proper separation, this oil vapor is pulled into the intake tract, where it can lower effective fuel octane and contribute to carbon buildup.

The IAG AOS separates oil from crankcase gases before they re-enter the intake system, returning the oil to the engine while allowing clean air to be recirculated. This helps reduce detonation risk, improve combustion consistency, and keep intake components cleaner over time.

Street-Friendly, Low-Maintenance Design

The IAG AOS is designed as a true closed-loop system, maintaining factory-style ventilation behavior. Because oil is continuously returned to the engine, there is no need for manual draining like with traditional catch cans.

This makes it well suited for street-driven vehicles, stock or lightly modified setups, and performance builds where reliability and ease of ownership are priorities.

Street Series vs Competition Series

IAG offers both Street Series and Competition Series AOS options depending on vehicle use and power level.

• Street Series AOS: Maintains PCV functionality and is suitable for street-driven vehicles, including those running E85 or high-ethanol fuel blends. Recommended for builds up to approximately 600 WHP.
• Competition Series AOS: Designed for competition-only vehicles producing over 600 WHP. This configuration vents crankcase gases differently and may require tuning changes.

If the vehicle is driven on public roads or emissions compliance is a concern, the Street Series AOS is the appropriate choice.

Key Benefits

• Reduces oil vapor entering the intake system
• Helps lower detonation risk by limiting oil ingestion
• Minimizes carbon and oil buildup in intercooler and intake components
• Continuous oil return design for low-maintenance operation
• Improves crankcase ventilation under boost

Vehicle Fitment Overview

This AOS is designed for early turbocharged Subaru models including WRX, STI, and Forester XT platforms. Full year-by-year compatibility is listed in the Vehicle Fitment tab above.

Please note that the IAG AOS is not compatible with most aftermarket strut tower bars.

What’s Included

• IAG Air / Oil Separator assembly
• Vehicle-specific mounting bracket
• Pre-formed oil and crankcase hoses
• Required fittings and installation hardware
• Installation instructions

Frequently Asked Questions

Why do I need an air / oil separator?

Turbocharged Subaru engines generate crankcase pressure that carries oil vapor into the intake system. Over time, this oil vapor can lower effective fuel octane, increase detonation risk, and cause carbon buildup in the intercooler and intake manifold. An AOS helps separate oil from crankcase gases before they are recirculated.

How is an AOS different from a catch can?

Traditional catch cans collect oil vapor but require manual draining. The IAG AOS uses a continuous oil return design that routes separated oil back into the engine, eliminating the need for routine draining while maintaining proper oil levels.

Is this AOS suitable for street-driven vehicles?

Yes. The Street Series IAG AOS is designed to retain factory-style PCV operation and is suitable for daily-driven vehicles, including those running E85 or other ethanol blends.

What is the difference between Street Series and Competition Series?

The Street Series AOS maintains closed-loop ventilation and does not require tuning changes. The Competition Series AOS is intended for competition-only vehicles producing higher power levels and may require ECU calibration changes.

Will this fit with a strut tower bar?

The IAG AOS is not compatible with most aftermarket strut tower bars. Fitment should be verified prior to installation.

Installation & Use Notes

• Professional installation is recommended
• Proper hose routing is critical for correct operation
• Competition Series AOS may require ECU calibration changes
• Always verify clearance with strut tower bars prior to installation

Recommended for Engine Builds

This IAG Air / Oil Separator is strongly recommended for Subaru EJ-based engine builds running increased boost, forged internals, or closed-deck configurations. Proper crankcase ventilation and oil vapor control are critical for maintaining engine reliability in high-performance applications.

• High-performance EJ25 short block builds
• IAG EJ25 long block engine packages

Learn more about IAG Performance engine and reliability solutions