Detonation is the single biggest threat to your WRX engine. This destructive knock happens when oil vapor from the stock PCV system contaminates your intake, lowering your fuel's effective octane and risking catastrophic failure. Pushing your car hard only makes the problem worse. The definitive solution is a purpose-built Air Oil Separator for your WRX. It intercepts these harmful vapors before they reach your engine, separating out the oil and sending only clean air into the combustion chamber. This isn't just an upgrade; it's essential protection for any modified or hard-driven WRX.
Key Takeaways
- Protect your engine from its biggest threat: An AOS prevents oil vapor from contaminating your intake, which is critical for maintaining your fuel's octane rating. This directly reduces the risk of detonation, a leading cause of engine failure in turbocharged Subarus.
- Opt for a maintenance-free solution: A true Air Oil Separator is a closed-loop system that automatically drains collected oil back into the engine. This "set-it-and-forget-it" design offers continuous protection, unlike a catch can that requires you to manually empty it.
- Build on a reliable foundation: Before adding parts that increase power, install an AOS to ensure your engine's long-term health. It’s a smart, preventative investment that supports all current and future modifications by keeping your engine running cleanly and efficiently.
Air Oil Separator 101: Why Your WRX Engine Needs One
If you’re serious about your WRX's performance and longevity, you’ve likely heard about an Air Oil Separator, or AOS. But what exactly does it do, and is it a necessary upgrade? In short, an AOS is a device designed to remove oil and water vapor from the crankcase gases that get recirculated back into your engine's intake. In a high-performance turbocharged engine, keeping that intake charge as clean and cool as possible is essential for making consistent power safely. An effective AOS ensures a cleaner air-fuel mixture, which is your first line of defense against performance-robbing issues.
How an AOS Guards Your Turbo Engine
Think of an AOS as a dedicated bodyguard for your engine. During normal operation, especially under pressure, some combustion gases (called "blow-by") slip past the piston rings and into the crankcase. These gases are contaminated with oil vapor. The stock system vents these gases back into your intake, but it also brings that oil along for the ride. This oil vapor can coat your intercooler, intake manifold, and valves, reducing efficiency. More critically, it lowers your fuel's effective octane rating. This increases the risk of engine knock, or detonation, which can cause catastrophic damage to your engine. An AOS intercepts these gases, separates the oil, and ensures only clean air re-enters the system, protecting your Subaru WRX upgrades and keeping the engine healthy.
Where the Factory PCV System Fails
Your WRX comes with a Positive Crankcase Ventilation (PCV) system from the factory, but it’s designed for stock power levels and daily driving. When you start adding power or driving your car harder on the street or track, the stock PCV system can quickly become overwhelmed. It simply can’t separate the oil vapor effectively enough under high crankcase pressure. This is where many owners consider a catch can, which collects the oil in a small container you have to empty manually. An AOS is a far more advanced and integrated solution. It actively separates the oil and automatically drains it back into the engine, making it a true set-it-and-forget-it part of your performance parts arsenal.
What an AOS Actually Does for Your WRX
Installing a quality Air Oil Separator is one of the smartest investments you can make for your engine's long-term health. The primary benefit is preventing detonation by keeping oil out of your combustion chamber, which helps maintain your fuel’s octane level. This allows for more consistent and reliable power delivery. It also keeps your intake system, including the intercooler and sensors, clean from oily residue, ensuring they function at peak efficiency. Unlike a basic catch can, a well-designed AOS is completely maintenance-free, as it returns the collected oil to the engine. Ultimately, an AOS provides peace of mind, protecting your engine from premature wear and potentially saving you from expensive repairs down the road.
How to Choose the Right Air Oil Separator for Your WRX
Picking the right Air Oil Separator for your WRX is about more than just grabbing the first one you see. This component plays a critical role in your engine's health and performance, so you want to make sure you’re investing in a quality part that’s right for your build. A little research upfront can save you from major headaches down the road. To make the best choice, focus on four key areas: the quality of the materials, vehicle-specific fitment, the maintenance design, and emissions compliance.
Why Materials and Build Quality Matter
An AOS is constantly exposed to heat, pressure, and oil vapors, so it needs to be built to last. Prioritize units made from durable materials like billet aluminum, which can handle the harsh conditions inside an engine bay without warping or cracking. The quality of the smaller components matters just as much. Look for kits that include high-grade, reinforced hoses and sturdy fittings. Cheaper rubber hoses can degrade quickly, leading to oil leaks and vacuum issues that compromise the whole system. Investing in well-constructed performance parts ensures your AOS will protect your engine reliably for years to come.
Will It Fit? Checking Compatibility
There is no one-size-fits-all Air Oil Separator. Engine bays and mounting points differ significantly between Subaru models and even across different generations of the WRX. It’s essential to choose an AOS kit designed specifically for your vehicle's year and model. A part made for a 2015 STI might not fit a 2022 WRX. Reputable manufacturers provide detailed fitment information, so double-check that your car is listed before you buy. Proper fitment guarantees that all the hoses are the correct length, the mounting brackets line up perfectly, and the unit functions as intended without interfering with other components.
Engine Compatibility: EJ, FA, and Beyond
Subaru’s iconic boxer engines have evolved, and the differences between the older EJ series and the modern FA series are significant, especially when it comes to the PCV system. An AOS designed for an EJ257 engine in a Subaru STI is engineered completely differently than one for an FA20 or FA24 engine in a newer WRX. The hose routing, mounting points, and even the internal design are tailored to handle the specific crankcase pressures and layout of each engine family. This isn't just a matter of bolt-on convenience; it's about functional compatibility. Choosing a kit specifically made for your vehicle's engine ensures it integrates seamlessly and performs its protective role effectively, whether you're modifying a track-ready STI or a daily-driven Forester XT.
Factoring in Design and Long-Term Maintenance
The best AOS is one you don’t have to think about after it’s installed. Some systems, particularly basic catch cans, require you to manually drain the collected oil periodically. If you forget, the can can overfill and stop working. Top-tier Air Oil Separators are designed to be completely maintenance-free. These systems automatically drain the separated oil back into the engine’s crankcase, creating a closed-loop system. This "set it and forget it" design ensures continuous protection for your engine without adding another task to your maintenance checklist. It’s a crucial feature for any daily driver or track car.
Is It Street Legal? A Note on CARB Compliance
If you live in California or another state with strict emissions standards, this step is non-negotiable. To keep your WRX street-legal, any aftermarket part that affects the emissions system must be CARB compliant. Look for an Air Oil Separator that has a CARB Executive Order (EO) number. This number proves the part has been tested and approved by the California Air Resources Board and won't cause your vehicle to fail an emissions test. Installing a non-compliant part can lead to fines and inspection issues, so always verify the CARB status when looking at Subaru WRX upgrades.
Our Top Air Oil Separator Picks for the WRX
When you start looking for an Air Oil Separator, you’ll find a few key players in the Subaru community. Each brand brings something different to the table, whether it’s a unique design, specific material choice, or a long-standing reputation. Choosing the right one depends on your build goals, budget, and even the climate you live in. While there are several solid options on the market, they aren’t all created equal. Understanding the differences will help you select the best AOS to protect your engine and support your performance goals.
Crawford Performance
We have to start with the original. The Crawford Performance Air Oil Separator is widely recognized as the benchmark for Subaru AOS technology. Its design is specifically engineered to handle the unique demands of the Subaru boxer engine, effectively preventing oil blow-by from contaminating your intake system. This keeps your engine running clean and maintains performance. What sets the Crawford AOS apart is its zero-maintenance design—there are no cans to empty or filters to clean. It’s a true set-it-and-forget-it solution that integrates seamlessly with your engine’s cooling system. For WRX owners looking for a proven, reliable, and maintenance-free system, this is the trusted choice.
The Gymkhana Legacy: Tested by Ken Block
The ultimate proof of a performance part's capability is seeing it perform under the most extreme conditions. The Crawford Performance AOS has a legacy that’s hard to beat, having been battle-tested in the world of professional motorsports and famously used in Ken Block’s legendary Gymkhana builds. This isn't just a part for daily driving; it's the same technology trusted to protect engines pushed to their absolute limits on closed courses. This heritage is why many consider it the original and definitive AOS for Subarus, setting the standard for engine protection. When you install one on your WRX, you're not just adding a part; you're adding a piece of race-proven engineering designed for ultimate reliability.
Technical Specs and Warranty
The Crawford AOS is engineered specifically for the demands of turbocharged Subaru engines. Its design is completely maintenance-free, creating a closed-loop system that automatically drains separated oil back into the crankcase—no cans to empty, ever. It also integrates with your car's cooling system, using engine heat to prevent condensation and oil from mixing into a thick, harmful sludge, which is a common issue in colder climates. This ensures the system functions perfectly year-round. At Crawford Performance, we stand behind our engineering with a comprehensive 12-month/12,000-mile warranty on our built engines, a testament to the quality and reliability we build into every component we design.
IAG Performance
IAG Performance is another well-known name that offers a range of Air Oil Separators for various Subaru models, including the latest WRX platforms. Their kits are designed to safeguard your engine by capturing oil and preventing it from entering the intake tract, which is crucial for maintaining long-term engine health. IAG offers different versions of their AOS, including their "Street Series" and "Competition Series," allowing you to choose a model that best fits your driving style and power output. They are a popular option among enthusiasts who are looking for comprehensive WRX upgrades and want a system tailored to their specific build.
Key Design Features
IAG's AOS kits are designed with a clear purpose: to safeguard your engine by capturing oil and preventing it from entering the intake tract. This is crucial for maintaining long-term engine health, especially in modified vehicles. They offer different models, like the "Street Series" and "Competition Series," which allows you to select a system that matches your car's power level and how you drive it. This flexibility makes them a popular choice for enthusiasts building out their cars with various performance parts. The core design focuses on actively separating oil from crankcase vapors to ensure a cleaner air-fuel mixture, which helps maintain consistent power and engine reliability.
Universal and Racing Applications
While some universal AOS kits exist for custom projects, racing applications demand precision-engineered solutions. The unique horizontal layout of the Subaru boxer engine creates specific challenges for oil control, especially under the high G-forces of a racetrack. This is why a model-specific system is so important. Crawford Performance is widely recognized as the benchmark for Subaru AOS technology because its design is specifically engineered to handle these demands. In a high-performance turbocharged engine, keeping the intake charge as clean and cool as possible is essential for making consistent power safely. For serious track builds and Subaru STI upgrades, a proven AOS isn't just an add-on; it's a foundational part of a reliable setup.
Perrin Performance
Perrin Performance is recognized for producing high-quality parts, and their Air Oil Separator is no exception. Known for its reliable construction and straightforward design, the Perrin AOS effectively manages oil vapor to keep your intake clean. It functions similarly to other top-tier systems by separating oil from crankcase gases and returning it to the engine. This makes it a solid choice for WRX owners who want to protect their engine with a dependable part from a reputable brand. If you’re building a car with various performance parts, Perrin’s AOS is often considered a reliable component to add to the mix.
Killer B Motorsport
Killer B Motorsport is praised for its innovative engineering, and their Air Oil Separator reflects that reputation. Their design is particularly effective at separating oil from air, ensuring your engine operates cleanly and efficiently. One of the standout features often noted by users is its performance in warmer climates, where its design helps manage oil vapors effectively. For WRX owners who prioritize thoughtful design and robust engineering, especially those who push their cars hard, the Killer B AOS is a popular and highly respected option. It’s a key component for anyone serious about proper engine tuning services.
Verus Engineering
Verus Engineering is known for its data-driven approach to design, and their Air Oil Separator for the WRX is a great example of this. Engineered to efficiently handle oil vapor, their AOS is built to prevent contamination of the intake system, which is essential for maintaining engine performance and reliability. The Verus Engineering AOS is recognized for its solid construction and effective design, ensuring only clean air is recirculated into the engine. This is especially important for modified vehicles where maintaining a pure air-fuel mixture is key to preventing detonation. For enthusiasts who appreciate meticulous engineering, Verus offers a well-crafted solution that stands up to the demands of high-performance driving and complements other performance parts.
GrimmSpeed
As a staple in the Subaru community, GrimmSpeed offers a highly regarded Air Oil Separator kit for the WRX. Their design focuses on effectively separating oil from crankcase gases, which helps preserve your fuel's octane rating and reduces the risk of engine knock. This makes it a vital upgrade for any performance-focused WRX owner. The GrimmSpeed AOS is often praised for its quality materials and user-friendly installation process, making it a popular choice for both DIY enthusiasts and professional installers. It’s a reliable option from a trusted brand that helps ensure your engine runs cleanly and efficiently, supporting both stock and modified setups looking for long-term protection.
AOS Installation and Maintenance: What to Expect
Once you’ve chosen the right Air Oil Separator, the next steps are installation and upkeep. While a quality AOS is designed for longevity, knowing what to expect will ensure you get the best performance and protection for your engine. From fitting the unit to performing seasonal checks, a little preparation goes a long way. Here’s a straightforward guide to installing and maintaining your AOS.
Installation Time and Process
So, how long does it take to install an AOS? For most WRX owners with basic hand tools and a bit of mechanical confidence, you can expect the job to take around two to three hours. The process is generally straightforward, as most high-quality systems are designed to be direct bolt-on upgrades. Reputable brands provide complete kits that include everything you need, from vehicle-specific hoses and clamps to detailed instructions, which simplifies the process significantly. You won't need any special engine tuning after the installation, making it a manageable weekend project that pays huge dividends in engine protection. Just follow the instructions carefully, and you'll have it installed without any major issues.
Handling Common Installation Hurdles
Installing an AOS can be a rewarding DIY project, but it’s wise to go in with a clear understanding of the process. Depending on your specific WRX model and the AOS kit, some steps can be more complex than others. Connecting certain hoses, for example, can be tricky in the tight confines of an engine bay. It’s important to follow the manufacturer’s instructions precisely. If you’re not comfortable with the installation, there’s no shame in having a professional mechanic who specializes in Subaru performance parts handle it for you. A correct installation is the foundation for a system that works flawlessly.
AOS Maintenance Tips for Every Climate
While many AOS units are virtually maintenance-free, your climate plays a role in long-term care. If you live in an area with freezing winter temperatures, you’ll want to pay closer attention. Cold weather can cause condensation to mix with oil vapor, creating a thick, milky sludge that can freeze and block the drain lines. To prevent this, make it a habit to check your AOS lines during the coldest months. Some owners also choose to insulate exposed lines as an extra precaution. Regularly cleaning the AOS, PCV, and drain lines ensures the system functions properly year-round, protecting your engine no matter the weather.
Dealing with Cold Weather Sludge (aka "The Milkshake")
If you live somewhere with harsh winters, you might encounter a creamy, yellowish gunk in your AOS lines, often called "the milkshake." This sludge is simply a mix of oil vapor and condensation that forms when temperatures drop. While it looks alarming, it’s a common occurrence in colder climates. The real problem arises when this mixture freezes, blocking the drain lines. A clogged AOS can't relieve crankcase pressure, which can lead to serious issues like blown seals or oil being forced out of the engine. To prevent this, make a habit of checking your AOS lines during the coldest months. Some owners even insulate the lines as an extra precaution, protecting their investment in high-quality performance parts.
Solving Common AOS Problems
A properly functioning AOS is a silent guardian for your engine, but knowing the signs of a potential issue can help you prevent major problems. If your AOS is failing or clogged, you might notice a few key symptoms. Keep an eye out for a sudden increase in oil consumption, blue or white smoke from your tailpipe, or a milky, sludgy residue under your oil cap. These are all indicators that oil is getting past the separator and into places it shouldn't be. Catching these signs early allows you to address the problem before it can lead to more significant engine trouble.
AOS vs. Catch Can: What's the Difference?
When researching blow-by solutions, you’ll see both Air Oil Separators and catch cans mentioned. While they serve a similar purpose, they operate differently. A catch can is a simpler device that "catches" oil and water vapor in a canister, which you must manually empty on a regular basis. An AOS is a more advanced, active system. It separates the oil vapor from the air and automatically drains the collected oil back into the engine's crankcase. This creates a closed-loop, maintenance-free system, making it a popular choice for Subaru WRX upgrades where convenience and consistent performance are key.
Is an Air Oil Separator Worth the Investment?
When you’re planning your build, every dollar counts. You want to know that the parts you’re adding deliver real, tangible benefits. So, is an Air Oil Separator worth the investment? For any Subaru owner who cares about long-term engine health and consistent performance, the answer is a clear yes. An AOS isn't just another bolt-on accessory; it's a foundational upgrade that protects your engine from the inside out.
Think of it as preventative maintenance that also enhances performance. By addressing the inherent weaknesses of the stock PCV system, an AOS prevents a host of problems down the road, from carbon buildup and reduced fuel economy to catastrophic detonation. It’s one of the smartest investments you can make, ensuring your WRX runs strong for years, whether you’re navigating daily traffic or pushing its limits on the track. It’s a critical piece of the puzzle for a reliable, high-performing Subaru.
Comparing Costs Across Brands
While price is always a consideration, it’s important to view an AOS as a long-term investment in your engine's health. The cost of a quality Air Oil Separator can vary, with prices for top-tier units generally falling between $250 and $500. For example, Crawford Performance AOS units typically range from $250 to $450, offering a proven, maintenance-free solution. Other popular brands, like IAG Performance, often price their models around the $500 mark. Instead of focusing solely on the price tag, consider what you’re getting for your money: superior materials, vehicle-specific engineering, and a design that provides continuous protection. A few hundred dollars for a reliable AOS is a small price to pay when compared to the potential cost of an engine rebuild down the line.
The Payoff: Long-Term Engine Protection
An Air Oil Separator acts as a dedicated bodyguard for your engine. Its primary job is to intercept the oil, fuel, and water vapors vented from the crankcase and separate them from the air before it’s recirculated into your intake. This process is crucial because those contaminants can wreak havoc on your engine. When oil vapor coats your intake valves and settles in the combustion chamber, it lowers the octane of your air/fuel mixture, which can lead to detonation—a major cause of engine failure in turbocharged Subarus. By ensuring only clean air re-enters the engine, an AOS helps your engine run better, last longer, and maintain its power over time. It’s one of the most effective performance parts for safeguarding your investment.
How an AOS Affects Performance
A healthy engine is a high-performing engine. The benefits of an AOS translate directly to how your WRX drives, whether it’s your daily commuter or a weekend track weapon. By keeping the intake system clean, an AOS ensures a more efficient combustion process. This means your engine can make power more consistently and safely, without the risk of power-robbing carbon buildup or detonation. For anyone modifying their Subaru, an AOS is a crucial supporting upgrade. It allows your other Subaru WRX upgrades, like a new tune or a bigger turbo, to perform optimally on a solid foundation of engine reliability. You get peace of mind knowing your engine is protected while you enjoy its full performance potential.
Long-Term Power Retention
Installing a quality Air Oil Separator is one of the smartest investments you can make for your engine's long-term health. The primary benefit is preventing detonation by keeping oil out of your combustion chamber, which helps maintain your fuel’s octane level. This isn't about adding a huge amount of horsepower; it's about preserving the power you already have, consistently and reliably. Over time, oil contamination can degrade performance, leading to a less responsive and less efficient engine. An AOS ensures that your engine is breathing clean air, allowing for more consistent power delivery every time you get behind the wheel. It’s a foundational part that supports every other modification you make.
Reducing Harmful Blow-By
During combustion, some gases inevitably "blow by" the piston rings and enter the crankcase. These gases are mixed with oil vapor, and the factory PCV system vents them back into your intake. An AOS intercepts these gases, separates the oil, and ensures only clean air re-enters the system. This process is vital for protecting your engine's internal components. Without it, that oily mist can coat your intercooler, throttle body, and intake valves, reducing their efficiency. By installing one of these essential performance parts, you’re actively preventing the buildup that robs your engine of its health and responsiveness.
Benefits for E85 Fuel Users
If you’re running E85 fuel to maximize performance, an Air Oil Separator becomes even more critical. Ethanol is effective because of its high octane rating and cooling properties, but its performance can be compromised by oil dilution. When oil vapor mixes with the intake charge, it can reduce the fuel's effectiveness and stability. An AOS prevents this oil contamination, ensuring your E85 fuel can do its job without interference. This leads to a cleaner, more efficient burn and more reliable power. For anyone serious about their setup, an AOS is a necessary supporting modification for any proper engine tuning services involving alternative fuels.
What About Your Warranty?
It’s a common concern: will adding an aftermarket part void your car’s warranty? The short answer is no, installing a quality AOS from a reputable manufacturer should not void your Subaru’s warranty. Thanks to the Magnuson-Moss Warranty Act, a dealer can’t deny a warranty claim just because you’ve installed an aftermarket part. They would have to prove that the specific part you installed was the direct cause of the failure. This is why choosing a well-engineered, vehicle-specific AOS is so important. A properly designed unit integrates seamlessly with your engine’s systems, making it highly unlikely to cause any issues. Stick with trusted brands, and you can protect your engine without worrying about your warranty.
Is an AOS Right for Your Build?
Ultimately, the right AOS comes down to your specific needs and how you use your WRX. Are you building a reliable daily driver, a competitive track car, or a rugged overlanding rig? Each application puts different demands on the engine. A daily driver benefits from the general engine protection an AOS provides, while a track car absolutely requires it to handle the high G-forces and extreme conditions that can overwhelm the stock PCV system. Before you buy, consider your driving style and future modification plans. Choosing an AOS that matches your build ensures you get the best protection and performance, forming a key part of your overall engine tuning services strategy.
Why Don't Subarus Come with an AOS from the Factory?
Subaru designs its vehicles for a broad range of drivers, and the factory-installed Positive Crankcase Ventilation (PCV) system is a perfect example of this. It's engineered to be a cost-effective and reliable solution for a stock vehicle under normal, everyday driving conditions. For the average commute, it does its job of managing crankcase gases and meeting emissions standards just fine. However, this system was never intended for the high-stress demands of performance driving. When you start adding power or pushing your WRX on the track, the stock PCV system is easily overwhelmed by the increased crankcase pressure, failing to effectively separate oil vapor from the air. This is why an AOS is considered a crucial upgrade for enthusiasts, not a standard factory part.
Is an AOS Necessary for a Stock WRX?
While a completely stock WRX used for daily driving might not "need" an Air Oil Separator to run, installing one is one of the smartest preventative measures you can take. The reality is that even at stock power levels, the turbocharged boxer engine produces significant blow-by. Over time, this leads to oil vapor contaminating your intake, lowering your fuel's octane, and increasing the risk of detonation. An AOS is a foundational upgrade that protects your engine from the inside out, ensuring its long-term health. If you have any plans for future performance parts, an AOS should be your very first investment to create a reliable platform for building power safely.
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Frequently Asked Questions
Is an Air Oil Separator the same thing as a catch can? While they both aim to solve the same problem, they work very differently. A catch can is a simple container that collects oil vapor, which you have to drain manually. An Air Oil Separator is a more advanced, active system that separates the oil and automatically drains it back into the engine. This makes an AOS a completely maintenance-free solution.
Do I really need an AOS if my WRX is completely stock? An AOS is most critical for modified cars, but even a stock WRX benefits from the long-term engine protection it provides. The factory PCV system can still allow oil vapor into your intake, especially if you drive your car hard. Think of an AOS as smart preventative care that keeps your engine clean and healthy from day one.
Does an Air Oil Separator add horsepower? An AOS does not directly add horsepower. Instead, it protects the power your engine already makes. By preventing oil from contaminating your air-fuel mixture, it reduces the risk of detonation and keeps your intake system clean. This ensures your engine runs efficiently and reliably, allowing you to safely use all of its available performance.
Will an AOS cause problems in freezing temperatures? This is a valid concern, as condensation in the system can freeze and cause blockages. However, well-designed AOS units, especially those that integrate with the engine's cooling system, are made to function in all climates. During the coldest months, it’s a good practice to occasionally check that the drain lines are clear, but a quality system is built to handle winter conditions without issue.
At what point in my build should I install an AOS? An AOS is one of the best foundational upgrades you can make, and it’s wise to install one early in your modification process. By adding it before or at the same time as a tune or other power-adding parts, you establish a baseline of engine protection. This ensures your engine remains healthy as you begin to increase its performance and put more stress on its components.