IAG Air / Oil Separator (AOS) Fits 2015-21 Subaru WRX, 2014-18 FXT

Part Details:

The IAG Air / Oil Separator (AOS) is engineered to improve crankcase ventilation and reduce oil vapor entering the intake system on later-model turbocharged Subaru platforms. By separating oil from blow-by gases before they are recirculated, this system helps maintain cleaner intake components, stabilize combustion, and support long-term engine reliability.

Unlike traditional catch cans that require manual draining, the IAG AOS features a continuous oil return design that routes separated oil back into the engine. This low-maintenance, closed-loop configuration preserves factory-style crankcase ventilation behavior while providing improved oil control under boost.

How the IAG AOS Improves Engine Reliability

Turbocharged Subaru engines generate crankcase pressure that carries oil vapor through the PCV and crankcase ventilation system. Without proper separation, this oil vapor is drawn into the intake tract, where it can reduce effective fuel octane, increase detonation risk, and contribute to carbon buildup in the intercooler and intake components.

The IAG AOS separates oil from crankcase gases before they re-enter the intake, returning the oil to the engine while allowing cleaner air to be recirculated. This improves combustion consistency and helps protect the engine during both daily driving and high-load operation.

Street-Friendly, Low-Maintenance Design

Because oil is continuously returned to the engine, the IAG AOS does not require routine draining like traditional catch cans. This makes it a practical upgrade for street-driven vehicles as well as modified and performance-focused setups.

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 closed-loop PCV operation and is suitable for street-driven vehicles, including those running E85 or other ethanol blends. Recommended for builds up to approximately 600 WHP.
• Competition Series AOS: Intended for competition-only vehicles producing higher power levels. This configuration prevents oil vapor from re-entering the intake but may require ECU calibration changes.

For street-driven or emissions-regulated vehicles, the Street Series AOS is the appropriate choice.

Key Features

• Direct-fit air / oil separator for supported Subaru turbo models
• Reduces oil vapor entering the intake system
• Improves crankcase ventilation under boost
• Continuous oil return design for minimal maintenance
• Suitable for street-driven and performance applications
• Durable construction with vehicle-specific fitment

Vehicle Fitment Overview

Designed for later-model turbocharged Subaru platforms including WRX and Forester XT. 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?

Oil vapor entering the intake system can lower effective fuel octane and increase the risk of engine knock, especially under boost. Over time, oil vapor also causes carbon buildup in the intercooler, intake manifold, and intake valves, which can lead to airflow disruption and performance issues.

What causes oil to enter the crankcase ventilation system?

The amount of oil present in the crankcase ventilation system depends on several factors, including boost pressure, piston-to-cylinder wall clearance, oil viscosity, engine temperature, and overall engine condition. Higher boost levels typically increase oil vapor production.

How is an AOS different from a catch can?

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

Which AOS is right for my vehicle?

Street-driven vehicles and builds under approximately 600 WHP should use the Street Series AOS. Competition-only vehicles producing higher power levels may benefit from the Competition Series AOS, which may require tuning changes.

Will this AOS 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 Notes

• Professional installation is recommended
• Proper hose routing is critical for correct operation
• Always verify vehicle configuration and clearance before installation

Recommended for Engine Builds

This IAG Air / Oil Separator is a key supporting upgrade for FA20-based engine builds, especially when increasing boost pressure or upgrading internal components. Improved oil vapor separation helps protect intake components and supports long-term engine durability.

• FA20 forged short block builds
• FA20 long block engine packages

Learn more about IAG Performance engine and reliability solutions