WRX STI Air Oil Separator Kit: The Ultimate Guide
Dreaming of a bigger turbo or a new exhaust for your WRX or STI? It's easy to get caught up in the chase for more power, but the smartest builds start with reliability. Before you add a single horsepower, you need to address a core weakness in the boxer engine's design. An Air Oil Separator prevents harmful oil vapor from contaminating your intake, protecting your engine from knock and power-robbing carbon buildup. Choosing the right WRX STI air oil separator kit isn't just another upgrade; it's the essential first step for building a powerful and durable car.
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 Does an Air Oil Separator Actually Work?
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.
Top Benefits of an AOS for Your WRX STI
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.
The Best Air Oil Separator Kits for Your 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.
Spotlight: The Crawford Performance AOS Kit
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.
Racing Heritage and Street-Legal Design
What sets the Crawford AOS apart is its deep roots in professional motorsport. This isn't just a part designed in a lab; it's a product of years of experience building some of the most demanding Subaru engines in the world, including those used in Ken Block's legendary Gymkhana cars. That level of testing ensures it can handle anything you throw at it. But its real genius lies in how it balances extreme performance with daily usability. Unlike some aftermarket solutions, the Crawford AOS is engineered to work seamlessly with your car's original PCV system. This integration is crucial because it keeps your vehicle emissions-compliant and street-legal, giving you the reliability of race-proven performance parts without compromising the practicality of your daily driver.
Spotlight: The GrimmSpeed AOS Kit
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.
Spotlight: The IAG Performance AOS Kit
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.
Spotlight: The Perrin Performance AOS Kit
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.
Other Notable Brands to Consider
Beyond the well-known names we've already covered, several other brands have earned a strong reputation in the Subaru community for producing high-quality Air Oil Separators. These companies bring their own engineering philosophies to the table, offering unique designs and features that cater to specific performance needs. Whether you prioritize cutting-edge materials, innovative separation techniques, or a design proven in the harshest racing conditions, there's likely a perfect fit for your build. Exploring these additional options ensures you have a complete picture of the market, helping you make the most informed decision for protecting your engine and securing your performance investment.
COBB Tuning
COBB Tuning is a household name for many Subaru enthusiasts, and their Air Oil Separator lives up to the brand's reputation for quality. Designed to integrate directly into the crankcase ventilation system, the COBB AOS effectively stops oil vapors from reaching your intake. This is key to preventing carbon buildup and reducing the potential for engine knock, especially in modified vehicles. Owners often appreciate its straightforward installation process and proven reliability, making it a popular choice for those seeking a dependable solution from a trusted brand. It’s a solid upgrade that provides essential protection for both daily drivers and more aggressively tuned cars.
Radium Engineering
Radium Engineering is known for its meticulous approach to design and manufacturing, and their Air Oil Separator kits are a prime example of this. They are engineered for maximum oil separation efficiency, often featuring a compact design that fits well within a crowded engine bay. Radium’s AOS kits are praised for their high-quality construction and innovative features, which can include heated lines to prevent condensation and sludge buildup in colder climates. For owners who value precision engineering and want a top-tier solution for their engine’s health, Radium offers a compelling and highly effective option that stands up to rigorous use.
Verus Engineering
Verus Engineering applies a motorsport-driven design philosophy to its products, and its Air Oil Separator is no exception. The Verus AOS is engineered to deliver reliable performance by efficiently separating oil from blow-by gases, ensuring only clean air is recirculated into your intake. The company places a strong emphasis on data-backed design and uses high-quality materials to ensure durability under demanding conditions. This focus on technical excellence makes their AOS a dependable choice for performance-minded Subaru owners who want a product that is both well-engineered and thoroughly tested to protect their engine during spirited driving or track use.
Killer B Motorsport
Killer B Motorsport has built a formidable reputation in the Subaru world, particularly for its robust oiling system solutions. Their Air Oil Separator is designed to withstand the extreme demands of high-performance applications, making it a favorite among serious enthusiasts and racers. The Killer B AOS is engineered to handle significant amounts of blow-by, ensuring oil vapor doesn't compromise your intake system, even in highly modified engines. The brand’s commitment to durability and performance is evident in their construction and design, making their AOS a top-tier choice for anyone building a car for the track or pushing for maximum reliable power.
How to Choose the Right AOS Kit for Your WRX STI
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.
Confirming Fitment for Your Subaru Model
Before you add an AOS kit to your cart, the most critical step is to confirm it will fit your specific Subaru. While many kits are designed for the popular WRX and STI platforms, the market offers solutions for a much wider range of models and years. Manufacturers provide detailed fitment guides, and it’s essential to cross-reference your vehicle’s exact year and model against their compatibility list. A part designed for a 2015 STI might not fit a 2022 WRX due to subtle changes in engine bay layout or mounting points. Taking a few minutes to verify this information will save you from the headache of returns and installation issues down the road.
Compatibility Beyond the WRX and STI
While the WRX and STI get most of the attention in the performance world, you don’t have to own one to benefit from an AOS. Many leading kits are engineered to fit other turbocharged and direct-injection Subaru models that share similar engine architecture. This means owners of vehicles like the Forester XT, Legacy GT, and even the Crosstrek can find an AOS to protect their engine. This broad compatibility ensures that no matter which Subaru you drive, you can implement the same foundational engine protection. If you're building an off-road machine, finding the right overlanding solutions starts with a reliable engine, and an AOS is a key part of that equation.
Covering Model Years from 2002 to Present
Whether you’re driving a classic bug-eye WRX or a brand-new model, there’s likely an AOS kit designed for your vehicle. Manufacturers understand the longevity of these cars and typically offer fitments that span decades. For example, it’s common to find kits that cover WRX models from 2002 all the way to the present and STI models from 2004 through their final model year. This extensive coverage ensures that owners of nearly any generation can find high-quality performance parts to safeguard their engine. Always check the product description for the specific year range to guarantee a perfect match for your car’s generation and avoid any installation surprises.
Why Build Quality and Materials Matter
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.
Temperature Resistance for Peak Performance
Your engine bay experiences a massive swing in temperatures, from freezing cold starts to the intense heat generated during aggressive driving. An effective AOS must perform flawlessly across this entire range. Cheaper materials can become brittle in the cold or warp under extreme heat, leading to leaks or complete failure. This is why robust construction is so important. For example, the Crawford V3 Air Oil Separator is built from airplane-grade aluminum and is rated to operate in temperatures from -20°F to 250°F. This level of resilience ensures that the separator maintains its function under the toughest conditions, whether you're on a race track or a frozen mountain pass, consistently protecting your engine by keeping the oil supply clean.
Installation: How Hard Is It, Really?
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.
Typical Installation Time and Tools
So, how much time should you set aside for this job? Realistically, an AOS installation can take anywhere from three to ten hours. This wide range depends heavily on your mechanical experience and the specific kit you choose. Some designs are more straightforward, while others might require removing the intake manifold, which adds significant time. You'll need a solid set of basic hand tools, including wrenches, various sockets, and screwdrivers. Before you start, I highly recommend reviewing the installation instructions thoroughly and watching a few tutorial videos for your specific model. And trust me, having a friend to help makes the process much smoother—an extra set of hands is invaluable when you're routing hoses in tight spaces.
Price vs. Value: What Should You Expect to Pay?
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.
Typical Price Ranges and Shipping
When you're budgeting for an Air Oil Separator, you'll find that most quality kits fall somewhere between $315 and $610. The final price tag often depends on the brand's specific design, the materials used, and the complexity of the system. For instance, premium options like the Crawford Performance AOS and the IAG Performance Air Oil Separators typically start around the $500 mark, reflecting their robust engineering. The good news is that once you’ve made your choice, you won’t have to wait long to get started. Most suppliers ship these kits out within 24 to 72 hours, which is a huge plus when you’re eager to get your new parts installed and give your engine the protection it deserves.
Don't Forget 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.
Considering Long-Term Support and Upgrades
An Air Oil Separator is more than just a part; it's a long-term investment in your engine's health. When you choose a kit, you're also choosing the company that stands behind it. A reputable manufacturer won't just sell you a product and disappear. They'll offer ongoing support for the life of the part. This includes having a clear path for obtaining replacement components if something wears out and even providing opportunities to upgrade as their technology improves. Thinking about this long-term relationship from the start can save you headaches and money down the line, ensuring your investment continues to protect your WRX or STI for years.
Availability of Replacement Parts
Over time, components in your engine bay are exposed to extreme heat cycles and vibrations. Hoses can become brittle, and fittings can wear. A key indicator of a quality manufacturer is whether they offer individual replacement parts for their AOS kits. Being able to purchase a new hose or a specific seal is far more convenient and cost-effective than having to buy an entirely new system. Some brands, like IAG Performance, sell replacement parts for some of their older models. This kind of support shows a commitment to the customer and the longevity of their products, giving you confidence that your AOS can be easily maintained.
Manufacturer Upgrade Programs
Performance engineering is always advancing. As companies refine their designs and materials, they often release improved versions of their products. The best brands reward their loyal customers by offering a clear upgrade path. For instance, if you own an older Crawford Performance V2 AOS, you can trade it in for a significant discount on a new V3 kit. This kind of program demonstrates a company's confidence in its product evolution and its appreciation for its customer base. It allows you to benefit from the latest technology without starting from scratch, ensuring your Subaru STI is always protected by the best equipment available.
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.
Gather Your Tools: What You'll Need for the Install
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.
Common Installation Mistakes (And How to Avoid Them)
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.
Your Post-Installation Checklist
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 Go-To AOS 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.
Debunking the "Maintenance-Free" Myth
The term "maintenance-free" gets thrown around a lot with Air Oil Separators, but it's important to understand what that really means. It’s used to distinguish a true AOS from a traditional catch can, which requires you to manually drain collected oil. A high-quality, closed-loop system, like the Crawford Performance Air Oil Separator, automatically returns that oil to the crankcase, eliminating the need for regular draining. However, "maintenance-free" doesn't mean "inspection-free." The hoses, clamps, and fittings are still subjected to intense heat and vibration in the engine bay. A quick visual check during your oil changes is all it takes to ensure every connection is secure and the lines are free from wear, guaranteeing your engine stays protected.
Key Signs 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.
Tips for Peak AOS 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.
Protect Your Investment: A Longer 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.
Why an AOS Helps 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.
The Data Behind AOS Performance
The benefits of an Air Oil Separator aren't just theoretical; they translate into measurable improvements in engine stability and performance. The most significant data point is its ability to preserve your fuel's octane rating. When oil vapor from blow-by enters your intake, it mixes with the air and fuel, effectively lowering the octane of the mixture. This contamination makes the fuel more prone to premature detonation, or engine knock. Your ECU detects this knock and immediately pulls ignition timing to protect the engine, which directly results in a loss of power. By installing a quality AOS, you ensure only clean air enters the combustion chamber, allowing your engine to safely use the aggressive timing maps from your tune. This is why an AOS is considered one of the most vital performance parts for any modified Subaru—it allows your engine to perform consistently at its peak potential.
Stop Harmful Carbon Buildup in Its Tracks
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.
Why an AOS is a Must-Have 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.