4 Best Air Oil Separators for WRX: Ranked & Reviewed
You've invested a lot in your WRX, from the car itself to every performance part you've carefully selected. But one of the most critical upgrades isn't about adding horsepower—it's about protecting it. Every turbocharged boxer engine produces oil vapor that contaminates the intake system, which can lead to reduced performance and long-term engine wear. An Air Oil Separator acts as your engine's bodyguard, preventing this harmful blow-by from causing issues like detonation and carbon buildup. Think of it as an insurance policy for your turbo, intercooler, and internal components. Finding the best air oil separator for WRX is a proactive step to ensure the reliability and longevity of your entire build, safeguarding your investment for years to come.
Key Takeaways
- An AOS is essential engine protection: This is a critical preventative upgrade for any WRX, not just track builds. By stopping oil vapor from contaminating your intake, it helps maintain consistent power, efficiency, and long-term engine health.
- Evaluate features, not just brand hype: Make an informed choice by comparing the core technical details. Prioritize high-quality materials, proven separation efficiency, and a maintenance-free design to get the most value from your investment.
- Choose the right AOS for your specific build: Your ideal system depends on your personal needs. Align your choice with your performance goals, budget, and mechanical skill level to ensure a perfect fit for you and your car.
Is an Air Oil Separator Worth It for Your WRX?
If you’re serious about the long-term health and performance of your Subaru, the short answer is yes. An Air Oil Separator (AOS) is one of the most effective preventative upgrades you can make, especially for a turbocharged boxer engine. It’s not just for track-day heroes or heavily modified builds; even a daily-driven WRX can benefit significantly. An AOS acts as a guardian for your engine, ensuring that only clean air circulates through your intake system, which is critical for maintaining power, efficiency, and reliability over time. Think of it as an investment that protects all your other performance parts and your engine itself from harmful contaminants.
How Does an Air Oil Separator (AOS) Work?
Your engine’s Positive Crankcase Ventilation (PCV) system is designed to relieve pressure from the crankcase, but it has a downside: it allows oil vapor and unburnt fuel—collectively known as "blow-by"—to get sucked into your intake manifold. An Air Oil Separator intercepts this vapor before it can reach your intake. The AOS rapidly cools the vapor, causing the oil and other contaminants to condense and separate from the air. The clean air is then sent back into the intake to be burned, while the collected oil is drained back into the oil pan. This creates a closed loop that protects your engine without requiring you to manually empty a can.
Key Benefits for Your Turbocharged Engine
The primary purpose of an AOS is to safeguard your engine. By preventing oil vapor from contaminating your intake system, you get several key benefits. First, it helps prevent detonation (or knock) by keeping oil out of the combustion chamber, which lowers your fuel's effective octane rating. Second, it reduces carbon buildup on your intake valves, which can restrict airflow and hurt performance over time. For any WRX owner, especially those who have invested in engine tuning services, an AOS ensures your engine runs as efficiently and powerfully as intended. It maintains the clean, cool air charge your turbo needs to perform at its best.
Signs You Need an AOS
How do you know if your WRX is suffering from excessive blow-by? One of the most common signs is noticeable oil consumption between changes, forcing you to top off your oil frequently. You might also see a puff of blueish smoke from your exhaust during a cold start or under hard acceleration. If you’ve ever taken apart your intake or intercooler piping and found a grimy film of oil inside, that’s another clear indicator. While a properly functioning PCV system is the first thing to check, these symptoms often point to the need for a more robust solution like an AOS to keep your engine clean and healthy.
Common AOS Myths, Debunked
One of the biggest myths is that an AOS is only necessary for high-horsepower race cars. The reality is that even stock WRX engines produce blow-by, and an AOS provides protection from day one. Another point of confusion is the difference between an AOS and a catch can. While a catch can also collects oil vapor, it must be manually emptied. A true Air Oil Separator, like the systems we design, is a maintenance-free solution that automatically returns the collected oil to the engine. This ensures your engine is always protected without adding another task to your maintenance checklist, making it one of the most practical Subaru WRX upgrades you can install.
The Best WRX Air Oil Separators, Compared
Choosing the right Air Oil Separator for your WRX can feel overwhelming, with several strong contenders on the market. Each brand brings a unique design philosophy and reputation to the table. Your final decision will likely come down to your specific performance goals, driving conditions, and budget. To help you sort through the options, we’ve compared four of the most talked-about AOS systems available for Subaru WRX models from 2002 to the present. We'll look at what the community is saying and what sets each one apart, giving you the information you need to protect your engine.
Crawford Performance V3 AOS System
Our V3 AOS system is a well-known name in the Subaru community. While some online discussions have questioned its design, particularly concerning the valve cover breathers, our system is engineered with a unique swirl-pot technology inspired by motorsports. This design uses centrifugal force to separate oil from air, aiming to provide a maintenance-free solution without a coolant loop, which simplifies installation. It’s built to be a durable, set-it-and-forget-it component among your other performance parts. By focusing on a simple and effective mechanical separation process, we designed it to protect your engine from oil blow-by under demanding driving conditions.
IAG Performance Street Series AOS
IAG Performance has built a strong reputation in the AOS market, becoming one of the most popular choices for WRX owners. You’ll frequently see their name recommended in forums and social media groups. This popularity comes from a track record of reliability and effective performance. Many enthusiasts consider the IAG Street Series a go-to solution for preventing oil from contaminating the intake system. Its widespread adoption means there is a wealth of community knowledge and installation guides available, making it a trusted option for both new and experienced Subaru modifiers.
Perrin Performance AOS
Perrin Performance is another key player offering a quality AOS for the WRX. While perhaps not as dominant in marketing as IAG, Perrin is recognized for producing effective and well-made components. Owners who choose Perrin often praise its solid construction and reliable function. It’s considered a competitive alternative that delivers good value for its price point. If you're looking for a dependable system from a long-standing brand in the Subaru aftermarket scene, Perrin is definitely a brand to keep on your shortlist.
Killer B Motorsport AOS
Killer B Motorsport is often praised for its thoughtful engineering and innovative designs. Their AOS is particularly noted for its effectiveness, and some users suggest it’s an especially trustworthy choice for those living in warmer climates where freezing temperatures aren't a concern. The brand has earned a loyal following by focusing on meticulous design details that cater to the specific needs of performance-tuned engines. For drivers who appreciate clever engineering and want a solution tailored for high-performance use, the Killer B system is a compelling option.
Comparing Price and Long-Term Value
When you're looking at the price tag, it's important to see it as an investment in your engine's longevity. Many top-tier Air Oil Separators, like IAG's popular models, are priced around the $500 mark. While this might seem like a significant upfront cost, it's a protective measure that can save you from much larger repair bills down the road. A quality AOS protects your turbo, intercooler, and valves from harmful oil deposits, ensuring all your WRX upgrades continue to perform as they should. The long-term value comes from maintaining engine efficiency, preventing detonation, and preserving the health of your entire system.
Key Features to Compare
Choosing the right Air Oil Separator for your WRX isn't just about picking a brand; it's about matching a component to your specific needs and performance goals. Not all AOS units are built the same, and the differences in design, materials, and functionality can have a real impact on your engine's health and output. To make an informed decision, you need to look past the marketing and compare the core features that truly matter. From the quality of the materials to the complexity of the installation, each aspect plays a role in the long-term value you'll get from your investment.
Think of it like choosing any other critical engine component. You wouldn't pick a turbo without understanding its specs, and the same principle applies here. We'll break down the seven key features to evaluate when comparing AOS systems. This will help you see how different models stack up and decide which one is the perfect fit for your build, whether you're aiming for a reliable daily driver or a track-ready machine.
Build Quality and Materials
The durability of an AOS starts with its construction. Look for units machined from high-grade materials like billet aluminum, which can withstand the intense heat and pressure within the engine bay. A well-built AOS won't just perform better; it will last longer, preventing leaks or failures down the road. The Crawford Performance Air Oil Separator is known as the original solution to Subaru's oiling issues, a reputation built on years of reliable engineering and robust design. High-quality hoses and fittings are just as important, ensuring a secure, leak-free connection that maintains the system's integrity under demanding driving conditions.
Separation Efficiency
The primary job of an AOS is to effectively separate oil and water vapors from crankcase gases before they re-enter your intake. Higher efficiency means a cleaner air-fuel mixture, which is critical for preventing detonation in a turbocharged engine. Top-tier brands have perfected their internal baffling and cyclonic separation designs to maximize this process. An efficient system protects your pistons, valves, and intercooler from harmful oil buildup, ensuring your engine runs smoothly and maintains its power output over time. This is the single most important function, so it’s worth investing in a unit with a proven track record.
Installation Difficulty
Let's be practical: some AOS kits are more straightforward to install than others. The complexity can range from a simple bolt-on process to a more involved job requiring you to navigate tight spaces and reroute multiple lines. Some kits, like Crawford's, are designed for a precise, snug fit that can be challenging but ultimately secure. Before buying, consider your own mechanical skill level. If you're not comfortable working on your engine, factor in the cost of a professional installation. Checking for model-specific installation videos or guides can also give you a clear idea of what you're getting into.
Maintenance Needs
An AOS should simplify your life, not add another tedious task to your maintenance checklist. Some designs require you to periodically drain collected oil, while others are engineered to be completely maintenance-free. For instance, the Crawford Performance V2 Universal AOS is designed to automatically drain oil back into the crankcase, making it a true set-it-and-forget-it solution. This design ensures the system is always working at its best without you ever having to open the hood, offering peace of mind and consistent engine protection.
CARB Compliance
If you live in a state with strict emissions regulations, like California, CARB (California Air Resources Board) compliance is non-negotiable. An AOS with a CARB Executive Order (EO) number has been tested and approved, meaning it won't cause you to fail an emissions test. Not all high-performance parts carry this certification, so it's crucial to check before you buy. While it may not be a factor for everyone, for those in emissions-regulated areas, it’s one of the most important features to verify to keep your vehicle street-legal.
Impact on Performance
A high-quality AOS does more than just protect your engine; it helps maintain its performance. By preventing oil from contaminating the intake charge, it keeps your effective octane level from dropping. This reduces the risk of engine knock, allowing your ECU to apply more aggressive timing and deliver consistent power. The Crawford Air Oil Separator Baja Edition, for example, is engineered to improve engine performance while providing maximum protection. A clean system is an efficient system, and an AOS is a foundational part of that equation for any modified Subaru.
Warranty and Support
A manufacturer's warranty is a direct reflection of its confidence in the product. Look for a solid warranty that covers defects in materials and workmanship, as this protects your investment. Beyond the warranty, consider the quality of customer support. Reputable companies that specialize in Subaru performance parts will have knowledgeable staff ready to answer your questions about installation or troubleshooting. Strong post-purchase support ensures that if you run into any issues, you’ll have the expert help you need to get your car running right.
Your Guide to AOS Installation and Maintenance
Installing an Air Oil Separator is a fantastic weekend project that can significantly protect your WRX's engine. While it might seem intimidating, taking the time to do it right will pay off for years to come. This guide breaks down the process, from gathering your tools to checking your work, so you can feel confident tackling the installation and keeping your system in top shape. Think of it as a long-term investment in your Subaru’s health and performance. With the right preparation and a bit of patience, you’ll have your AOS installed and running smoothly, giving you peace of mind on the road and the track.
Tools and Preparation
Before you even pop the hood, the most important step is to read the installation manual for your specific AOS kit from start to finish. Every kit is slightly different, and knowing what’s ahead will save you a lot of headaches. Gather your standard tools, including a socket set, screwdrivers, and pliers. For a smoother installation, consider a set of multi-directional hose clamp pliers, which make reaching those awkward factory clamps much easier. Lay out all the performance parts from your kit to make sure everything is accounted for. Working in a well-lit space and giving yourself plenty of time will make the entire process less stressful and more rewarding.
Step-by-Step Installation Guide
While you should always follow the manufacturer's instructions, most AOS installations on a WRX follow a similar pattern. You’ll start by safely disconnecting the battery. Next, you'll likely need to remove the top-mount intercooler to access the PCV and crankcase ventilation hoses. Once you have clear access, you can mount the AOS canister in its designated spot in the engine bay. The most critical part is routing the new hoses correctly to ensure oil-laden air is sent through the separator before clean air returns to the intake. This prevents oil from contaminating your intake system and lowering your fuel's octane rating. After all hoses are connected and secured, you can reinstall the intercooler and other Subaru WRX upgrades before reconnecting the battery.
Avoiding Common Installation Mistakes
The most frequent issues during an AOS installation come down to hose routing and fitment. Some high-quality hoses can have a tight fit, which is great for preventing leaks but can be challenging to install. Take your time and be patient; never force a connection. A little silicone spray or even warm water can make hoses more pliable and easier to slide onto fittings. Double-check that none of the hoses are kinked or resting against hot components like the turbo or exhaust manifold. Finally, ensure every single clamp is tight. A loose clamp can lead to a vacuum leak, which will trigger a check engine light and hurt your engine’s performance.
Your Maintenance Checklist
One of the best things about a quality AOS is that it’s virtually maintenance-free. However, a quick check during your regular oil changes is always a good idea. Give the hoses a once-over to look for any signs of cracking, brittleness, or swelling. Confirm that all the connections are still secure and there are no signs of oil leaks around the fittings or the canister itself. It’s also smart to ensure your PCV system is working properly, as a faulty PCV valve can cause oil consumption issues that an AOS can’t fix. Keeping an eye on these simple things will ensure your AOS continues to protect your engine effectively, complementing professional engine tuning services.
Monitoring Performance After Installation
After you’ve installed your AOS, you’ll want to confirm it’s doing its job. The first step is to start the car and let it idle for a few minutes while you check for any fluid leaks or audible vacuum leaks (a hissing sound). Take the car for a gentle test drive to make sure everything feels normal. The real proof will come over time. The next time you remove your intercooler or check your throttle body, you should notice a significant reduction in the oily film that typically coats these components. This is the clearest sign that your AOS is successfully capturing oil vapor and keeping your intake tract clean, which is a major concern for many Subaru owners looking to maintain peak performance.
How to Choose the Right AOS for Your WRX
Picking the right Air Oil Separator for your WRX is about more than just grabbing the first one you see online. It’s a key investment in your engine's health and performance, so you want to make a choice that aligns with your specific needs. Think of it like choosing a new set of tires—what works for a daily commute won’t be the best fit for a track day. The ideal AOS for you depends on how you drive, your budget, and how comfortable you are working on your car.
To make the right call, you need to weigh a few key factors. We’ll walk through everything from your performance ambitions to your mechanical skill level. By considering each of these points, you can confidently select an AOS that will protect your engine and support your goals, whether you're building a reliable daily driver or a weekend track machine. Let’s get into what you should think about before you add one to your cart.
Define Your Performance Goals
First, what are you trying to achieve with your WRX? Your answer will guide you to the right AOS. If your car is primarily a daily driver, your main goal is likely long-term engine protection and reliability with minimal maintenance. However, if you’re building a car for autocross or track days, you’ll need an AOS designed for maximum efficiency under high-G forces and extreme engine conditions. A system that works perfectly for street driving might not keep up with the demands of a high-horsepower build. Aligning your choice with your specific performance needs ensures you get the protection you need without overspending on features you won’t use.
Consider Your Budget
Air Oil Separators are an investment, with quality units typically costing several hundred dollars. While it can be tempting to go for the cheapest option, it’s important to consider the long-term value. A well-engineered AOS from a reputable brand is an investment in your engine’s longevity. Cheaper alternatives might use lower-quality materials or have less effective separation, which could lead to issues down the road. Think about the total cost of ownership, including potential maintenance or replacement. Viewing the purchase as a crucial piece of engine insurance will help you choose a unit that provides lasting peace of mind and reliable performance for all your WRX upgrades.
Evaluate Your DIY Skill Level
Be honest about your comfort level with working on your car. Some AOS kits are relatively straightforward, designed for a simple weekend installation with basic tools. Others are more complex and may require routing hoses through tight spaces or modifying other components. Reading installation guides or watching videos for the specific kit you’re considering can give you a clear idea of what you’re getting into. If a particular installation seems too challenging, remember to factor in the cost of having a professional shop handle it. There’s no shame in leaving it to the experts, especially when it comes to protecting your engine.
Factor in Long-Term Reliability
An AOS is a set-it-and-forget-it part for many, so reliability is key. You want a unit that will do its job consistently without requiring constant attention. Look for brands with a proven track record in the Subaru community. Reading reviews and checking forums for long-term feedback can reveal how a particular AOS holds up over time. Some owners look for hard data and proof to compare brands, so pay attention to manufacturers who are transparent about their testing and results. A reliable AOS will give you confidence that your engine is protected, mile after mile.
Double-Check Vehicle Compatibility
This might seem obvious, but it’s a critical final check. The Subaru WRX has gone through several generations, and there are subtle but important differences between model years. An AOS designed for a 2015+ WRX will not fit a 2008-2014 model, and vice versa. Always confirm that the kit you’re buying is specifically made for your vehicle’s year and model. Reputable manufacturers will clearly list which Subaru models their products fit. If you’re ever in doubt, don’t hesitate to contact the seller or manufacturer directly to confirm compatibility before you make a purchase.
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Frequently Asked Questions
What's the real difference between an Air Oil Separator and a catch can? Think of it this way: a catch can is like a small trash bin for your engine's blow-by that you have to empty yourself. It collects the oil and fuel vapors but requires you to manually drain it periodically. An Air Oil Separator is a more advanced, self-sufficient system. It not only separates the contaminants but also automatically drains the collected oil back into the engine, making it a truly maintenance-free solution that continuously protects your engine without any extra work from you.
Will installing an AOS void my Subaru's warranty? This is a common concern, and the short answer is that it shouldn't. Legally, a manufacturer has to prove that an aftermarket part directly caused a specific failure to deny a warranty claim. Installing a high-quality AOS from a reputable brand is a protective measure for your engine. Most reasonable dealerships understand this and view it as a responsible upgrade. However, it's always a good practice to have a conversation with your service advisor if you have any concerns.
Is an AOS really necessary for a completely stock WRX? Absolutely. While heavily modified cars produce more blow-by, every engine, including a stock one, produces some. The boxer engine design is particularly susceptible to oil vapor making its way into the intake. Installing an AOS on a stock car is one of the best preventative measures you can take. It keeps your intake valves clean from the start and prevents the gradual performance loss that comes from carbon buildup, ensuring your engine stays healthy for the long haul.
Can I install an AOS myself, or should I go to a professional? If you're comfortable with basic hand tools and have some experience working on your car, installing an AOS is a very manageable weekend project. The key is to read the instructions thoroughly before you start and take your time. However, if you're new to working in the engine bay or the idea of removing your intercooler feels daunting, there's no shame in having a trusted shop handle the installation. The most important thing is that it's done correctly.
How does an AOS affect my car's performance and tuning? An AOS supports performance by ensuring your engine runs on a clean, pure air-fuel mixture. When oil vapor enters the combustion chamber, it effectively lowers your fuel's octane rating, which can lead to engine knock or detonation. This forces your car's computer to pull timing, which reduces power. By keeping oil out of the intake, an AOS helps maintain your fuel's octane and allows your tune to perform consistently and safely, delivering the power you expect.