What Is a Subaru Air Oil Separator & Why You Need One

When you start modifying your Subaru, it’s easy to get excited about parts that add horsepower, like a new exhaust or a tune. But before you chase bigger numbers, there’s a foundational upgrade every owner should consider first. Without it, adding more power can actually increase the risk of engine damage. The component in question is an Air Oil Separator. So, what is a Subaru air oil separator and why is it so critical? It’s a system that prevents oil vapor from contaminating your air-fuel mixture, a problem that gets worse as you increase boost. This protects your engine from power-robbing carbon buildup and catastrophic detonation, ensuring your performance gains are both safe and reliable.

Key Takeaways

• Prevent Oil Contamination to Protect Your Engine: An Air Oil Separator is essential for intercepting oil vapor from your crankcase before it can coat your intake system. This is the single most effective way to maintain engine health, especially for turbocharged Subarus like the WRX and STI.
• Stop Detonation and Ring Land Failure at the Source: When oil contaminates your fuel, it lowers the octane rating and causes carbon buildup, leading to engine knock. An AOS prevents this process, directly addressing a primary cause of catastrophic ring land failure in Subaru engines.
• Invest in a System Designed for Separation: Not all solutions are equal. Choose a true, closed-loop AOS over a simple catch can, and look for critical features like a heated design to manage condensation and internal baffles or a swirl pot to maximize oil separation.

What is a Subaru Air Oil Separator?

If you’re serious about protecting your Subaru’s engine and getting consistent power, an Air Oil Separator (AOS) is one of the most important upgrades you can make. Think of it as a gatekeeper for your engine's intake system. An AOS is a device that intercepts the air flowing through your engine's crankcase ventilation system and separates the oil vapor from it. This process ensures that only clean, dry air makes its way into your engine for combustion.

For turbocharged Subaru engines, especially those in the WRX and STI models, an AOS isn't just a nice-to-have accessory—it's a fundamental component for long-term health and performance. It addresses a core issue with the boxer engine's design, preventing a host of problems that can rob you of power and eventually lead to costly repairs. Let's break down exactly how it works and why it's so effective.

How an AOS Protects Your Engine

Your engine naturally produces pressure in the crankcase, which contains a mix of air and oil vapor. This pressure needs to be vented, and on a stock Subaru, it's routed directly back into your intake. The problem is, that oil vapor goes along for the ride. When this oily mist coats your intake valves, intercooler, and sensors, it lowers your fuel's octane rating and can lead to detonation, or engine knock.

An AOS protects your engine by collecting that oil vapor before it ever reaches the intake. It captures the oil, separates it from the air, and safely returns the liquid oil to your engine's oil pan. This keeps critical components clean and helps maintain the effective octane of your fuel, which is essential for safe and reliable engine tuning. By preventing oil contamination, an AOS helps your engine last longer and perform more consistently.

The Simple Science of Separating Oil from Air

So, how does an AOS actually pull the oil out of the air? The process is surprisingly straightforward and relies on basic physics. As the crankcase gases enter the AOS unit, they are forced through a series of chambers or a swirl pot. This design uses centrifugal force—or spinning force—to sling the heavier oil droplets against the walls of the container.

As the oil droplets collect on the walls, they condense back into liquid form and drain down to the bottom of the unit. From there, the clean oil is automatically sent back into your engine's oil system. Meanwhile, the now-clean air continues on its path to the intake manifold, ready for combustion. This simple but effective separation ensures your engine is only breathing what it’s supposed to: clean air. It’s one of the most vital performance parts for maintaining a healthy, high-performing Subaru engine.

Why Your Subaru Needs an Air Oil Separator

An Air Oil Separator (AOS) is more than just an add-on; it's one of the most effective preventative measures you can take to protect your Subaru's engine. While your car comes with a Positive Crankcase Ventilation (PCV) system from the factory, its job is simply to vent pressure. It doesn't stop oil vapor and other contaminants from being recirculated back into your engine's intake. This might be adequate for a stock daily driver, but for anyone who pushes their car—whether on the track, trail, or even during spirited daily driving—that recirculated oil becomes a serious problem.

Over time, this process leads to a cascade of issues that degrade performance and threaten engine health. An AOS intercepts these harmful vapors before they can cause damage. By installing one, you’re not just adding another part; you’re actively preserving your engine’s power, efficiency, and longevity. It’s a foundational upgrade that addresses the root cause of several common Subaru engine problems, making it an essential component for both stock and modified vehicles. Think of it as an investment that pays off by preventing costly repairs down the road.

Stop Carbon Buildup on Intake Valves

Every time your engine runs, oil vapor from the crankcase is pulled into your intake manifold. This vapor coats everything it touches, including the back of your intake valves. On direct-injection engines, found in many modern Subarus, there's no fuel spraying on the valves to wash this gunk away. As a result, the oil bakes into hard carbon deposits.

This carbon buildup acts like a sponge for fuel, disrupts airflow, and can even prevent the valves from sealing properly. The result is a gradual loss of power and efficiency that you might not even notice until it becomes severe. An AOS captures that oil vapor before it ever reaches the intake, keeping your valves clean and ensuring your engine breathes freely.

Prevent Engine Knock and Detonation

Carbon buildup does more than just restrict airflow; it creates a dangerous environment inside your combustion chamber. These deposits can flake off or create "hot spots" that glow red hot, igniting the air-fuel mixture before the spark plug fires. This uncontrolled explosion is known as pre-ignition or detonation, and it's incredibly destructive to your engine's internal components.

Detonation is a major cause of serious engine damage, including the notorious "ring land failure" common in Subarus. By preventing oil from contaminating the intake system, an AOS helps maintain the effective octane of your fuel and eliminates the carbon hot spots that trigger these damaging events. This is especially critical for turbocharged models like the Subaru WRX and STI.

Extend Your Engine's Life and Performance

An AOS is one of the smartest investments you can make for the long-term health of your Subaru. By stopping carbon buildup and preventing detonation, it saves your engine from a "slow death" caused by oil contamination. A cleaner engine is a more efficient and reliable engine. It maintains its power over time, requires less maintenance, and is far less likely to suffer from catastrophic failure.

Ultimately, an AOS helps your engine operate as it was designed to, free from the harmful byproducts that degrade performance. It’s a preventative measure that protects your investment, whether you’re building a high-performance track car or a dependable overlanding solution. You get peace of mind knowing your engine is protected from the inside out.

What Happens if You Don't Use an AOS?

Skipping an Air Oil Separator might seem like a way to save a few dollars on your build, but it exposes your engine to risks that can lead to significant performance loss and costly repairs down the road. The factory PCV (Positive Crankcase Ventilation) system is designed for stock power levels and daily driving. Once you start adding performance parts or pushing your Subaru on the track or trail, that factory system quickly becomes overwhelmed.

Without an AOS, oil vapor and unburned fuel—collectively known as blow-by—are routed directly back into your engine's intake. This isn't just a matter of keeping your engine bay clean; this process directly degrades the quality of the air and fuel mixture your engine needs to run efficiently and safely. Over time, this contamination builds up, slowly choking your engine's performance and setting the stage for much more serious mechanical failures. Think of an AOS not as an optional accessory, but as essential insurance for your engine's long-term health, especially for modified WRX, STI, and Forester XT models.

How Oil Contaminates Your Intake System

The unique design of the Subaru boxer engine makes it particularly prone to letting oil vapor slip past the PCV valve and into the intake tract. During normal operation, high pressure in the crankcase forces these oily gases out. The factory system recirculates them back into the intake manifold to be burned off. The problem is that this vapor doesn't stay a vapor. As it travels through the cooler intake system, it condenses into liquid oil, coating everything in its path—the throttle body, intake manifold, intercooler, and, most critically, the intake valves. This creates a sticky, grimy buildup that disrupts airflow and compromises engine efficiency.

The Risk of Lower Octane and Performance Loss

When that oil vapor gets pulled into the combustion chamber, it mixes with your air and fuel. This contamination effectively lowers the octane rating of your fuel. Even if you’re filling up with premium 93 octane, the oil dilution means your engine is combusting a lower-quality mixture. Your Subaru's ECU is smart enough to detect the potential for engine knock that this causes and will pull ignition timing to protect the engine. This protective measure directly results in a loss of horsepower and torque. You're essentially leaving power on the table because the engine can't operate at its peak efficiency, a problem that quality engine tuning services can identify but only an AOS can truly fix.

Avoid Long-Term Engine Damage

The occasional loss of power is one thing, but consistent, low-octane combustion leads to a far more dangerous condition: detonation. Detonation, or engine knock, is an uncontrolled explosion inside the combustion chamber rather than a smooth burn. This event sends a shockwave through the engine that can crack piston ring lands—a notoriously common and catastrophic failure point for Subaru engines. It can also damage pistons, bearings, and cylinder heads. By preventing oil from contaminating your fuel mixture in the first place, an AOS maintains your fuel's intended octane rating, drastically reducing the risk of detonation and protecting your engine from severe internal damage.

AOS vs. Catch Can: What's the Real Difference?

When you explore engine protection, you’ll quickly find the AOS vs. catch can debate. While both manage oil blow-by, they function very differently, and one offers a far more complete solution for your Subaru. Understanding this distinction is key to choosing the right protection for your engine, whether it's a daily driver or a track-ready machine. Let's break down how each system works and why the AOS consistently comes out on top.

How Each System Works

A catch can is a simple reservoir that "catches" oil and fuel vapors from your engine’s PCV line. The problem is that it only collects this gunk. You have to manually drain the can, and the oil it collects is permanently removed from your engine's crankcase, creating a risk of running low on oil during spirited driving. An Air Oil Separator (AOS) is a smarter, closed-loop system. It intercepts blow-by, separates the oil from the vapor, and automatically returns clean oil to the engine's oil pan. This ensures your oil level remains consistent, providing reliable protection for all your performance parts.

The AOS Advantage for Subaru Engines

The AOS design has a clear advantage for Subaru's boxer engine. Using features like centrifugal force, it spins blow-by rapidly, forcing heavier oil droplets to separate from lighter vapors. The collected oil then drains back into the crankcase. This constant recycling is critical, as it means your engine is never starved of lubrication, even under high-G cornering on the track. By maintaining optimal oil levels and keeping the intake clean, an AOS directly supports consistent performance and engine longevity. It’s a foundational part of many Subaru WRX upgrades for a reason.

Clearing Up Common Myths

While you’ll find plenty of arguments online, the expert consensus favors the AOS. The most common myth is that a catch can provides enough protection. While it’s better than nothing, it’s an incomplete solution that creates a new problem: the risk of running the engine low on oil. An AOS solves the core issue of blow-by without this drawback. It reroutes oil back into the oil system where it belongs, instead of letting vapors contaminate the intake manifold and compromise your air-fuel mixture. For anyone serious about performance and reliability, the choice is clear.

Which Subarus Benefit Most from an AOS?

While an Air Oil Separator is a valuable addition to nearly any Subaru engine, some models stand to gain more than others. The primary factor is whether your engine is turbocharged. Forced induction systems create higher crankcase pressures, which significantly increases the amount of oil vapor pushed through the PCV system and into your intake. This makes an AOS less of a simple upgrade and more of a necessity for maintaining performance and protecting your engine from harmful oil contamination.

If you drive your Subaru hard, have performance modifications, or simply want to ensure your engine stays as clean and efficient as possible for the long haul, an AOS is one of the smartest investments you can make. It’s a foundational part that supports every other performance modification by keeping the engine’s intake charge pure and potent. Let’s break down which models see the biggest benefits.

Turbocharged Engines: WRX & STI (2002-Present)

If you own a turbocharged Subaru, especially a WRX or STI, an AOS should be at the top of your upgrade list. These engines are particularly susceptible to oil consumption, and the problem only gets worse with performance modifications that increase boost. The higher pressure forces oil vapor directly into your intake tract, coating your intercooler, throttle body, and intake valves with sludge. This contamination lowers your fuel's effective octane rating, which can lead to detonation and potential engine damage. Installing a high-quality AOS is the most effective way to protect your investment and ensure you get consistent power from your Subaru WRX upgrades.

Essential Upgrades for Forester XT & Legacy GT

The need for an AOS isn't limited to the WRX and STI. Other turbocharged models, like the Forester XT and Legacy GT, share similar engine architecture and face the same risks of oil contamination. For owners who use these vehicles for spirited driving, towing, or have added modifications, an AOS becomes a crucial component for engine reliability. It prevents the gradual performance degradation caused by carbon buildup and ensures your engine is breathing clean, oil-free air. Think of it as essential preventative maintenance that supports the long-term health of your engine, especially if you’re planning further performance parts installations.

Do Naturally Aspirated Engines Need One?

What about non-turbo models like the Crosstrek, Impreza, or BRZ? While these naturally aspirated engines don't generate the same level of crankcase pressure as their turbocharged counterparts, they still benefit from an AOS. The PCV system still vents oil vapor into the intake, just at a lower volume. Over time, this can still lead to carbon buildup on intake valves, which is a known issue on direct-injection engines. While you might get by with regularly replacing your PCV valve on a stock vehicle, adding an AOS is a proactive step toward better long-term engine health and efficiency. It’s a smart move for any owner planning to keep their Subaru for years to come.

What to Look for in a High-Quality AOS

Not all air oil separators are built the same. When you’re investing in a part that directly protects your Subaru's Boxer engine, you need to know it has the right technology to do the job effectively. A cheap, poorly designed unit can give you a false sense of security while failing to capture the harmful contaminants it's supposed to. The best systems incorporate specific design features that work together to ensure only clean, oil-free air makes it back into your intake manifold. These aren't just bells and whistles; they are critical components for maximizing oil separation and preventing long-term engine wear.

When you're shopping for an AOS, look past the marketing claims and focus on three key engineering principles: a heated design to manage condensation, a sophisticated internal chamber system to separate oil, and features that maximize efficiency, like a swirl pot. These elements are the foundation of the most effective performance parts on the market and are essential for any serious Subaru build, whether you're hitting the track or exploring a new trail. Understanding what makes these features so important will help you choose a unit that provides real protection for your investment.

Why Heated Systems Matter

One of the most critical features of a high-quality AOS is its ability to manage condensation. Top-tier units achieve this by connecting to your Subaru’s coolant system, which keeps the AOS at a consistent operating temperature. Why does this matter? The crankcase vapors entering the AOS contain not just oil, but also water and unburnt fuel. In a cold, unheated can, these vapors condense and mix with the collected oil, creating a thick, milky sludge. A heated system prevents this by keeping the unit hot enough to vaporize the water and fuel, allowing them to be safely vented back into the intake as vapor while only the oil is separated and returned to the crankcase.

Understanding Dual Chamber and Labyrinth Designs

The inside of a premium AOS isn't just an empty can. It contains a complex series of baffles and chambers designed to force oil out of the air. Advanced units often feature dual-chamber or labyrinth designs that create a long, complicated path for crankcase gases to travel through. As the air navigates these passages, the heavier oil droplets can't make the sharp turns. They collide with the internal surfaces, coalesce, and drain away. This intricate design is key to separating the maximum amount of oil before the air is recirculated. The best part is that modern designs achieve this high level of efficiency while remaining compliant with emissions regulations, so you don't have to compromise performance for legality.

How Swirl Pot Design Maximizes Efficiency

To further improve separation, many high-end air oil separators incorporate a swirl pot. This feature uses physics to its advantage by forcing the incoming air and oil mixture into a vortex, or a swirling cyclone pattern. This cyclonic action slings the heavier oil droplets outward against the walls of the chamber through centrifugal force, where they stick and drain down to be collected. This process is incredibly effective at separating even very fine oil mist from the air. When combined with a labyrinth or chambered design, a swirl pot ensures that your engine receives the cleanest possible air, which is vital for maintaining performance in demanding applications and getting the most out of your Subaru WRX upgrades.

Top Air Oil Separator Brands for Your Subaru

When you start looking for an Air Oil Separator, you’ll find a few key players in the Subaru performance world. Each brand brings a slightly different design philosophy to the table, but all aim to solve the same core problem: keeping oil out of your intake system. Choosing the right one often comes down to your specific build, budget, and performance goals. Let’s look at some of the most trusted names and what makes their products stand out.

Crawford Performance

As the creators of the original AOS for Subaru, we designed our system to directly address the notorious oiling issues of the boxer engine. The Crawford Air Oil Separator is engineered to reduce oil consumption and maintain the efficiency of your engine's combustion cycle. By effectively separating oil vapor from the PCV system, it prevents harmful detonation that can lead to catastrophic damage like ringland failure. Our AOS is a foundational part of any reliable build, from a daily driven WRX to a track-prepped STI, ensuring your engine runs cleaner and lasts longer under pressure.

IAG Performance

IAG Performance is another highly respected name in the community, known for its well-engineered solutions. Their AOS focuses on effectively managing blow-by gases, which are a natural byproduct of combustion. The system is designed to capture these gases, separate the oil mist from the air, and drain the clean, collected oil back into the engine's crankcase. This closed-loop approach provides an elegant and low-maintenance solution for keeping your intake, turbo, and intercooler free from oil residue. Many builders trust IAG for its robust construction and proven effectiveness in demanding applications.

PERRIN Performance & COBB Tuning

PERRIN Performance and COBB Tuning also offer popular AOS solutions that protect your engine. PERRIN emphasizes how its AOS prevents oil from entering the intake tract, which is critical for avoiding engine knock and detonation. By keeping the air-fuel mixture pure, their system helps safeguard vital engine components and can save you from expensive repairs down the road. Similarly, COBB Tuning’s AOS is designed to regulate oil vapor evenly across the cylinders, contributing to smoother engine operation and potentially more stable turbo response. Both brands offer reliable options for enthusiasts looking to protect their investment.

How to Choose the Right AOS for Your Build

Selecting the right Air Oil Separator is about more than just picking a brand; it’s about finding the perfect match for your specific Subaru and how you drive it. An AOS is a critical component for engine health, especially in a performance build, so you want to get this choice right the first time. The best AOS for a daily driven Forester XT will be different from one designed for a track-prepped STI. To make the best decision, you need to consider three key factors: your vehicle’s exact model and year, your performance goals, and whether you plan to install it yourself or have a professional handle it. Thinking through these points will ensure you invest in a part that delivers the protection and performance your engine deserves, keeping it running strong for years to come.

Match the Part to Your Model and Year

The first rule of buying any performance part is ensuring it actually fits your car. Subaru engines and engine bays have changed quite a bit over the years, and an AOS is not a universal part. A kit designed for a 2004 WRX won’t have the right brackets or hose lengths for a 2022 model. Reputable manufacturers design model-specific kits to guarantee a perfect fit and proper function. For example, the Crawford Performance AOS was the original solution developed specifically for Subaru's unique oiling system challenges. When shopping, always use your vehicle’s model and year to filter your options. This ensures all mounting points line up, the hoses are correctly routed without kinks, and the system integrates seamlessly with your existing engine components.

Consider Your Performance Goals and Driving Style

How you use your Subaru directly impacts which AOS is right for you. If you have a modified WRX or STI that sees regular track days or aggressive driving, your engine produces more crankcase pressure and blow-by. For these high-performance builds, a robust, high-efficiency AOS is essential to handle the increased demand. An AOS is an extremely important upgrade for any modified turbocharged Subaru. On the other hand, if you have a lightly modified daily driver or an overlanding Crosstrek, a standard, high-quality unit will provide all the protection you need. Align your choice with your build to ensure you’re getting the right level of protection without overspending on features you won’t use.

DIY vs. Professional Installation

The final piece of the puzzle is installation. Are you comfortable working on your own engine, or would you rather leave it to an expert? Many AOS kits are designed for straightforward installation by a skilled home mechanic with the right tools. However, routing hoses correctly and ensuring there are no leaks is critical for the system to work properly. If you’re not confident in your mechanical skills, professional installation is the best path forward. A technician who specializes in Subarus will get the job done efficiently and correctly, giving you peace of mind. Many performance shops that offer engine tuning services also perform expert parts installations, ensuring your new AOS functions perfectly from day one.

Installing and Maintaining Your New AOS

Once you’ve chosen the right Air Oil Separator for your Subaru, proper installation and upkeep are key to getting the performance and protection you paid for. An AOS isn’t a “set it and forget it” part; it’s an integral component of your engine’s health that requires a little attention to do its job effectively. The good news is that maintaining a quality AOS is straightforward. By following a few simple steps for installation and maintenance, you can ensure your engine stays clean and powerful for years to come, whether you’re commuting to work or pushing the limits on the track.

Key Installation and Warranty Info

Proper installation is the foundation for your AOS to work correctly. While many enthusiasts are comfortable with a DIY approach, we strongly recommend professional installation. An AOS integrates with your PCV and coolant systems, and incorrect routing of hoses can lead to serious engine problems. A professional technician ensures everything is connected perfectly, preventing leaks and guaranteeing optimal function from day one. Plus, many performance parts warranties, including our own 12-month/12,000-mile engine warranty, require installation by a certified shop to protect your investment.

Your Simple Maintenance Checklist

Keeping your AOS in top shape doesn't require a lot of work, just consistency. The most important thing you can do is stick to a regular maintenance schedule. First, continue with regular oil changes using a high-quality, fully synthetic oil. This is especially critical for modified engines, as synthetic oil provides superior protection and longevity. While you’re under the hood, perform a quick visual inspection of the AOS and its hoses. Look for any signs of cracking, brittleness, or leaks. Most high-quality units are designed to be easily disassembled for cleaning, which you should consider doing periodically to clear out any accumulated gunk and ensure the internal pathways are clear.

How to Monitor Performance and Spot Issues

Staying in tune with your vehicle will help you spot potential issues before they become major problems. A failing AOS can show several signs, such as a sudden increase in oil consumption, blue smoke from the exhaust (especially on startup), or visible oil in your intercooler or intake piping. If you notice any of these symptoms, get your system inspected immediately. Oil slipping past a faulty AOS can lower your fuel's octane rating, leading to engine "knock" or "detonation." This phenomenon can cause severe damage to your pistons and rings. Addressing these issues often requires expert engine tuning services to restore performance and prevent further damage.

Related Articles

• All About Subaru's Original Air Oil Separator & Why You Need One
• 4 Best Air Oil Separators for WRX (Ranked)
• 2008-2014 Subaru WRX Air Oil Separator V2 Crawford Performance

Frequently Asked Questions

Is an AOS really necessary if my Subaru is completely stock? Yes, it's one of the most valuable preventative upgrades you can make, even on a stock engine. Your Subaru's engine produces oil vapor as a natural part of its operation, and the factory system allows that vapor back into your intake. Over time, this causes carbon to build up on your valves, which slowly reduces efficiency and power. An AOS stops this process from the start, preserving your engine's health and performance for the long term.

Will installing an Air Oil Separator add horsepower to my car? An AOS doesn't create new horsepower, but it plays a critical role in preventing power loss. When oil contaminates your fuel, it lowers the effective octane, forcing your car's computer to reduce ignition timing to prevent engine knock. This directly results in less power. By keeping the intake charge clean, an AOS allows your engine to operate consistently at its full, intended potential without the computer needing to intervene.

What kind of maintenance does a quality AOS require? A well-designed, heated AOS is a nearly maintenance-free system. Unlike a catch can that requires you to manually drain collected gunk, an AOS automatically returns clean oil to the engine. Your only task is to visually inspect the hoses for any signs of wear or cracking during your regular oil changes to ensure the system remains sealed and functions perfectly.

Can a faulty or poorly installed AOS cause any problems? Absolutely. An improperly installed AOS with kinked or leaking hoses can fail to relieve crankcase pressure, potentially leading to oil leaks or other engine issues. Similarly, a low-quality unit may not separate oil effectively, giving you a false sense of security while your intake continues to get contaminated. This is why choosing a reputable brand and ensuring professional installation is so important for protecting your engine.

Will installing an AOS void my vehicle's warranty? Legally, a manufacturer cannot void your entire vehicle warranty simply because you've installed an aftermarket part. For a warranty claim to be denied, the dealer must prove that the AOS was the direct cause of the specific failure. However, to ensure you are fully protected, we always recommend professional installation by a certified technician, which provides peace of mind and proper documentation.