Subaru Air Oil Separator: A Complete Buyer's Guide
Your Subaru’s Boxer engine has a dirty little secret: oil blow-by. During normal operation, a fine mist of oil vapor escapes past the pistons and gets sucked back into your engine’s intake. Over time, this oily gunk bakes onto your intake valves, robbing your engine of power and efficiency. This is especially true for modern direct-injection engines. The solution is a simple but incredibly effective component. A Subaru Air Oil Separator (AOS) acts as a guardian for your engine, intercepting this vapor before it can cause harm. It’s one of the most critical preventative upgrades you can make to ensure long-term performance and reliability.
Key Takeaways
- Protect your engine by preventing carbon buildup: An Air Oil Separator is a critical defense for your Subaru, especially for direct-injection models. It stops oil vapor from contaminating your intake valves, which helps maintain consistent power and engine efficiency.
- Choose a set-it-and-forget-it solution: Unlike a catch can that requires manual draining, a quality AOS is a self-draining system. It automatically separates and returns oil to the engine, offering continuous, hassle-free protection against blow-by.
- Prioritize quality design and proper installation: For the best results, select an AOS with durable construction and a dual-chamber design. Correct installation high in the engine bay is just as important, as it ensures the system can drain properly and function as intended.
What Is a Subaru Air Oil Separator?
A Subaru Air Oil Separator, or AOS, is a performance part designed to protect your engine and keep it running efficiently. Think of it as a gatekeeper for your engine’s intake system. During normal operation, your engine produces oil vapor, also known as "blow-by," which can get sucked into the intake. An AOS intercepts this oily mist, separating the oil from the air. It then sends the clean air back where it belongs and safely returns the collected oil to the engine's oil pan. This simple but crucial process prevents a host of problems, from power loss to long-term engine damage, making it one of the most effective performance parts you can install.
How an AOS Protects Your Engine
The main job of an AOS is to stop oil blow-by from contaminating your engine’s intake system. This contamination leads to carbon buildup on critical components like intake valves and pistons. Over time, this buildup restricts airflow, reduces engine efficiency, and can even lower your fuel's octane rating, which increases the risk of engine knock. A quality Subaru Air Oil Separator acts as a closed-loop filtration system. It captures the oil vapor before it can cause harm, ensuring only clean air enters the combustion chamber. This helps maintain optimal performance, prevents power loss, and keeps your engine running healthier for longer.
The Problem with Oil Vapor Contamination
Oil vapor contamination is a particularly significant issue for modern Subaru engines, especially those with direct fuel injection (DFI), like the WRX models from 2015 to the present. In DFI engines, fuel is injected directly into the cylinder, so it never washes over the intake valves to clean them. This makes them highly susceptible to carbon buildup from oil vapor. When oil coats the intake valves, it bakes into hard carbon deposits that disrupt airflow and can cause serious performance issues. An AOS solves this by separating the oil from the air before it ever reaches the valves, providing a vital defense against the carbon buildup that plagues these advanced engines.
AOS vs. Catch Can: What's the Difference?
While both an AOS and a catch can aim to manage oil blow-by, they operate very differently. A catch can is a simple container that collects oil vapor, but it requires you to manually drain it every few hundred miles. If you forget, it can overflow and send all that collected oil straight into your intake. An AOS, on the other hand, is a complete, self-draining system. It automatically separates the oil and returns it to the engine, making it a true set-it-and-forget-it solution. This maintenance-free design is why many enthusiasts consider an AOS a superior and more reliable upgrade for long-term engine protection.
Why Your Subaru Needs an Air Oil Separator
An Air Oil Separator (AOS) is more than just an add-on; it’s a fundamental upgrade for protecting your Subaru’s engine and maintaining its performance. By addressing the engine’s natural process of creating “blow-by,” an AOS prevents a host of issues that can reduce power, increase oil consumption, and lead to costly repairs down the road. Think of it as essential insurance for your Boxer engine, especially if you’ve invested in other performance modifications. It works behind the scenes to keep your engine running clean and strong, whether you’re on the track, the trail, or your daily commute.
Installing an AOS is one of the most effective preventative measures you can take. It ensures that the air entering your engine is as clean as possible, which is critical for combustion efficiency and the longevity of sensitive components. While all engines produce some level of blow-by, the unique design of the Subaru Boxer engine makes it particularly important to manage these oil vapors effectively. For any Subaru owner serious about performance and reliability, an AOS isn't a luxury, it's a necessity that pays for itself by preventing future problems.
Improve Performance and Reduce Carbon Buildup
Modern Subaru engines, particularly direct-injection models like the WRX, are prone to carbon buildup on the intake valves. This happens because an oily mist from the engine’s crankcase, known as blow-by, gets recirculated into your intake. Since fuel is injected directly into the cylinder, it never washes over the valves to clean them. Over time, this oily vapor bakes onto the valves, forming hard carbon deposits that restrict airflow, hurt fuel economy, and reduce overall performance. An AOS intercepts this mist, ensuring only clean air enters your intake. This simple modification is one of the most effective ways to maintain the power and responsiveness of your Subaru WRX.
Forget Maintenance with a Self-Draining Design
One of the biggest advantages of an Air Oil Separator is its set-it-and-forget-it design. Unlike a traditional catch can that you have to manually empty, an AOS is a closed-loop, self-draining system. It automatically separates the oil from the air and drains the collected oil right back into the engine’s oil pan where it belongs. This means you get constant, reliable protection without adding another maintenance task to your checklist. You can drive with confidence knowing your AOS is always working to protect your engine, without you ever having to check or empty a container. It’s a smart, efficient solution for any Subaru owner looking for hassle-free performance parts.
Extend Engine Life and Lower Oil Consumption
Keeping oil vapor out of your engine’s intake system does more than just keep things clean; it directly contributes to the long-term health of your engine. When oil enters the combustion chamber, it lowers the octane rating of your fuel, which can lead to detonation, or “engine knock.” This phenomenon can cause catastrophic damage to your pistons and cylinder walls. By preventing oil contamination, an AOS helps your engine run more smoothly and reliably, reducing wear and tear on critical components. This is crucial for extending engine life, especially in tuned or high-performance applications. It’s a key supporting mod for anyone considering engine tuning services.
Prevent Harmful Intake Valve Contamination
At its core, an Air Oil Separator acts as a highly efficient filter for your engine’s crankcase ventilation system. The blow-by gases that escape past the piston rings are full of unburnt fuel and oil vapor. The AOS is designed to capture these gases, swirl them through a series of chambers or baffles, and force the heavier oil droplets to separate from the lighter air. The clean air is then sent back into your engine’s intake to be burned, while the collected oil is safely returned to the oil pan. This process is vital for preventing the sticky, performance-robbing gunk that contaminates intake manifolds, sensors, and valves, ensuring your Subaru STI runs at its peak.
Which Subaru Models Benefit Most from an AOS?
While nearly every Subaru can gain from an Air Oil Separator, the benefits are most pronounced in turbocharged and direct-injection models. These engines naturally produce higher levels of crankcase pressure, pushing more oil vapor into your intake system. An AOS intercepts this vapor, preventing a host of performance-robbing issues down the line. Whether you’re hitting the track on weekends or navigating your daily commute, installing an AOS is one of the smartest moves you can make for your engine’s health and longevity. It’s a foundational upgrade that ensures your engine breathes clean air, allowing it to perform at its best.
A Must-Have for WRX & STI (2002-Present)
If you drive a WRX or STI, an Air Oil Separator isn't just a nice-to-have accessory; it's an essential component for protecting your engine. These high-performance turbocharged models are engineered for power, which means they generate significant crankcase pressure, especially under spirited driving. This pressure forces oil vapor through the PCV system and directly into your intake, lowering your fuel's octane rating and leading to potential detonation. By installing a quality AOS, you ensure that only clean, dry air enters the combustion chamber. This simple step protects your engine during aggressive driving and is a critical part of any plan for WRX upgrades.
Protecting Other Turbocharged Models
The WRX and STI aren't the only Subarus that benefit from an AOS. Turbocharged models like the Forester XT and Legacy GT share the same fundamental engine design and are equally vulnerable to oil contamination. Even if you use your Forester primarily for daily driving or overlanding solutions, oil vapor is constantly circulating through the intake system. Over time, this leads to carbon buildup on intake valves, which reduces airflow and hurts performance and fuel economy. An AOS acts as a permanent guardian for your engine, ensuring it stays clean and runs efficiently for years to come, making it a wise investment for any turbo Subaru owner.
Compatibility for Crosstrek & Impreza
You don't need a turbo to see the advantages of an AOS. Newer naturally aspirated models, including the Crosstrek and Impreza, often feature direct-injection technology. While great for efficiency, direct injection is known for causing carbon buildup on intake valves because fuel is no longer sprayed over them to keep them clean. An AOS helps solve this problem by capturing the oil vapor before it has a chance to bake onto the valves. For owners planning long-term reliability and consistent performance, adding an AOS is a proactive step toward maintaining engine health and avoiding costly carbon cleaning services. It’s a smart way to support your Crosstrek upgrades.
Performance Gains for Your Specific Model
Regardless of your specific Subaru model, an AOS delivers universal benefits by creating a healthier environment for your engine. By preventing oil vapor from contaminating the air-fuel mixture, it helps maintain your fuel's effective octane rating, which is crucial for preventing detonation and allowing your ECU to operate at peak efficiency. This clean operation is the bedrock of all other modifications. Whether you're adding a new exhaust or getting a custom tune, a quality AOS ensures your engine is in the best possible condition to handle more power. It’s a foundational part that supports every aspect of your vehicle’s performance and reliability.
How to Choose the Right Air Oil Separator
Not all air oil separators are created equal. When you’re ready to invest in one for your Subaru, you’ll find plenty of options on the market. Making the right choice comes down to understanding a few key design elements that separate a high-quality AOS from a basic catch can. A well-designed unit will not only perform better but also last longer, giving you peace of mind whether you're on your daily commute or pushing your car on the track. Look for a model built specifically for your engine that considers factors like mounting location, chamber design, and overall durability.
Prioritize Key Features Like Dual Chambers
One of the most critical features to look for in an AOS is a dual-chamber design. This isn't just a fancy add-on; it's essential for efficient oil separation. The first chamber works to slow down the pressurized crankcase gases, allowing the oil vapor to condense into larger droplets. The second chamber then refines this process, ensuring only clean air is sent back into your intake. This multi-stage approach is far more effective at removing contaminants than a simple, single-chamber catch can. A system with this design ensures your engine runs smoothly by keeping oil out of the combustion process where it doesn't belong.
Evaluate Construction and Durability
The materials and build quality of an AOS directly impact its performance and lifespan. Your Subaru’s engine bay is a harsh environment with constant temperature fluctuations and vibrations. You need a part that can handle it. The best performance parts are constructed from high-grade, durable materials designed to withstand these conditions without failing. For example, the Crawford Performance Air Oil Separator is engineered specifically for Subaru engines, making it a reliable choice for both daily driving and high-performance racing. Always choose a separator that is built to match the demands of your driving style and your vehicle.
Understand Proper Mounting and Drainage
Proper installation is just as important as the quality of the AOS itself. For the system to function correctly, it needs to be mounted high in the engine bay. This placement allows gravity to do its job, letting the separated oil drain back into the engine crankcase. If an AOS is mounted too low or without a clear drainage path, oil can accumulate inside the unit, defeating its purpose and potentially causing other issues. A well-designed kit will include the correct mounting hardware and clear instructions to ensure you get the placement right, allowing the self-draining system to work flawlessly.
Compare Brands and Customer Feedback
Before you make a final decision, take some time to research different brands and see what other Subaru owners are saying. Customer feedback provides real-world insight into a product's effectiveness, ease of installation, and long-term reliability. Look for reviews from people who own the same Subaru model as you, whether it's a WRX, STI, or Crosstrek. For instance, high customer satisfaction, often reflected in 5-star ratings, is a strong indicator of a quality product. This feedback can help you confidently choose an AOS that delivers on its promises and is trusted by the community.
Buying and Installing Your AOS: What to Expect
Once you've decided an Air Oil Separator is the right move for your Subaru, the next step is choosing a quality unit and getting it installed. This process is straightforward, but a few key considerations will ensure you get the best possible results. From understanding the advantages of a proven design to knowing the long-term upkeep, let's walk through what to expect. Making an informed choice here will protect your engine and give you peace of mind whether you're on the street, track, or trail.
The Crawford Performance AOS Advantage
When you choose a Crawford Performance Air Oil Separator, you’re getting a part with a serious legacy. As the original and proven solution for Subaru engines, our AOS is engineered to solve the core problem of oil blow-by effectively. It works by separating oil vapor from the crankcase gases and returning the liquid oil to the engine, keeping your intake, intercooler, and valves clean. This simple function is critical for preventing detonation and maintaining consistent power, especially in a modified or hard-driven engine. Our design ensures your engine is protected, allowing you to get the most out of your Subaru WRX upgrades or other performance modifications without compromise.
Installation: Should You DIY or Hire a Pro?
Proper installation is key to making your AOS work correctly. The unit needs to be mounted high in the engine bay, allowing gravity to do its job and drain the separated oil back into the crankcase. The hose routing is also crucial for creating an effective system. If you're comfortable working on your car and have the right tools, you can certainly handle the installation yourself. However, if you're unsure about any step, having a professional do the work is a great choice. A pro will ensure everything is mounted securely and routed perfectly, guaranteeing optimal performance. Our team offers expert engine tuning services and can provide the precision installation your vehicle deserves.
Cold Weather Performance and Upkeep
If you live in a colder climate, there's one maintenance point to keep in mind with any AOS. In freezing temperatures, condensation within the lines can turn to ice and create blockages. This can prevent the system from working correctly and, in rare cases, build up crankcase pressure. To avoid this, it's a good practice to periodically check and clean the PCV and drain lines, especially during the winter months. This is a simple preventative step that ensures your AOS continues to protect your engine year-round, no matter the weather. Think of it as routine upkeep, just like any other part of maintaining a high-performance vehicle.
Factoring in Price, Warranty, and Reliability
An Air Oil Separator is an investment in your engine's health, and it's important to choose a product that offers real value. While prices can vary, a well-engineered AOS from a reputable brand is worth it for the reliability it provides. At Crawford Performance, we stand behind the quality of our components, which are designed and tested to withstand the demands of both daily driving and track use. This commitment to quality is reflected in everything we build, from our AOS kits to our fully warrantied engine builds. When you invest in our performance parts, you're not just buying a component; you're buying confidence and a proven solution for your Subaru.
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Frequently Asked Questions
Is an AOS necessary if my Subaru is completely stock? Yes, absolutely. Every engine, stock or modified, produces oil blow-by. An Air Oil Separator is a preventative part that protects your engine from the long-term effects of this process, like carbon buildup and reduced fuel octane. For modern direct-injection engines, like those in the 2015+ WRX, it's especially important for preventing performance loss, even if you never plan on adding other modifications.
What's the real difference between an AOS and a catch can? The main difference comes down to maintenance and efficiency. A catch can is a simple container that collects oil vapor and requires you to manually drain it every few hundred miles. If you forget, it can overflow. An AOS, on the other hand, is a complete system that separates the oil and automatically drains it back into the engine. It’s a true set-it-and-forget-it solution that provides constant protection without adding another task to your to-do list.
Do I need to get my car tuned after installing an AOS? No, a tune is not required after installing an Air Oil Separator. The AOS is designed to improve the quality of the air entering your engine, but it doesn't change the air-to-fuel ratio or other parameters that would require an ECU retune. It simply helps your engine run cleaner and more efficiently with its current setup, making it a great foundational part that supports your existing tune or any future modifications.
Will installing an Air Oil Separator void my Subaru's warranty? Legally, a manufacturer cannot void your warranty simply because you've installed an aftermarket part. They must prove that the specific part you installed caused a specific failure. Since a quality AOS is designed to protect your engine from harmful oil contamination and detonation, it actually promotes engine health and longevity. It's a modification that works to preserve your engine, not harm it.
What happens in cold weather? Do I need to do anything special? In freezing temperatures, condensation inside the AOS lines can potentially turn to ice and cause a blockage. While a well-designed system minimizes this risk, it's a good practice to periodically check the lines during the winter months to make sure they are clear. This simple check ensures the system can breathe properly and continue protecting your engine, no matter how cold it gets outside.