How to Buy the Right Subaru AOS Kit for You

Your Subaru is more than just a car; it's an investment in performance and reliability. To protect that investment, you need to address a hidden threat: oil contamination in your intake system. The factory PCV system allows oil vapor to coat your intercooler, intake manifold, and valves, which can lead to reduced power and, in worst-case scenarios, catastrophic engine failure. An Air Oil Separator acts as your engine's bodyguard, filtering out this harmful oil before it can cause damage. Think of it not as a performance modification, but as essential engine insurance. This article will explain why it's a critical upgrade and help you confidently buy a Subaru AOS kit for long-term peace of mind.

Key Takeaways

• An AOS is a Protective Mod, Not a Performance Part: Its primary job is to prevent oil vapor from contaminating your intake system. This is crucial for maintaining your fuel's octane rating, preventing engine knock, and ensuring the long-term health and reliability of your Subaru's engine.
• Match the Kit to Your Build and Climate: The best AOS for you depends on your specific needs. A stock daily driver has different requirements than a modified track car, and if you live in a cold climate, a heated kit is essential to prevent sludge buildup and blockages.
• Plan Your Installation Before You Buy: Decide whether you'll handle the installation yourself or hire a professional early on. Factoring in the cost of labor or the time and tools needed for a DIY project will give you a realistic budget and prevent surprises down the road.

What is a Subaru Air Oil Separator (AOS) and Why is it a Must-Have Mod?

If you’ve spent any time in the Subaru community, you’ve likely heard the term “AOS” thrown around. An Air Oil Separator, or AOS, is one of the most important supporting modifications you can make to your engine. Its job is simple but critical: it separates oil vapor from the air that your engine recirculates from the crankcase back into the intake. Subaru’s boxer engines are known for producing a significant amount of oil blow-by, especially under spirited driving conditions. An AOS intercepts this oily air, condenses the oil, and returns it to the engine, ensuring only clean air re-enters your intake system. This simple process is key to maintaining engine health, performance, and longevity.

How an AOS Protects Your Subaru Engine

The main purpose of an AOS is to protect your engine from the harmful effects of oil blow-by. Without one, oil vapor travels through the PCV system and coats the inside of your intercooler, intake manifold, and valves. This oily residue can lower the effective octane of your fuel, which increases the risk of detonation—a destructive force that can lead to catastrophic engine failure. By installing an AOS, you ensure that the air mixing with your fuel is as clean and cool as possible. This helps maintain your fuel's octane rating and prevents the buildup that robs your engine of efficiency. It’s a foundational part of a reliable build, making all your other performance parts work better.

Key Benefits: More Power, Longer Engine Life

A cleaner engine is a happier, more powerful engine. By preventing oil from contaminating the intake tract, an AOS helps you achieve more consistent and reliable power. When your engine isn’t fighting detonation, the ECU can maintain optimal ignition timing, giving you the full performance you expect. This is especially crucial for turbocharged models. Over the long term, the benefits are even greater. Reducing oil consumption and preventing carbon buildup on valves and pistons significantly extends the life of your engine. For anyone invested in their vehicle, whether it’s a daily driven WRX or a track-prepped STI, an AOS is a smart move for preserving your investment and enjoying reliable performance from your Subaru STI upgrades.

Telltale Signs Your Subaru Needs an AOS

How do you know if your Subaru is a candidate for an AOS? While any modified Subaru will benefit from one, there are a few clear signs to watch for. The most obvious is high oil consumption, forcing you to top off your oil between changes. You might also notice a puff of blueish smoke from the exhaust during hard acceleration or deceleration, which is a sign of oil burning in the combustion chamber. Even if you don’t see these symptoms, the harmful process is still happening internally. An AOS is preventative medicine for your engine, stopping problems like detonation and ringland failure before they start. If you’re planning a tune, installing an AOS first is a great way to ensure your engine tuning services deliver safe, reliable results.

Does Your Subaru Model Need an AOS Kit?

While an Air Oil Separator is a smart upgrade for nearly any Subaru, its importance varies depending on your specific model and how you drive. The unique design of the Subaru boxer engine, combined with the stress of turbocharging and direct injection, makes these cars particularly susceptible to oil blow-by. This happens when combustion pressure forces unburnt fuel and oil vapor past the piston rings and into the crankcase. From there, the factory PCV (Positive Crankcase Ventilation) system sends this oily mist directly back into your intake. This isn't just messy; it's detrimental to performance and long-term reliability. The oil vapor coats your intercooler, intake manifold, and valves, which lowers the effective octane of your fuel and increases the risk of engine knock. If you push your car on the track, enjoy spirited daily driving, or have invested in other performance parts, an AOS shifts from a "nice-to-have" to a foundational part of your build. It’s one of the best preventative measures you can take to protect your engine’s health for the long haul. Let's break down which models benefit the most.

Essential for WRX and STI Models

If you own a WRX or STI, the short answer is yes, you absolutely need an AOS. These cars are built for performance, and their turbocharged boxer engines are notorious for producing significant oil blow-by, especially under boost. When that oily vapor gets recirculated back into your intake, it coats your intercooler, intake manifold, and valves, effectively lowering the octane rating of your air/fuel mixture. This increases the risk of detonation, or engine knock, which can cause catastrophic engine failure. Installing an AOS is a critical step when planning upgrades for your WRX or STI, ensuring your engine breathes clean air and maintains its power and reliability.

Protecting Your Turbocharged Forester XT

Don’t let the family-friendly exterior fool you—if you have a turbocharged Forester XT, Outback XT, or Legacy GT, your engine faces the same risks as its sportier cousins. The turbocharger puts the engine under similar stress, creating the same oil blow-by that can harm performance and longevity. An AOS is crucial for protecting your investment, especially if you use your Subaru for towing or demanding overlanding adventures. By separating the oil from the intake air, you prevent performance degradation and reduce the carbon buildup that can lead to costly repairs down the road. It’s a simple modification that provides serious peace of mind for any turbocharged Subaru.

Benefits for NA and High-Mileage Subarus

What about naturally aspirated (NA) models like the Crosstrek or Impreza? While the need for an AOS isn't as urgent as with a turbo model, it’s still a highly beneficial upgrade, particularly for high-mileage vehicles. All engines experience some level of blow-by, and this effect worsens over time as piston rings and seals wear down. Adding an AOS to naturally aspirated models like the Crosstrek helps reduce oil consumption, keeps intake components cleaner, and ultimately contributes to a longer, healthier engine life. Think of it as a long-term investment in your Subaru’s reliability, helping you get the most out of your engine for years to come.

Comparing the Top Subaru AOS Kit Brands

Choosing an AOS kit often comes down to your specific build, driving style, and brand trust. While several excellent options exist, they each have unique strengths. Let's look at how the top brands stack up so you can find the perfect fit for your Subaru.

Crawford Performance: The Baja-Proven Choice

If you push your Subaru to the limit, especially off-road, the Crawford Performance AOS is engineered for you. Our Baja Edition Air Oil Separator Kits are specifically designed to keep oil out of the air intake system on naturally aspirated models like the Crosstrek, Forester, and Impreza. This focus on N/A engines is a key distinction, providing a purpose-built solution where others often concentrate on turbo models. Battle-tested in the grueling Baja 1000, these kits are built to withstand the harshest conditions, making them the ultimate choice for any overlanding build or daily driver that demands reliability. They are engineered to protect your engine when it matters most.

IAG Performance: A Popular Street Option

IAG Performance has built a strong reputation in the Subaru community, particularly among owners of turbocharged models. Their Air Oil Separators are a popular modification for the WRX, STI, Forester XT, and Legacy GT. IAG focuses on creating a comprehensive system designed to "safeguard your engine" from the harmful effects of oil blow-by, especially under high boost. Their kits are well-regarded for street performance and track day applications, offering a reliable solution for owners looking to protect their modified engines. With various configurations available, they provide a versatile option for many different build goals, from daily driving to more aggressive setups.

Mishimoto: Reliable and Widely Available

Mishimoto is a household name in the automotive aftermarket, known for producing a wide range of reliable performance parts. Their catch cans and AOS kits are a go-to for many Subaru owners who want a dependable, well-supported product. A major advantage of choosing Mishimoto is their no-questions-asked lifetime warranty, which provides excellent peace of mind. Their kits are engineered to be effective and are widely available through most performance part retailers. This makes them a solid, accessible choice for enthusiasts who value strong customer support and the backing of a large, established brand for their build.

Perrin Performance: Trusted Engineering

Perrin Performance is another long-standing and respected brand in the Subaru tuning world. They are known for their meticulous engineering and high-quality manufacturing, and their AOS kits are no exception. Perrin products are often favored by enthusiasts who appreciate thoughtful design and proven performance. Their AOS systems are engineered to efficiently separate oil vapor in a compact and easy-to-install package. For many builders, the Perrin name is synonymous with quality and reliability, making their AOS kit a premium choice for those who want a component that is as well-engineered as the rest of their vehicle.

Budgeting for Your AOS Kit: A Cost Breakdown

Let's talk numbers. The price of an Air Oil Separator kit can range from a couple hundred to over five hundred dollars, and it’s easy to wonder if the more expensive options are truly worth it. The final cost depends on the brand, design complexity, materials, and features like heating elements. While it might be tempting to go for the cheapest option, remember that an AOS kit is an investment in your Subaru’s long-term health. A well-engineered kit provides crucial protection for your engine, preventing oil contamination that can lead to detonation, reduced performance, and costly repairs down the road.

When setting your budget, think about your car’s setup and how you drive it. A daily driven WRX has different needs than a track-prepped STI pushing high boost. Your goal is to find the sweet spot between your budget and the level of protection your engine requires. We’ll break down the typical costs, from solid entry-level options to high-performance systems, and touch on the often-overlooked cost of professional installation. This will give you a clear picture of the total investment required to properly equip your Subaru.

Entry-Level Kits

When you see "entry-level," don't mistake it for "low-quality." These kits are designed to be dependable and effective for a wide range of applications, from daily driving to spirited weekend runs. They provide excellent protection by performing the core function of separating oil from crankcase vapors, just without the premium features or complex construction of higher-priced models. For most Subaru owners with stock or lightly modified engines, an entry-level kit is more than enough. For example, our Baja-proven AOS kits were engineered to withstand the most grueling off-road racing conditions, proving that a straightforward, robust design can deliver exceptional results.

High-Performance Kits

If you have a heavily modified or track-focused build, a high-performance AOS kit is a wise investment. These kits often command a price tag around $500 and justify the cost with advanced features and superior materials. You’ll typically find CNC-machined billet aluminum components, more complex multi-chamber separation designs for maximum efficiency, and sometimes integrated heating elements to prevent condensation and sludge buildup in colder climates. These systems are engineered to handle the increased blow-by from high-horsepower engines, ensuring your intake and fuel mixture remain as clean as possible under extreme conditions. For serious enthusiasts, the added protection is well worth the premium.

Factoring in Professional Installation Costs

Don't forget to account for installation when budgeting for your AOS kit. While the kit itself is the main purchase, getting it installed correctly is critical to its performance. The process involves tapping into your engine's PCV system, which can be a straightforward job for some kits or a more involved one for others. If you aren't comfortable working on your engine, it's best to have a professional handle it. Labor costs can vary, but expect to pay for one to three hours of work from a qualified technician. Getting a quote from a shop that specializes in Subarus will ensure there are no surprises and that your new AOS kit functions perfectly from day one.

Key Features to Look for in an AOS Kit

When you’re ready to invest in an Air Oil Separator, it’s easy to get lost in the details. Not all kits are built the same, and the right one for your Subaru depends on your build, your climate, and how you drive. Focusing on a few key features will help you cut through the noise and choose a kit that delivers reliable performance for years to come. Let’s break down what really matters.

Separation Design and Efficiency

The single most important job of an AOS is to effectively separate oil vapor from the air returning to your intake. A well-designed system prevents oil from contaminating your intake tract, which is crucial for preventing detonation and maintaining the long-term health of your Subaru’s engine. Look for kits with a proven multi-chamber or cyclonic design that can handle the high crankcase pressures of a modified engine. The more efficient the separation, the cleaner your engine will run, directly impacting both performance and longevity.

Heated vs. Unheated: A Cold Climate Guide

If you live somewhere with cold winters, this feature is non-negotiable. In low temperatures, condensation can mix with the collected oil and create a thick, milky sludge inside the AOS and its lines. This gunk can cause blockages, rendering the system useless and potentially damaging your engine. A heated AOS kit uses engine coolant to warm the unit, preventing this condensation buildup and ensuring the system functions properly year-round. For drivers in warmer climates, an unheated kit is usually sufficient, but for anyone facing freezing temperatures, a heated model is a smart investment.

Materials and Build Quality

An AOS is a part you install and want to forget about, so durability is key. The best kits are constructed from high-grade materials like billet aluminum that can withstand the intense heat and pressure of the engine bay. Pay attention to the quality of the hoses, fittings, and mounting brackets, as these are common failure points on cheaper kits. Many top-tier performance parts are tested in extreme conditions like off-road racing, which is a great indicator of their strength and reliability for both aggressive track use and daily driving.

Maintenance and Oil Drainage

Finally, consider the long-term ownership experience. Your AOS will need occasional checks and cleaning, so a design that allows for easy access is a huge plus. Some kits require significant disassembly just to access hose clamps or drain ports. Look for a system with a straightforward oil return line that sends collected oil back to the crankcase, eliminating the need for manual draining. This self-servicing feature makes maintenance much simpler and ensures your AOS is always working to protect your engine.

Common AOS Kit Myths, Busted

Let's clear the air on a few common misconceptions about Air Oil Separator kits. These myths can keep you from making one of the most effective upgrades for your Subaru's long-term health and performance. By understanding the facts, you can make a confident and informed decision for your build.

Myth: "AOS Kits Are Only for Race Cars"

This is one of the biggest myths out there. While an AOS is absolutely essential for a track-driven Subaru, it offers huge benefits for a daily driver, too. Your stock engine still produces oil blow-by, which can lower your fuel's octane rating and lead to carbon buildup on your valves and pistons. A quality AOS kit prevents this by keeping oil out of your intake system. Using one of our Baja Air Oil Separator AOS kits helps your engine run cleaner and more efficiently, protecting it from harmful contaminants and preserving its power and longevity, whether you're on the street or the trail.

Myth: "All AOS Kits Are Created Equal"

It’s easy to assume that all AOS kits do the same job, but their design and effectiveness can vary dramatically. The quality of materials, the efficiency of the separation process, and features like heating elements all play a critical role in how well a kit performs. Some kits are simple catch cans, while others are complex, multi-chambered systems designed for maximum oil separation under extreme conditions. There's a reason certain brands have built a strong reputation in the market. To see how different designs stack up, you can explore a comparison of the best Air Oil Separators for WRX and choose the one that truly fits your performance goals.

Myth: "DIY Installation is Impossible"

The thought of installing an AOS kit can seem intimidating, but it’s a very achievable project for most home mechanics. You don't need a professional lift or a shop full of specialized equipment to get the job done right. With a good set of tools and clear instructions, you can handle the installation in your own garage. The key is to follow a reliable guide that walks you through each step, from preparing your workspace to routing the hoses correctly. Our step-by-step DIY Subaru AOS install guide provides pro tips and a full tool list to make the process straightforward and successful.

AOS Kit Installation: DIY or Pro?

Once you’ve picked out the right AOS kit, the next big question is how to get it installed. This is one of those mods where the answer really depends on your comfort level with a wrench and how much time you have on your hands. While a DIY installation can be a rewarding weekend project that saves you some cash, it’s not a simple bolt-on. The process involves tapping into your engine’s PCV system, which is critical for engine health.

A professional installation offers peace of mind, ensuring every hose is perfectly routed and every connection is secure. On the other hand, tackling it yourself gives you a deeper understanding of how your engine works. Before you decide, let’s break down what’s involved in both paths so you can make the best choice for you and your Subaru.

What You'll Need for a DIY Install

If you're ready to install the AOS kit yourself, having the right tools and a clear plan is essential. The core of the job involves connecting the separator to your engine's PCV system, which often requires removing components like the intercooler or intake manifold to get enough access. You’ll need a good socket set, pliers for hose clamps, and potentially some hose cutters. Patience is probably the most important tool you can have. We’ve put together a complete step-by-step guide that walks you through the entire process, complete with a full tool list and pro tips to make your installation go smoothly.

When to Call a Professional

There’s absolutely no shame in handing your keys to a professional. If you’re short on time, don’t have a garage full of tools, or simply aren’t comfortable digging into your engine bay, a pro install is the way to go. Experienced technicians have done this job countless times. They know the quirks of the Subaru engine layout and can ensure the shortest, cleanest hose routing for optimal performance and a tidy look. This is especially true for more complex builds or right-hand-drive models where space is tight. Getting it done right the first time by an expert prevents potential headaches down the road.

Common Hurdles and Time Commitment

Be realistic about the time and effort involved. For a first-timer, an AOS installation can easily become an all-day project. It’s one of those jobs that can test your patience, with tight spaces and tricky hose connections that can be frustrating to manage. It’s smart to set aside a full afternoon or weekend morning so you don’t feel rushed. Rushing leads to mistakes, like forgetting to tighten a clamp or routing a hose incorrectly. Taking your time, following instructions carefully, and staying organized will make the difference between a successful install and a weekend of frustration.

Where to Buy Your Subaru AOS Kit

Once you’ve decided which Air Oil Separator is right for your build, the next step is finding a reliable place to purchase it. You have a few great options, each with its own set of benefits. Whether you prefer the expertise that comes from buying directly from the manufacturer, the convenience of a large online retailer, or the hands-on help from a local shop, you can find a source that fits your needs. The most important thing is to buy from a reputable seller to ensure you get an authentic, high-quality part that will protect your engine for years to come.

Directly from Crawford Performance

Buying directly from the source is often the best way to guarantee you’re getting a product engineered specifically for your vehicle. We offer a range of specialized performance parts, including our popular Baja Edition Air Oil Separator Kits. These are designed to meet the unique demands of naturally aspirated Subaru models like the Crosstrek, Forester, and Impreza. When you purchase from us, you get more than just a part; you get the peace of mind that comes with our extensive R&D and a product that’s been tested in the most demanding conditions. It’s the most direct way to ensure a perfect fit and function for your build.

Trusted Online Retailers

For those who enjoy comparison shopping, trusted online retailers are a fantastic resource. Websites that specialize in performance parts often carry a wide selection of AOS kits from various brands, allowing you to see different options side-by-side. This can be helpful if you’re looking to compare features or find a kit that fits a specific budget. The key here is "trusted." Be sure to buy from well-known, reputable sites to avoid counterfeit products. Check reviews, look for clear return policies, and make sure they are an authorized dealer for the brand you’re interested in.

Your Local Performance Shop

If you value face-to-face advice and professional support, your local performance shop is an unbeatable option. The experts there can offer personalized recommendations based on your specific Subaru model, your driving style, and your performance goals. This is also the ideal choice if you’re planning on having the AOS kit professionally installed. You can purchase the part and schedule the installation all in one place, ensuring the job is done right. Building a relationship with a local shop provides long-term benefits, giving you a go-to resource for future upgrades and engine tuning services.

Choosing the Perfect AOS Kit for Your Build

Now that you understand why an AOS is a critical upgrade, it's time to select the right one for your build. This decision is more personal than you might think; the perfect kit for a daily-driven Crosstrek used for weekend adventures is different from one built for a track-focused WRX STI. The market is full of options, but narrowing them down is simple when you know what to look for. Your choice hinges on three key factors: your specific engine type, your current and future modification plans, and the quality and support offered by the manufacturer.

Getting this right means you’re not just buying a part; you’re investing in your engine’s long-term health and performance. An AOS works in harmony with the rest of your engine setup, so compatibility is everything. A mismatched kit can be inefficient at best and problematic at worst. We’ll break down each of these decision points to help you confidently choose a kit that fits your Subaru like a glove and supports your performance goals. This will ensure your engine is protected whether you're on the trail, at the track, or just running errands.

Match the Kit to Your Engine

First and foremost, the AOS kit must be compatible with your Subaru’s engine. A kit designed for a turbocharged FA20 engine in a WRX won't work correctly on a naturally aspirated FB25 in a Forester. Manufacturers design these systems with specific engine bays, PCV systems, and crankcase pressures in mind. For example, our Baja Edition AOS Kits are a core part of our overlanding solutions, engineered specifically for naturally aspirated Subarus like the Crosstrek and Impreza. Always double-check the product description to confirm it fits your exact model and year. This is the most critical step to ensure your AOS performs as intended and provides maximum protection.

Consider Your Current Mods

Your existing modifications play a huge role in choosing the right AOS. If your Subaru is mostly stock, a street-series kit is likely sufficient. However, if you've started adding performance parts—like a larger turbo, a front-mount intercooler, or an E85 tune—you've increased the engine's power and, consequently, its crankcase pressure. This higher pressure can lead to more oil blow-by, which a standard AOS might struggle to handle. For modified vehicles, especially those seeing track time, a competition- or race-spec kit is a much better choice. It’s designed for higher flow and better separation efficiency under extreme conditions, protecting your investment.

Check for Warranty and Support

A quality AOS kit is an investment, and it should be backed by a solid warranty and reliable customer support. A manufacturer that offers a warranty—typically one year for many brands—is showing confidence in their product's materials and engineering. This is your assurance against defects and premature failure. At Crawford Performance, we stand behind the engine protection our parts provide, which is why we offer an exclusive 12-month/12,000-mile warranty on our engine builds. Beyond the warranty, look for clear installation guides and accessible technical support. If you run into an issue, you want to know that expert help is just a phone call or email away.

Related Articles

• All About Subaru's Original Air Oil Separator & Why You Need One
• 4 Best Air Oil Separators for WRX (Ranked)
• 2008-2014 Subaru WRX Air Oil Separator V2 Crawford Performance

Frequently Asked Questions

Is an Air Oil Separator the same thing as a catch can? While they serve a similar purpose, they function differently. A catch can is a simpler device that collects oil vapor and condensation in a container that you have to manually empty on a regular basis. An Air Oil Separator is a more advanced, self-servicing system that separates the oil from the air and drains the collected oil right back into the engine. This makes an AOS a true install-and-forget solution for engine protection.

Will installing an AOS on my Subaru void my factory warranty? This is a common concern, but installing an aftermarket part like an AOS does not automatically void your entire vehicle warranty. Thanks to consumer protection laws, a manufacturer must prove that the specific aftermarket part you installed was the direct cause of a specific failure. Since an AOS is designed to improve engine reliability and prevent damage, it's highly unlikely to cause a warranty issue.

Do I need to get my car tuned again after installing an AOS? No, an AOS does not require a new tune. It's a supporting modification that improves the quality of the air entering your engine but doesn't change the air-to-fuel ratio or other parameters that a tune adjusts. However, it is an excellent modification to perform before you get a tune, as it ensures your engine is running on clean air, which allows your tuner to create a safer and more reliable calibration.

How often does an AOS need to be cleaned or maintained? A well-designed AOS requires very little maintenance. Because it automatically drains the collected oil back into the engine, you don't have to worry about emptying it. It's a good practice to visually inspect the hoses for any signs of cracking or blockages during your regular oil changes, but beyond that, the system takes care of itself. If you have a heated kit, it will also prevent sludge from building up in cold weather.

My Subaru is completely stock. Is an AOS still a worthwhile investment? Absolutely. The Subaru boxer engine produces oil blow-by right from the factory, even without any performance modifications. Installing an AOS on a stock vehicle is one of the best preventative measures you can take. It protects your engine from the long-term effects of oil contamination and carbon buildup, ensuring it stays healthy and performs consistently for years to come. Think of it as a foundational investment in your car's longevity.