When you start modifying your WRX or STI, it’s easy to get caught up in the chase for more power. A bigger turbo, a new exhaust, and a custom tune are exciting upgrades. But before you add a single horsepower, you should focus on reliability. The single most important foundational mod for any tuned Subaru is an Air Oil Separator. It addresses a core weakness in the boxer engine's design by preventing oil vapor from contaminating your intake system. This protects against engine knock and carbon buildup. Choosing the right WRX STI air oil separator kit is the first step in building a powerful and durable car. This guide will help you understand why it's so critical.
Key Takeaways
- An AOS is a critical engine safeguard: It actively intercepts oily blow-by to prevent engine knock, maintain consistent power, and stop performance-robbing carbon buildup on your intake valves.
- Invest in a true closed-loop system: Unlike a catch can that needs manual draining, a quality AOS is a maintenance-free solution that automatically returns oil to the engine, making it a superior choice for long-term reliability.
- Proper installation and routine checks are vital: A successful installation requires careful attention to detail to avoid common mistakes like pinched lines. Afterward, make inspecting the AOS hoses and connections part of your regular oil change routine to ensure it's always working correctly.
What is an Air Oil Separator (and Why Does Your WRX STI Need One)?
If you’re serious about the health and performance of your Subaru, an Air Oil Separator (AOS) is one of the most important upgrades you can make. Think of it as a guardian for your engine. During normal operation, especially in turbocharged boxer engines, pressure in the crankcase forces oil vapor and unburnt fuel—a mixture called "blow-by"—past the piston rings. Your car's stock PCV (Positive Crankcase Ventilation) system routes this messy vapor back into your intake to be re-burned.
The problem? This oily mist coats everything it touches. It gums up your intake manifold, intake valves, and intercooler, which reduces cooling efficiency and restricts airflow. More critically, oil vapor lowers the effective octane of your air/fuel mixture, increasing the risk of detonation or "knock," which can be catastrophic for your engine. An AOS intercepts this blow-by, separating the oil from the air before it can cause any harm. It’s a non-negotiable mod for any owner looking to protect their investment, whether you’re hitting the track or just want peace of mind on your daily drive. For any modified Subaru WRX, an AOS is essential for long-term reliability.
How an AOS Works
The function of an Air Oil Separator is elegantly simple. It’s plumbed into your engine's crankcase ventilation lines, acting as a filter between the crankcase and the air intake. As the hot, oily blow-by gases enter the AOS unit, they are forced through a series of internal baffles or a centrifugal chamber. This process causes the heavier oil and fuel droplets to cool, condense, and separate from the much lighter air. The clean air is then safely routed back into the intake manifold to be burned in the combustion cycle, while the collected oil is drained back down into the engine’s oil pan. This creates a closed-loop system that keeps your engine’s oil supply where it belongs and ensures only clean air reaches the cylinders.
Key Benefits of Installing an AOS
Installing an AOS is one of the smartest preventative measures you can take for your WRX STI. The primary benefit is the protection it offers against detonation. By preventing oil from contaminating your intake charge, you maintain your fuel's octane rating, allowing your engine to run safely at its peak potential. This also keeps your intercooler and intake tract clean, ensuring consistent and efficient performance without power-robbing carbon buildup. Over time, this contributes to a healthier engine that requires less internal cleaning and maintenance. An AOS also helps reduce oil consumption, as it returns oil that would otherwise be burned off. It’s a foundational part of any reliable build, complementing other engine tuning services by creating a more stable environment for performance.
AOS vs. Catch Can: What's the Difference?
While they serve a similar purpose, an Air Oil Separator and a catch can are not the same. A catch can is a simpler, passive device. It intercepts blow-by and uses baffles to collect oil, which then pools in a reservoir at the bottom of the can. The key difference is that this collected oil just sits there until you manually drain it. If you forget, the can can overfill, sending all that collected gunk straight into your engine—the very problem you were trying to prevent. An AOS, on the other hand, is an active, closed-loop system. It not only separates the oil but automatically drains it back into the engine. This makes it a true "set it and forget it" solution that requires no regular maintenance. For any serious Subaru STI build, the reliability and maintenance-free design of an AOS make it the superior choice.
Comparing the Best AOS Kits for the WRX STI
With so many options on the market, picking the right Air Oil Separator for your WRX STI can feel like a tough decision. Each brand brings a unique design philosophy and set of features to the table, catering to different build priorities and budgets. Some focus on intricate multi-chamber systems for maximum separation, while others prioritize simple installation and compatibility with factory components. To help you make an informed choice, we’re breaking down four of the most respected AOS kits available for the Subaru WRX STI. We’ll look at what makes each one stand out, from the materials they use to their core separation technology. This comparison will give you a clear picture of which kit aligns best with your performance goals and your specific vehicle, whether it’s a daily driver or a dedicated track car.
Crawford Performance
Crawford Performance is a name that’s practically synonymous with Subaru engine protection, and their AOS kits are a testament to that legacy. The Crawford Performance Air Oil Separator is engineered to solve the notorious oiling system issues in Subaru boxer engines. Constructed from aerospace-grade aluminum with CNC-machined components, this kit is built for extreme durability. Its signature feature is a dual-chamber, swirl-pot design that uses centrifugal force to efficiently separate oil from crankcase vapors. This process ensures only clean air re-enters your intake, which is critical for maintaining engine health and consistent power, especially under high-G cornering or hard acceleration. It’s a professional-grade solution trusted in motorsport for its reliability.
GrimmSpeed
GrimmSpeed takes a thoughtful approach to engine health with its Air/Oil Separator, designed to integrate seamlessly with your Subaru’s existing systems. A key feature of the GrimmSpeed Air/Oil Separator Kit is its ability to retain the factory PCV valve, which is essential for maintaining proper crankcase pressure and engine function as intended by Subaru engineers. This design choice makes it a solid option for owners who want effective oil separation without altering the stock PCV system. The kit is engineered to maximize efficiency and minimize oil vapor, offering a reliable way to protect your intake and prevent performance-robbing buildup. It’s a well-rounded choice for both daily driven and modified STIs.
IAG Performance
IAG Performance is a major player in the Subaru aftermarket, and their Air Oil Separators are among the most popular choices for WRX STI owners. Known for their robust construction and efficient performance, IAG Performance Air Oil Separators are designed to effectively manage oil blow-by and prevent it from contaminating the intake system. This helps reduce the risk of engine knock and carbon buildup on your valves. IAG offers several versions of their AOS, including their Street Series and Competition Series, allowing you to choose a kit that perfectly matches your car’s power level and usage. Their reputation for quality and proven results makes them a go-to for many performance enthusiasts.
Perrin Performance
Perrin Performance offers a well-engineered Air Oil Separator designed to improve engine reliability and performance. The Perrin Performance Air Oil Separator works by collecting oil vapors from the crankcase and routing them through a series of baffles that cause the oil to condense and drain back into the engine. This process ensures that your intake tract, turbo, and intercooler remain free of oil residue, which helps maintain consistent power and engine efficiency. Perrin’s commitment to quality materials and smart design makes their AOS a dependable choice for any WRX STI owner looking to protect their investment and ensure their engine runs cleanly for years to come.
How to Choose the Right AOS Kit
Once you’ve decided an Air Oil Separator is the right move for your WRX or STI, the next step is picking the right one. With several excellent options on the market, the choice can feel overwhelming. But breaking it down into a few key factors makes it much easier to find the perfect fit for your build, budget, and mechanical skill level.
The best AOS kit for you isn't necessarily the most expensive or the one with the most features. It's the one that delivers reliable performance, fits your long-term goals, and gives you confidence in your engine's health. Think about what matters most to you—is it bulletproof construction, a simple installation process, or a maintenance-free design? By evaluating each kit against these criteria, you can make an informed decision that protects your investment and supports your performance goals for years to come. Let's look at the four main things to consider.
Build Quality and Materials
The engine bay of a tuned Subaru is a harsh environment, so your AOS needs to be tough. Look for kits constructed from high-grade materials like billet or aerospace-grade aluminum. These materials are strong and dissipate heat effectively, ensuring the separator can withstand high temperatures and engine vibrations without failing. Pay attention to the details, like CNC-machined components and quality fittings, which indicate precision engineering. A well-built unit, like the Crawford Performance Dual Chamber AOS, is designed for durability, ensuring it will protect your engine reliably whether you're on the track or a daily commute.
Ease of Installation
The complexity of installing an AOS kit can vary significantly from one brand to another. Some kits are designed for a straightforward, bolt-on installation that a confident DIYer can handle in an afternoon. Others may require more involved steps, like removing the intake manifold, which can add time and complexity. Before buying, review the installation instructions or watch a tutorial video if available. A kit with clear instructions, pre-cut hoses, and logical routing will make the process much smoother and reduce the risk of mistakes like pinched lines, ensuring your new performance parts function correctly from the start.
Price vs. Value
While it’s tempting to go for the cheapest option, price should be weighed against the overall value the kit provides. AOS kits typically range from around $300 to over $600, and that price difference often reflects the quality of materials, design complexity, and engineering invested. A less expensive kit might save you money upfront, but if it fails to separate oil effectively or develops leaks, it’s not providing real value. Think of a quality AOS as an investment in your engine's longevity. Spending a bit more on a proven, well-engineered system can save you from costly engine repairs down the road.
Long-Term Maintenance
One of the biggest advantages of a modern AOS over a traditional catch can is the lack of required maintenance. The best systems are designed to be completely maintenance-free. They drain the separated oil back into the crankcase, so you never have to worry about checking levels or emptying a container. This "set it and forget it" design is a huge plus for convenience and peace of mind. When choosing a kit, confirm that it operates as a closed-loop system. This ensures your engine is consistently protected without adding another task to your regular maintenance checklist, letting you focus on driving.
Installing Your WRX STI Air Oil Separator
Installing an Air Oil Separator is a popular and rewarding project for many Subaru enthusiasts. While the process can be done in a home garage with the right tools and a bit of patience, it requires careful attention to detail. Taking your time and following instructions closely will ensure your AOS functions correctly and protects your engine for years to come. This guide will walk you through the key phases of the installation, highlighting common challenges and how to avoid them.
Tools and Preparation
Before you start turning wrenches, gather all your tools and lay out the parts from your AOS kit. You’ll typically need a good socket set, wrenches, hose cutters, and pliers. Having the manufacturer’s instructions handy is essential, as installation steps can vary between kits. If your WRX STI has a top-mount intercooler (TMIC), pay close attention to hose routing. It’s easy to accidentally pinch coolant or vacuum lines under the intercooler or charge pipe, which can cause issues later. Double-check that every hose has a clear, unobstructed path before you start tightening everything down.
A Step-by-Step Guide
While every AOS kit is slightly different, the core installation process involves tapping into your engine's PCV system. This often requires removing parts like your intake manifold and engine harness bracket to access the necessary connection points. One of the most common hurdles is removing the factory one-time-use QC fitting on the PCV system; it can be stubborn, so be prepared to apply steady, careful pressure. Always refer to the specific guide included with your Subaru STI upgrades for detailed, model-specific instructions. Following the correct sequence is key to a smooth and successful installation.
Avoiding Common Installation Mistakes
Learning from the experiences of others can save you a major headache. One of the most frequent mistakes is forgetting to plug the radiator fans back in after moving them for access. Another common oversight is not properly burping the coolant system if your AOS kit involves coolant lines. This can lead to overheating, so take the time to do it right. Some enthusiasts also recommend swapping out any plastic PCV fittings included in a kit with a more durable metal alternative. This small upgrade can prevent future cracks or leaks, ensuring a more reliable setup.
Post-Installation Checks
Once everything is reassembled, the job isn’t quite done. Before starting the car, do a thorough visual inspection. Check that all clamps are tight, hoses are secure, and no tools have been left in the engine bay. After starting the engine, let it idle and check for any coolant or oil leaks. Listen for any unusual noises that might indicate a vacuum leak. For ultimate peace of mind and to ensure your engine is running optimally with its new hardware, consider scheduling a professional check-up or dyno tune. Proper engine tuning services can confirm everything is working together perfectly.
How to Maintain Your Air Oil Separator
Installing an Air Oil Separator is a fantastic step toward protecting your Subaru's engine, but it’s not a "set it and forget it" component. Just like any other high-performance part on your WRX or STI, your AOS needs regular attention to do its job effectively. Consistent maintenance ensures it continues to prevent oil contamination and protect your engine from harmful blow-by. A few simple checks can save you from major headaches down the road and keep your engine running at its best. Think of it as essential upkeep for a critical system—because that’s exactly what it is.
Your Maintenance Checklist
It’s tempting to think that if your engine is running smoothly, everything is fine. However, skipping AOS maintenance can lead to gradual issues that aren't immediately obvious. The best approach is to build a routine. During every oil change, or at least every 3,000 to 5,000 miles, run through a quick checklist. Start by visually inspecting all the hoses for cracks, brittleness, or loose connections. Ensure all clamps are tight and secure. Check the drain line to make sure it’s clear of any blockages or kinks that could prevent oil from returning to the crankcase. This simple, proactive check is the key to catching small problems before they become big ones.
What to Look For During Inspections
When you’re inspecting your AOS, details matter. Pay close attention to the hose routing, making sure nothing is pinched by the charge pipe or other engine components, as this can restrict flow and cause pressure issues. If you have a top-mount intercooler (TMIC), it’s a good idea to periodically check for any oil film inside the intercooler and at the throttle body. A light coating can be normal over time, but excessive oil might signal that your AOS isn't separating vapor effectively or that there's another issue with your PCV system. Catching this early helps maintain the efficiency of your intercooler and keeps your intake clean, ensuring your performance parts deliver the power you expect.
Troubleshooting Common Issues
If you notice a change in performance or suspect an issue with your AOS, start with the basics. Many problems can be traced back to simple installation mistakes. Forgetting to plug in a radiator fan or not properly burping the coolant system after an install can cause overheating issues that seem unrelated at first. Double-check that all connections are secure and that the system is sealed. Sometimes, what appears to be an AOS failure is actually a vacuum leak elsewhere. Understanding how your specific system works is crucial for effective diagnosis. If you’re running into persistent problems, getting a professional opinion from a tuner can help you pinpoint the exact cause and get your car back to running perfectly.
Optimizing for Peak Performance
Your AOS is designed to protect your engine under demanding conditions, and proper maintenance ensures it can always perform at that level. The Crawford Performance Air Oil Separator is engineered to be the ultimate solution for Subaru's oiling system issues, but even the best designs benefit from regular care. Keeping the unit and its lines clean and clear of sludge guarantees it can handle extreme heat and pressure without failure. This isn't just about preventing problems; it's about ensuring your engine consistently delivers the power and reliability you built it for. Regular maintenance is the key to unlocking the long-term value and protective benefits of your investment.
The Payoff: How an AOS Protects Your Engine
Installing an Air Oil Separator is more than just another modification; it's a foundational upgrade for the health and longevity of your Subaru's engine. By addressing the factory PCV (Positive Crankcase Ventilation) system's shortcomings, an AOS provides critical protection that pays dividends in performance, reliability, and peace of mind. Think of it as an insurance policy for the heart of your car, ensuring that every other performance part you install can function at its best in a clean, stable environment. Whether you're building a daily driver or a track-focused machine, the benefits of an AOS are immediate and long-lasting. It’s a crucial step in safeguarding your engine from the harmful oil vapors that the stock system allows into your intake, creating a more efficient and durable powertrain.
Extend Engine Life
Your engine's PCV system is designed to vent crankcase pressure, but in the process, it pulls oil vapor and other contaminants into your intake. Over time, these abrasive materials can accelerate wear on internal components. An AOS intercepts this vapor, separating the oil and returning it to the oil pan while allowing clean air to pass through. This simple but effective process reduces engine sludge and minimizes wear on pistons, rings, and bearings. By ensuring your engine is lubricated with clean oil and breathes clean air, you’re directly contributing to a longer, healthier lifespan for your entire powertrain and other performance parts.
Maintain Consistent Power
One of the biggest enemies of a turbocharged engine is detonation, or engine knock. When oil vapor contaminates the air-fuel mixture in the combustion chamber, it effectively lowers the fuel's octane rating. This increases the risk of pre-ignition, forcing the ECU to pull timing to protect the engine, which in turn reduces power output. An AOS keeps the intake charge pure and free of oil, maintaining your fuel's intended octane level. This allows for more consistent and reliable power delivery, ensuring your engine operates at its full potential and gets the most out of professional engine tuning services.
Prevent Harmful Carbon Buildup
Carbon buildup is a significant issue, especially for the direct-injection engines found in modern WRX models. Without an AOS, oil vapor bakes onto the back of the intake valves and inside the intake manifold. Since fuel is injected directly into the cylinder, there's no gasoline to wash the valves clean. These carbon deposits can restrict airflow, disrupt fuel atomization, and lead to a noticeable loss in performance and fuel efficiency. Installing an AOS is the most effective way to prevent this buildup, keeping your engine’s airways clear and ensuring it breathes efficiently. It's a must-have for any list of Subaru WRX upgrades.
A Smart Investment for Your Build
If you plan on modifying your Subaru, an AOS should be one of your first purchases. It’s a proactive measure that safeguards your entire investment. Every dollar you spend on a bigger turbo, upgraded fuel system, or custom tune is better protected when the engine is running in a clean, oil-free environment. An AOS prevents a host of potential problems that could otherwise lead to costly repairs down the road. It provides the stable foundation needed for reliable performance, making it an essential component for any serious build and one of the smartest Subaru STI upgrades you can make.
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Frequently Asked Questions
Do I really need an AOS if my WRX STI is completely stock? Yes, absolutely. The issues caused by oil blow-by are inherent to the boxer engine's design, not just a problem for modified cars. Installing an Air Oil Separator on a stock vehicle is one of the best preventative measures you can take. It protects your engine's long-term health from day one by keeping your intake tract and valves clean, which helps maintain factory performance and reliability for years to come.
What's the real difference between an AOS and a catch can? The biggest difference comes down to maintenance and reliability. A catch can is a simple reservoir that collects oil, but you have to remember to drain it manually. If you forget and it overfills, it can dump all that collected gunk right back into your engine. An AOS is an active, closed-loop system that not only separates the oil but automatically drains it back into the engine's oil pan, making it a truly maintenance-free solution.
Will installing an AOS add horsepower to my car? An AOS doesn't directly create more horsepower, but it is essential for maintaining the power you already have. By preventing oil from contaminating your fuel and air mixture, it stops the ECU from pulling timing to prevent engine knock. This results in more consistent, reliable power delivery, especially under hard driving. It’s a supporting modification that allows your engine to perform at its best, safely.
Can I install an AOS myself, or is it a job for a professional? If you are comfortable working on your car and have a decent set of tools, installing an AOS is a very manageable DIY project. The most important thing is to follow the manufacturer's instructions precisely and take your time to ensure every hose is routed correctly and every connection is secure. If you're not confident in your mechanical skills, having a professional technician do the installation is always a great way to ensure it's done right.
Will installing an aftermarket part like an AOS void my Subaru warranty? This is a common and valid concern. In the United States, the Magnuson-Moss Warranty Act states that a manufacturer cannot void your vehicle's warranty simply because you installed an aftermarket part. For a warranty claim to be denied, the dealer would have to prove that the specific aftermarket part you installed was the direct cause of the failure. That said, individual dealer policies can vary, so it's always wise to be aware of the law and your rights.