You've invested a lot in your WRX, carefully choosing every part to build your perfect car. But a hidden threat is working against your engine with every drive. Your turbocharged boxer engine naturally creates oil vapor that contaminates your intake system, robbing you of power and risking long-term damage. Finding the best air oil separator for your WRX is like hiring a bodyguard for your engine. It intercepts this harmful blow-by before it causes issues like carbon buildup or detonation. Think of it as an insurance policy for your turbo, 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 a Smart Mod 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) Actually 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.
Why Your Turbocharged Engine Needs an AOS
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.
Engine Risks Without an AOS
Running your WRX without an AOS means you're letting a steady mist of oil vapor circulate through your intake. This isn't just a minor inconvenience; it directly impacts your engine's health. As this oil enters the combustion chamber, it contaminates the air-fuel mixture and effectively lowers your fuel's octane rating. This significantly raises the risk of detonation, also known as engine knock, which can lead to catastrophic internal damage over time. For those of us who have carefully selected performance parts to build a reliable and powerful car, skipping an AOS essentially negates those efforts by compromising the quality of the air your engine depends on.
Preventing Throttle Body Malfunctions
One of the most direct and frustrating consequences of oil blow-by is a compromised throttle body. Over time, the oil vapor condenses and forms a sticky sludge on the throttle plate and its housing. This buildup can prevent the plate from moving freely, causing a range of issues from a lazy throttle response and rough idling to a persistent check engine light. In more severe cases, the throttle could get stuck open or closed, which is a serious safety concern. An AOS is the definitive solution, preventing this residue from ever forming. It ensures a critical component of your engine operates smoothly, protecting your investment in other WRX upgrades.
Warning Signs: When Your WRX Needs 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 Air Oil Separator 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.
Is an AOS Always Necessary?
Let's get straight to the point: for any turbocharged Subaru, an AOS is one of the smartest preventative measures you can take. While it might seem like an upgrade reserved for track builds, the reality is that even a completely stock, daily-driven WRX benefits from day one. Your engine produces harmful blow-by regardless of how you drive, and an AOS is the most effective line of defense against it. By preventing oil vapor from contaminating your intake, it helps maintain consistent power, efficiency, and long-term engine health. For anyone who has already invested in performance modifications or engine tuning services, an AOS is non-negotiable; it protects your investment by ensuring your engine operates on clean air, just as it was tuned to.
Comparing the Top Air Oil Separators for Your WRX
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.
A Closer Look: Crawford Performance V3 AOS
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.
Key Features and Endorsements
Known as 'The Original' and 'The Proven' solution, the Crawford Performance Subaru Air Oil Separators are engineered to do one job exceptionally well: protect your engine. The core function is to stop oil blow-by, preventing oil vapor from contaminating your intake and intercooler. This keeps critical components clean and helps maintain your engine's health and longevity. By ensuring only clean air enters the combustion chamber, our AOS helps preserve your fuel's octane rating and prevents the carbon buildup that can degrade performance over time. It’s a straightforward, effective design trusted by enthusiasts to safeguard their investment, whether on the street or the track.
Pricing and Model Availability
We offer a wide range of AOS kits tailored for nearly every modern Subaru platform, including specific applications for the WRX, STI, Crosstrek, Impreza, and Forester from 2002 to the present. With over 35 different Air Oil Separator products available, you can find a version designed for your exact model year and modification level. This extensive compatibility ensures a precise fit and proper function without guesswork. Pricing varies depending on the specific kit and application, but each is built to provide lasting protection for your engine. This variety makes it easy to find the right setup for your build, from a daily driver to a dedicated track car.
A Closer Look: 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.
Key Features and Pricing
The IAG Street Series AOS is recognized for its heated, multi-baffle design that uses engine coolant to warm the canister. This feature is engineered to prevent the condensation and milky sludge that can accumulate in colder climates, ensuring the collected oil remains fluid enough to drain back into the engine properly. It’s a complex system that many owners trust for its consistent performance in various conditions. In terms of cost, most IAG Air Oil Separators are priced at around $499.99. This places it in the premium category, reflecting its intricate design and the brand's strong reputation within the Subaru community.
Model Compatibility and Add-Ons
A significant reason for IAG's popularity is its broad compatibility across the Subaru lineup. You can find specific kits designed for nearly every WRX, STI, Forester XT, and Legacy GT model from 2002 to the present, making it an accessible option for a large segment of enthusiasts. While the system is designed for a clean installation, it does require tapping into coolant lines to function. Its widespread use means there is a large support network of community-created guides for those looking to install one of the most common performance parts on the market.
A Closer Look: 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.
A Closer Look: 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.
Price vs. Value: Which AOS Is the Best Investment?
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.
What to Look for in a WRX Air Oil Separator
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.
Materials and Build Quality: What to Check
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.
How Well Does It Actually Separate Oil?
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.
How Hard Is It to Install?
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.
What Maintenance Does an AOS Require?
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.
Is It Street Legal? Understanding 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.
Will an AOS Affect Your WRX's 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.
What to Know About Warranty and Customer 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.
Installing and Maintaining Your Air Oil Separator
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.
Getting Started: Tools and Prep for Installation
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.
Common Installation Mistakes and How to Avoid Them
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.
A Simple Maintenance Checklist for Your AOS
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.
AOS Maintenance in Cold Climates
If you live in a region where temperatures regularly drop below freezing, your approach to AOS maintenance needs a little extra attention. While an Air Oil Separator is designed to handle condensation, extreme cold can introduce a new variable: freezing. The mixture of oil and water vapor that the AOS separates can form a thick, milky sludge. In frigid weather, this sludge can freeze inside the lines, creating a blockage. This is a serious issue because a blocked AOS can cause crankcase pressure to build up, potentially leading to blown seals or gaskets. Understanding this risk is the first step to preventing it and ensuring your engine remains protected year-round.
Understanding Potential Clogging
The primary job of an AOS is to condense oil and water vapor out of your crankcase ventilation gases. In cold weather, this condensation process is much more pronounced. The resulting emulsified oil-water mixture, often called "mayo" by enthusiasts, is thicker than oil alone and has a much higher freezing point. If this sludge freezes within the drain or PCV lines, it effectively plugs the system. Instead of relieving crankcase pressure, the system now traps it. This can force oil past your engine's seals, leading to leaks or even catastrophic failure in a worst-case scenario. Regular checks are crucial to catch any buildup before it becomes a major problem.
How to Prevent Frozen AOS Lines
Preventing your AOS lines from freezing is all about proactive inspection and insulation. During the winter months, make it a habit to regularly check the PCV and drain lines for any signs of thick, milky buildup. Cleaning these lines as needed will stop a clog from forming. For an extra layer of defense, consider insulating any lines that are exposed to direct cold airflow, such as those routed near the front of the engine bay or behind the intercooler. You can use thermal wrap or insulated sleeving to help keep the lines warm enough to prevent freezing. This simple step ensures your investment in high-quality performance parts continues to protect your engine, no matter how low the temperature drops.
What to Check After You Install Your AOS
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.
What Are 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.
What's Your Budget for an AOS?
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.
How Comfortable Are You with DIY Projects?
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.
Choosing an AOS for 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.
Will It Fit? Confirming 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.