Why Your Subaru Needs an Air/Oil Separator

Every turbo Subaru makes blow-by. Most of it ends up in the intake. An air/oil separator is the fix — if it is designed correctly.

What blow-by does to a turbo EJ or FA

Combustion pressure slips past the piston rings on every power stroke. That pressure — carrying atomized oil with it — pressurizes the crankcase. The PCV system vents it back into the intake tract, which means oil mist goes where only air and fuel should be.

Three problems follow.

Octane loss. Oil in the combustion chamber lowers the effective octane of your fuel. Lower octane means detonation, and detonation kills turbo EJ and FA motors. This is the big one.

Intercooler contamination. Oil coats the inside of the intercooler, and an oil-coated intercooler transfers heat worse than a clean one. Intake temperatures climb. Power drops. Knock threshold drops with it.

Valve deposits. On direct-injection engines like the FA20 DIT, there is no fuel washing the intake valves. Oil vapor bakes onto them as carbon. Over time, deposits restrict airflow and hurt drivability.

Turbocharged engines make more blow-by than naturally aspirated ones — higher cylinder pressures push more past the rings. A stock WRX or STI driven hard generates a meaningful amount of oil vapor. A modified one generates more.

Catch can vs. closed-loop AOS

A catch can traps the oil in a container you drain by hand. It works, with two caveats: you have to remember to empty it, and in cold climates the collected oil and condensation can freeze or sludge.

A closed-loop air/oil separator separates the oil from the crankcase vapor and returns it to the sump automatically. No draining, no maintenance interval, no frozen catch can in January. The intake stays clean and the oil stays in the engine where it belongs.

For a street car — especially a daily driver — the closed-loop design is the right answer. A catch can is a maintenance item you will eventually forget. A separator you never touch is protection that never lapses.

What to look for in an AOS

Coolant heating. The separator should be heated by engine coolant. Cold separators condense water vapor into sludge. Heated ones stay above the dew point and separate oil cleanly in all weather.

Real baffling. Separation happens through baffling and directional changes, not magic. A can with an inlet and an outlet and nothing between them is a catch can with better marketing.

Correct PCV integration. The separator has to work with the factory PCV system, not against it. Crankcase ventilation is an emissions and engine-health system. Deleting it or disrupting it creates new problems.

Sized for the application. A separator designed for a stock motor on a 500-whp build will be overwhelmed. Match the unit to your power level and how the car gets driven.

When it matters most

Every turbo Subaru benefits. These cases benefit more.

Modified cars. More boost means more cylinder pressure, which means more blow-by. If you've added a bigger turbo, an intercooler, or a tune, the stock PCV system is working harder than it was designed for.

Track and hard street use. Sustained high RPM and high load generate the most blow-by. A car that sees track days is exactly the car that needs separation the most.

Cold climates. Short trips in winter are the worst case for condensation in the crankcase. A heated, closed-loop separator handles this. A catch can full of winter sludge does not.

High mileage. Ring seal degrades with miles. A 150,000-mile EJ makes more blow-by than it did new. An AOS is cheap insurance on an aging motor.

One thing it doesn't need: a tune. An air/oil separator doesn't change airflow, fueling, or boost. It's a bolt-on that works with the factory calibration.

Crawford Performance invented the Subaru-specific air/oil separator. We designed it because nothing on the market separated oil correctly on a turbo EJ — so we built the one that did. It is heated, baffled, closed-loop, and it goes back to the sump on its own. If your Subaru is turbocharged, it should have one.

---