If you own a modern direct-injection Subaru like the 2015+ WRX, you’ve probably heard about the issue of carbon buildup on intake valves. Unlike older engines that used fuel to keep valves clean, direct injection leaves them vulnerable to deposits from recirculated crankcase vapors. This buildup restricts airflow and can slowly reduce your engine's power and efficiency. The most effective defense is stopping the oil from ever reaching the valves in the first place. An Air Oil Separator does exactly that. We'll explore the specific air oil separator benefits for direct-injection engines and show you how this single part can prevent costly maintenance down the road.
Key Takeaways
- An AOS is foundational engine protection: It actively prevents oil vapor from contaminating your intake, which helps stop power-robbing carbon buildup, reduces the risk of engine knock, and preserves your Subaru's long-term health.
- Prioritize features for a maintenance-free solution: A true Air Oil Separator is not a simple catch can. Look for critical features like an automatic oil return and engine coolant heating to ensure the system works effectively without requiring you to manually empty it.
- A critical upgrade for turbocharged and modified cars: While beneficial for any Subaru, an AOS is essential for models like the WRX and STI or any vehicle with performance parts. These engines produce more blow-by, and an AOS is the best defense for protecting your investment and ensuring reliable power.
What Is an Air Oil Separator and How Does It Work?
An Air Oil Separator, or AOS, is a crucial component for protecting and maintaining the performance of your Subaru's engine. Think of it as a dedicated filter for your engine's crankcase ventilation system. Its main job is to remove oil vapor and other contaminants from the air that gets recirculated back into your engine's intake. This simple but effective process prevents a host of issues that can rob your engine of power and reduce its lifespan, especially in turbocharged and modified vehicles. By keeping your intake clean, an AOS ensures your engine breathes only fresh air, allowing it to run at its full potential.
Understanding Engine Blow-By
During normal engine operation, high-pressure gases from the combustion chamber inevitably "blow by" the piston rings and enter the crankcase. This process is completely normal, but the problem is that these gases carry a fine mist of hot oil vapor with them. Your car's factory PCV (Positive Crankcase Ventilation) system is designed to vent these gases back into the intake manifold to be burned off. However, it doesn't stop the oil vapor from coming along for the ride. When this oily mist coats your intake system, it can lower your fuel's effective octane rating, leading to reduced power and an increased risk of engine knock or detonation—a major concern for any performance engine tuning.
How an AOS Filters Oil from Air
An Air Oil Separator works by intercepting the blow-by gases before they reach your intake. Inside the AOS unit, the oily air is forced through a series of baffles or a centrifuge that causes it to change direction rapidly. Because oil droplets are heavier than air, they can't make the sharp turns. They separate from the air, collect inside the AOS, and are safely drained back into the engine's oil pan. The now-clean air continues on its path to the intake manifold. This ensures only air and fuel are entering your combustion chamber, allowing for a more efficient and powerful engine cycle. It’s one of the most effective performance parts you can install to preserve engine health.
Key Benefits of Installing an Air Oil Separator
Installing an Air Oil Separator (AOS) is one of the most effective modifications you can make to protect your Subaru’s engine, whether it’s a daily driver or a fully built track car. The primary job of an AOS is to remove oil vapor and other contaminants from the crankcase gases before they are recirculated back into your intake. This simple function has a cascade of positive effects that preserve engine health, maintain performance, and reduce long-term maintenance needs.
By keeping your intake system clean, an AOS prevents a host of issues that commonly plague boxer engines, especially turbocharged and direct-injection models. These benefits range from preventing power-robbing carbon buildup on intake valves to maintaining a stable air-fuel ratio for consistent combustion. Think of it as an investment in your engine's longevity and reliability. It ensures that your engine is breathing the cleanest air possible, allowing it to operate at its full potential while protecting critical components from harmful deposits. For anyone serious about performance, an AOS is a foundational part of a healthy engine setup.
Prevent Oil Contamination in Your Intake
Every time your engine runs, a small amount of combustion gas, or "blow-by," slips past the piston rings and into the crankcase. The factory Positive Crankcase Ventilation (PCV) system is designed to vent this pressure back into the intake manifold to be burned off. The problem is, these gases are full of hot oil vapor. An Air Oil Separator intercepts these vapors, using a series of baffles to separate the liquid oil from the air. The clean air continues to the intake, while the collected oil is safely drained back into the engine. This process keeps your entire intake tract—from the turbo inlet to the intercooler and throttle body—free from the oily film that can accumulate over time and attract other contaminants.
Reduce Carbon Buildup on Intake Valves
For modern direct-injection (DI) Subaru engines, like those in the 2015+ WRX, carbon buildup on intake valves is a significant concern. In a traditional port-injected engine, gasoline is sprayed over the back of the intake valves, which helps keep them clean. In a DI engine, fuel is injected directly into the cylinder, so the valves never get this cleaning effect. When oily crankcase vapors are recirculated, they bake onto the hot intake valves, forming hard carbon deposits. These deposits can restrict airflow, disrupt fuel atomization, and ultimately reduce engine power and efficiency. An AOS is the best defense against this, as it stops the oil from ever reaching the valves in the first place.
Maintain a Healthy Air-Fuel Ratio
Oil vapor doesn't just create a mess; it also degrades the quality of your air-fuel mixture. When oil enters the combustion chamber, it effectively lowers the octane rating of your fuel, making it more prone to detonation or "knock." Your vehicle's ECU is designed to detect this and will pull ignition timing to protect the engine, which directly reduces horsepower and throttle response. By installing an AOS, you ensure that only clean air is entering the intake manifold. This helps maintain a consistent and optimal air-fuel ratio, allowing your engine to run safely and efficiently. It’s a critical step for anyone who has invested in professional engine tuning services and wants to preserve their results.
Restore Lost Power and Efficiency
All of these benefits work together to help your engine perform at its best. A clean intake system allows for cooler, denser air to enter the engine, which is essential for making power. Preventing carbon buildup ensures that airflow isn't restricted, and maintaining a stable air-fuel ratio allows the engine to operate at peak efficiency. Over time, an engine without an AOS will slowly lose power as oil contaminates the system and carbon deposits form. Installing one of our performance parts like an AOS not only prevents this degradation but can also help restore power that has already been lost due to these issues, making it a smart upgrade for both new and high-mileage vehicles.
How an Air Oil Separator Protects Your Engine
An Air Oil Separator isn't just another bolt-on part; it's a critical component for protecting the heart of your Subaru. By effectively managing blow-by, an AOS provides a multi-layered defense against common issues that can rob your engine of power, efficiency, and longevity. Think of it as an investment in your engine's long-term health, ensuring it runs cleanly and powerfully for years to come. Whether you're pushing your car on the track, exploring a new trail, or just enjoying your daily commute, an AOS works continuously to keep your engine in top condition. It addresses the root cause of many performance-related problems, making it one of the most valuable performance parts you can install.
Improve Combustion and Airflow
A clean intake is essential for optimal engine performance. An AOS ensures that only clean, filtered air reaches your combustion chamber by capturing oil vapor from the crankcase before it can enter the intake system. This process prevents a sticky film of oil from coating your intercooler, intake manifold, and sensors. When these components stay clean, your engine can breathe more freely, leading to more consistent and efficient combustion. This simple but effective filtering action helps maintain the power and responsiveness you expect from your Subaru, ensuring every combustion cycle is as clean as possible.
Reduce Detonation and Premature Wear
Engine detonation, or knock, is one of the most damaging things that can happen to a high-performance engine. It occurs when the air-fuel mixture ignites prematurely, creating uncontrolled explosions inside the cylinders. When oil vapor enters the combustion chamber, it effectively lowers your fuel's octane rating, dramatically increasing the risk of detonation. An AOS prevents this by keeping oil out of the mix. This protects your pistons, rods, and bearings from extreme stress and premature wear. Proper engine tuning services are crucial for performance, and an AOS helps maintain the stable conditions a good tune relies on.
Lower Your Long-Term Maintenance Needs
Installing an Air Oil Separator is a proactive step that can save you significant time and money on future maintenance. By preventing oil and carbon from building up on your intake valves—a common issue on direct-injection engines—you can avoid costly and labor-intensive valve cleaning services down the road. A cleaner engine is also a more efficient one, which means you’ll maintain better fuel economy and consistent power over the life of your vehicle. This is especially important for owners of performance models like the Subaru WRX, where maintaining peak condition is a top priority.
Air Oil Separator vs. Catch Can: What’s the Difference?
When you start looking for ways to manage blow-by, you’ll quickly run into two options: the air oil separator (AOS) and the catch can. Both are designed to pull oil vapor out of your crankcase ventilation system before it can contaminate your intake. However, they handle the job in fundamentally different ways. A catch can is a simple, passive solution, while an AOS is an active, integrated system that offers a more complete and maintenance-free approach to protecting your engine. Understanding these differences is key to choosing the right upgrade for your Subaru.
Automatic Draining vs. Manual Emptying
The biggest distinction between an AOS and a catch can comes down to what happens to the oil after it’s separated. A catch can is exactly what it sounds like: a can that collects the oil, water, and fuel vapors. This means you have to manually empty it on a regular basis. If you forget, it can overfill and send that collected sludge right back into your intake, which is worse than having no protection at all.
An air oil separator, on the other hand, is a true "set it and forget it" solution. It’s a closed-loop system that automatically drains the separated oil back into the engine’s oil pan. This eliminates the need for routine maintenance and ensures your system is always working to protect your engine.
The Importance of Heated Units
Another critical feature that sets a high-quality AOS apart is heating. The blow-by coming from your crankcase isn't just oil; it's a mix of oil, unburnt fuel, and water vapor. In colder weather, or even during short trips where the engine doesn't fully warm up, this mixture can condense into a thick, milky sludge inside a standard catch can.
Premium Air Oil Separator kits solve this problem by using engine coolant to heat the unit. This keeps the vapors hot, preventing condensation and ensuring that only liquid oil is separated and returned to your engine. Water and fuel vapors are burned off in the combustion cycle as intended. This feature is essential for maintaining performance and reliability in all driving conditions.
Which Is Better for Daily Driving?
So, do you really need an AOS for a stock Subaru that you just drive to work? Many experienced owners say yes, calling it one of the best preventative upgrades you can make. For direct-injection engines found in newer models like the WRX, preventing carbon buildup on the intake valves is crucial for long-term health. An AOS keeps the intake tract clean from the start, helping maintain power and efficiency over the life of the vehicle.
While some might see it as overkill for a car that isn't tracked, think of it as an investment. Whether you're building a high-performance track car or a reliable rig for overlanding adventures, protecting your engine is always a smart move. It reduces the chances of detonation, maintains a proper air-fuel ratio, and can save you from costly maintenance down the road.
Which Subaru Models Need an Air Oil Separator?
While an Air Oil Separator is a smart upgrade for nearly any Subaru, certain models and driving styles benefit more than others. The amount of blow-by your engine produces is the key factor, and this can be influenced by everything from factory turbochargers to your weekend hobbies. Let’s look at which vehicles need an AOS the most.
Turbocharged Engines: WRX & STI (2002-Present)
If you drive a turbocharged Subaru, an AOS should be at the top of your upgrade list. Models like the WRX and STI, from 2002 to the present, are especially prone to oil blow-by due to the increased pressure created by the turbo. This forces more oil vapor and unburnt fuel past the piston rings and into the crankcase. Without an AOS, that oily mist gets recirculated directly into your intake, leading to carbon buildup and reduced performance. Installing one is a foundational step in protecting your engine and ensuring it runs cleanly. It’s one of the most effective Subaru WRX upgrades you can make for long-term health.
Modified and High-Performance Builds
Are you planning to add power? Any modification that increases horsepower—whether it’s a larger turbo, higher boost pressure, or a custom tune—will also increase crankcase pressure. This means more blow-by and a greater need for an effective air-oil separation system. For high-performance and track builds, an AOS isn’t just a recommendation; it’s essential for protecting your investment. It keeps your intake clean, your air-fuel ratio stable, and prevents detonation under heavy load. When you invest in quality performance parts, an AOS ensures you get the reliable power you paid for.
Daily Drivers and Overlanding Rigs
You don’t need a track-ready STI to benefit from an Air Oil Separator. Many owners install an AOS on a completely stock car to improve long-term reliability and engine life. Even for a daily driver, preventing oil from coating your intake valves and sensors is a smart move. This is also true for overlanding solutions on models like the Crosstrek and Forester. Off-road driving often involves sustained RPMs and steep angles, which can worsen oil blow-by. An AOS provides peace of mind by protecting your engine from contamination, no matter where your adventures take you.
How to Choose the Right Air Oil Separator
Not all air oil separators are created equal. When you start looking, you'll find a lot of options, and choosing the right one is critical for protecting your engine and getting the performance you paid for. Think of a high-quality AOS as an investment in your Subaru’s long-term health, whether you’re building a dedicated track car or a dependable overlanding rig. To make the right choice, you need to focus on a few key features that distinguish a great AOS from a mediocre one. Getting these details right ensures your system will work effectively in all driving conditions, keeping your intake clean and your engine running at its peak for years to come.
Look for Engine Coolant Heating
One of the most important features to look for is engine coolant heating. An effective AOS will have lines that circulate hot engine coolant through its body. This process keeps the unit warm, preventing the oil and water vapors collected from the crankcase from condensing into a thick, milky sludge, especially in colder climates. Without this heating element, the collected gunk can clog the system and render it ineffective. A heated unit ensures the oil remains in a liquid state, allowing it to separate properly from the air and drain back into the engine as intended.
Prioritize an Automatic Oil Return
The main difference between a catch can and a true air oil separator is what happens to the oil after it’s captured. While a catch can simply stores the blow-by, requiring you to empty it manually, a proper AOS automatically returns the clean oil to your engine’s oil pan. This "set it and forget it" design is incredibly convenient, as it means you can stick to your regular oil change schedule without adding another maintenance task to your list. An automatic return system ensures your engine always has the right amount of oil while the AOS continuously protects it from contamination.
Check for Quality and Proper Fitment
An air oil separator is not a part where you want to compromise on quality. Look for units made from durable materials like billet aluminum that can withstand the harsh conditions of an engine bay. Precision engineering ensures there are no leaks and that the internal baffling system is effective at separating oil from the air. Equally important is ensuring the AOS is designed specifically for your Subaru model, whether it’s a WRX or a Forester. Proper fitment guarantees that all the hoses, brackets, and connections line up perfectly for a clean, professional installation and optimal performance.
Explore Crawford Performance AOS Kits
When you’re searching for a reliable unit that ticks all the boxes, our Crawford Performance AOS kits are a great place to start. Designed and tested on our own race-winning Subaru builds, they incorporate engine coolant heating, an automatic oil return, and model-specific fitment for a seamless installation. Our kits are trusted by enthusiasts and professional racers alike to protect their engines under the most demanding conditions. While other brands like IAG and Perrin also offer functional solutions, choosing a well-engineered system is the best way to safeguard your investment and support your performance parts upgrades.
Debunking Common Air Oil Separator Myths
When you’re researching engine upgrades, it’s easy to run into conflicting advice. Air oil separators are a perfect example—there’s a lot of misinformation out there that can make it hard to decide if one is right for your Subaru. Let’s clear the air and tackle some of the most common myths I hear about AOS systems. Understanding the facts will help you make an informed decision to protect your engine’s health and performance for the long haul.
Myth: Stock Engines Don't Need One
One of the biggest misconceptions is that an AOS is only necessary for heavily modified or track-driven cars. The reality is that every engine, including a completely stock one, produces blow-by. This process allows oil vapor to enter your intake system, which can coat your intercooler, intake manifold, and valves over time. This buildup makes your engine run less efficiently and can slowly rob it of power. Installing an AOS is a preventative measure that keeps your intake clean and helps your engine maintain its factory performance level from day one.
Myth: The Factory PCV System Is Enough
Your Subaru comes with a Positive Crankcase Ventilation (PCV) system from the factory, so why add anything else? While the stock PCV system does its job, it’s designed as a compromise between cost, emissions regulations, and performance. It’s adequate, but it’s not optimized for separating oil vapor effectively, especially under spirited driving conditions. Aftermarket performance parts like an AOS are engineered specifically to do one job exceptionally well: remove oil from the crankcase vapor before it can contaminate your intake. Think of it as an extra layer of protection the factory system simply can’t provide.
Myth: Oil Buildup Only Affects Modified Cars
Many drivers assume oil buildup is only a problem for cars running higher boost or other significant modifications. However, oil contamination is a known issue for all turbocharged engines, including the Subaru EJ and FA series found in the WRX and STI. Even on a daily driver, owners often find a surprising amount of oil coating their intercooler and intake tract. This oil lowers the effective octane of your air-fuel mixture, which increases the risk of detonation—a dangerous condition that can cause serious engine damage. An AOS helps prevent this, making it a smart investment for any turbocharged Subaru.
What to Know About Installation and Maintenance
Once you've chosen the right air oil separator, the next step is getting it on your car. You might be wondering about the complexity of the installation and what kind of upkeep is required. The good news is that an AOS is one of the more straightforward and low-maintenance performance parts you can add to your Subaru. It delivers significant engine protection without adding a lot of complexity to your build. Let's walk through what you can expect.
Why Professional Installation Matters
While many air oil separator kits are designed to be DIY-friendly, professional installation offers complete peace of mind. For enthusiasts who are comfortable under the hood, the process is often straightforward and doesn't require specialized tools. However, if you want to ensure every hose is perfectly routed and every connection is secure, letting an expert handle it is the best way to go. A professional install guarantees your AOS will function flawlessly from the start, preventing potential issues like vacuum leaks that could compromise performance. It’s a great way to protect your investment and get back on the road or trail without any guesswork.
Do You Need a Tune After Installation?
This is a common question, and the answer is simple: you generally do not need an engine tune after installing an air oil separator. An AOS works by restoring your engine's PCV system to optimal function—it doesn't alter your air-fuel ratio or boost pressure in a way that requires recalibrating the ECU. It simply ensures the air entering your engine is cleaner and free of oil vapor. While other modifications like upgraded turbos or injectors require professional engine tuning services to adjust for the changes, an AOS is a supporting modification that helps your engine run as it was designed to, only cleaner and more efficiently.
Simple Long-Term Maintenance
One of the biggest advantages of an air oil separator is its "set it and forget it" design. Unlike a catch can that needs to be manually emptied, an AOS automatically drains the separated oil back into the engine's oil pan. This closed-loop system means there is no regular maintenance for you to perform. The only thing you need to keep up with is your vehicle's standard oil change schedule. By installing an AOS, you're not adding another item to your maintenance checklist; you're simply making your engine healthier in the long run with a part that takes care of itself.
Upgrades That Pair Well With an Air Oil Separator
An Air Oil Separator is a foundational upgrade that supports nearly every other performance modification you can make. By ensuring your engine breathes clean, oil-free air, an AOS allows other components to function at their peak. If you’re planning to add more power or improve reliability, think of an AOS as the essential first step that helps your other investments perform as intended. It’s particularly crucial when you start introducing modifications that increase cylinder pressure and, consequently, engine blow-by.
Upgraded Turbos and Higher Boost
If you’re installing a larger turbo or increasing the boost on your stock unit, an AOS is non-negotiable. Higher boost levels force more air into the cylinders, creating more power but also generating significantly more crankcase pressure. This pressure pushes a greater volume of oil vapor and unburnt fuel past the piston rings as blow-by. Without an effective AOS, this oily mist will coat your intercooler, intake manifold, and valves. This reduces your intercooler's efficiency and lowers your fuel's effective octane rating, which can lead to detonation. An AOS protects your engine and ensures your Subaru WRX upgrades deliver clean, consistent power.
High-Performance Engine Builds
For anyone investing in a high-performance or fully built engine, an AOS is essential for protecting that investment. Modified engines with forged internals are built to handle extreme stress, but they often produce more blow-by, especially under track conditions or aggressive driving. An AOS keeps the intake charge pure, which is critical for preventing detonation that could damage expensive components. By separating the oil from the crankcase vapor, you ensure your engine is only burning a precise mixture of air and fuel. This maintains performance, improves reliability, and is a key part of any serious build using top-tier performance parts.
ECU Tuning and Intercooler Upgrades
An ECU tune is designed to optimize your engine’s performance by carefully adjusting fuel, timing, and boost parameters. However, its effectiveness depends on consistent and clean airflow. When oil vapor coats the inside of your intercooler, it acts as an insulator, reducing its ability to cool the charge air. Hotter air is less dense and more prone to knock. An AOS keeps your intercooler and sensors clean, allowing your tune to perform as the tuner intended. This ensures the power gains from your engine tuning services are both reliable and repeatable, giving you the safe, consistent performance you paid for.
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Frequently Asked Questions
Is an AOS really worth it for a stock car I just drive daily? Absolutely. Think of it as a long-term investment in your engine's health. Every Subaru engine produces blow-by, even a brand new one. An AOS prevents the slow, gradual buildup of oil in your intake system that can reduce efficiency and power over time. For newer direct-injection models like the WRX, it's especially important for preventing carbon buildup on the intake valves, which is a common issue that can be expensive to fix later on.
Will installing an Air Oil Separator void my vehicle's warranty? This is a common concern, but installing a quality aftermarket part like an AOS does not automatically void your vehicle's warranty. Legally, a manufacturer must prove that the specific aftermarket part you installed was the direct cause of a failure. Since a properly installed, high-quality AOS is designed to protect your engine from harmful oil contamination and detonation, it actually promotes engine health rather than causing damage.
What's the main reason to choose an AOS over a simpler catch can? The biggest advantage is that an AOS is a complete, maintenance-free system. A catch can simply collects oil and requires you to manually empty it on a regular basis. If you forget, it can overflow and send all that collected sludge into your engine. A true AOS, on the other hand, automatically drains the separated oil back into the engine, so you can set it and forget it. Premium units are also heated by engine coolant, which prevents harmful sludge from forming in cold weather.
Can an AOS actually improve my car's performance, or does it just protect the engine? While its primary job is protection, that protection directly supports performance. By keeping oil out of your intake, an AOS helps your engine run more efficiently and can even restore power that has been lost due to contamination. A clean intake system allows for a cooler, denser air charge, and preventing oil from entering the combustion chamber helps maintain a stable air-fuel ratio. This allows your engine to produce consistent power and reduces the risk of the ECU pulling timing to prevent knock.
Do I need to get my car retuned after installing an AOS? No, a tune is not required after installing an air oil separator. An AOS is considered a supporting modification that helps your engine's crankcase ventilation system work more effectively. It doesn't fundamentally change your engine's air-fuel ratio, boost pressure, or other parameters that would require an ECU recalibration. It simply ensures the air entering your engine is cleaner, allowing your current setup—whether stock or tuned—to run more reliably.