Best Oil Catch Can for Subaru WRX and STI

The best oil catch can for the WRX and STI is one built specifically for the platform. Oil blow-by is a known characteristic of the EJ and FA engine family. Combustion gases push oil vapor into the intake system over time and the stock PCV system does not stop it. An oil catch can, also called an Air Oil Separator or AOS, intercepts that vapor before it reaches the intake. This guide covers what to look for and which option is worth buying for your Subaru WRX or Subaru STI.

What an Oil Catch Can Actually Does

Combustion gases and oil vapor push past the piston rings and into the crankcase during normal engine operation. The PCV system then routes that vapor back into the intake to be burned. On the WRX and STI, this process deposits an oily residue on the intake valves, intercooler, and intake manifold over time. That buildup reduces airflow, hurts throttle response, and contributes to carbon deposits on the valves that become progressively harder to remove.

On direct injection platforms like the FA20 in the 2015 to 2021 WRX, the problem is worse. Fuel no longer washes over the intake valves the way it did on port-injected engines. Oil vapor from the PCV system coats the valves and there is nothing to clean it off. The deposits build up faster and the performance impact shows sooner. An oil catch can sits in the PCV line and intercepts that vapor before it re-enters the intake. For anyone still weighing whether an AOS is worth it for your WRX or STI, the dedicated post covers that decision in detail.

Oil Catch Can vs Air Oil Separator: Is There a Difference

This question comes up constantly in the WRX and STI community. A basic oil catch can is a simple reservoir in the PCV line. It collects blow-by but returns nothing to the crankcase. The reservoir needs to be emptied periodically or it overflows back into the intake.

An Air Oil Separator actively separates oil from the air using baffles and chambers designed to force the vapor through a separation process. The clean oil is returned to the crankcase. Cleaner air goes back into the intake. Nothing accumulates in a reservoir that needs draining. For the WRX and STI, a proper AOS is the better long-term solution. The return-to-crankcase design handles the volume of blow-by the EJ and FA platforms produce more effectively than a basic catch can setup.

What to Look for in an Oil Catch Can for WRX and STI

Platform-specific fitment is the most important factor. Generic catch cans require custom plumbing that does not always route cleanly on the WRX and STI engine layout. Hose runs that are too long, too short, or poorly routed create vacuum leaks and defeat the purpose of the upgrade. A purpose-built unit connects to the correct ports without modification and routes cleanly within the factory engine bay.

Separation efficiency determines how much oil vapor actually gets caught versus passing through into the intake. A unit with low separation efficiency lets a significant percentage of the blow-by pass straight through. The baffling design, chamber volume, and internal construction all affect how well the unit actually works under boost and high engine loads.

Build quality matters for long-term reliability. The unit lives in a hot engine bay and handles pressurized blow-by every time the engine runs. Cheap materials degrade under heat cycling. Fittings loosen. Reservoirs crack. A quality unit is built to last the life of the engine without requiring replacement.

The reservoir design determines how much maintenance the system requires. A collect-and-empty catch can needs to be checked and drained regularly. Neglecting it means the reservoir fills and blow-by passes straight through into the intake anyway. A return-to-crankcase AOS eliminates that maintenance requirement entirely.

Why IAG Is the Standard for WRX and STI

IAG builds their AOS specifically for the EJ and FA Subaru platform. The design is not adapted from a universal catch can. It is engineered around the specific PCV routing, port locations, and blow-by volume of the WRX and STI engine family. The fitment is clean, the hose runs are correct for the engine bay, and the installation does not require custom fabrication.

The separation efficiency of the IAG AOS is proven across thousands of builds. The internal baffling and dual-chamber design on the higher-spec units forces oil vapor through a separation process that pulls a high percentage of the oil out of the air stream before it can reach the intake. What returns to the intake is significantly cleaner than what the stock PCV system delivers.

The return-to-crankcase design means the separated oil goes back where it belongs instead of collecting in a reservoir. The system is self-managing. There is no reservoir to check, no risk of overflow, and no maintenance interval to track. For WRX and STI owners who want a proper solution rather than a generic catch can adapted to fit, IAG is the standard recommendation across the community. Browse IAG oil catch cans for WRX and STI to see available options for your platform.

Which IAG AOS Fits Your WRX or STI

IAG makes dedicated AOS units for different WRX and STI generations. The FA20 WRX, the EJ-based STI, and the 2022 and newer FA24 WRX each have their own unit. The plumbing connects to specific ports on each engine and fitment is not interchangeable between platforms.

Getting the right unit for the right engine matters before ordering. Compare IAG AOS options for WRX and STI to confirm the correct fitment for your model year and engine.

When to Install an Oil Catch Can on a WRX or STI

Blow-by accumulation starts from the first mile. The sooner an AOS is installed, the less residue builds up in the intake system. For most WRX and STI owners, this is a smart early install rather than a build-stage upgrade. Waiting until the car is fully built means years of oil deposits have already accumulated on the intake valves and intercooler.

For built engines and high-boost setups, the AOS becomes even more critical. Blow-by increases with boost and cylinder pressure. A proper AOS protects the intake system and the internals on any build pushing past stock power levels. The WRX engine reliability guide covers how supporting modifications like the AOS contribute to long-term engine health. Our post on most common WRX engine failures covers what happens when supporting mods are skipped on a platform that needs them.

Find the Right Oil Catch Can for Your WRX or STI

A purpose-built AOS from IAG is the right call for the WRX and STI platform. Generic catch cans work in theory but the platform-specific fitment, proven separation efficiency, and return-to-crankcase design of the IAG unit make it the clear choice for any serious build. Installing it early protects the intake system from the start and removes one more variable from long-term reliability.

Browse IAG oil catch cans for WRX and STI to find the right unit for your platform, or explore all available oil catch can options for WRX and STI to compare what is available.