Air Oil Separator 101: How It Works & Why You Need It
If you own a newer Subaru with a direct-injection (DI) engine, like the FA20 or FA24 in the WRX, you need to be aware of a serious issue: carbon buildup. Because fuel is injected directly into the cylinder, it never washes over the intake valves to keep them clean. This makes your engine extremely vulnerable to performance-robbing oil vapor from the crankcase. For these modern engines, an air oil separator is no longer just a good idea—it's an essential modification for protecting your investment. We'll cover exactly why DI engines need this defense and guide you through choosing the right system for your Subaru.
Key Takeaways
- Focus on Engine Protection, Not Power Gains: An AOS is a defensive modification that preserves your engine's health and existing horsepower. It prevents oil contamination in the intake system, which stops carbon buildup and reduces the risk of engine knock.
- Opt for a Self-Draining System Over a Catch Can: Unlike a catch can that requires regular manual emptying, an AOS is an integrated, closed-loop system. It automatically separates oil and returns it to the engine, offering a reliable, low-maintenance solution for long-term protection.
- Installation Details Dictate Effectiveness: A successful AOS installation depends on correct placement and hose routing. Mounting the unit high in the engine bay is essential for proper gravity-fed drainage, ensuring the system functions correctly and protects your engine as designed.
What Does an Air Oil Separator (AOS)?
If you’ve modified your Subaru, you’ve likely heard about adding an Air Oil Separator, or AOS. So, what exactly is it? Think of an AOS as a gatekeeper for your engine's intake system. Your engine naturally produces "blow-by," which is a mix of air, unburnt fuel, and oil vapor that slips past the piston rings and into the crankcase. The factory PCV (Positive Crankcase Ventilation) system vents these gases back into your intake to be re-burned, which is fine for a stock vehicle.
However, on a performance-tuned engine, especially a turbocharged one like a WRX or STI, this process introduces a problem. The oil vapor coats the inside of your intake, intercooler, and valves, leading to carbon buildup. This can lower your fuel's octane rating and increase the risk of premature detonation, or "knock"—an engine killer. An AOS intercepts these crankcase gases, separates the oil from the air, and sends clean air back to the intake while returning the collected oil to the engine. This simple but effective process protects your engine, maintains performance, and gives you peace of mind whether you're on the track or a trail.
How an Air Oil Separator Filters Out Oil Vapor
The magic of an AOS happens through physics. When blow-by gases enter the AOS unit, they are forced into a rapid swirling motion, creating a vortex. Because oil droplets are heavier than air, this centrifugal force slings them against the outer walls of the separator's chamber. As the oil droplets collide with the walls and internal baffles, they condense and coalesce, trickling down to the bottom of the unit. The now-cleaner air, being much lighter, continues its path through the center of the vortex and is routed back into your engine’s intake. It’s a continuous, self-operating process that effectively filters your engine's crankcase emissions on the fly.
How Much Oil Does an AOS Actually Capture?
There isn't a single answer to how much oil an AOS captures, as the amount depends on your engine's condition, your driving habits, and your specific build. A stock engine might only produce a fine mist, while a highly modified, high-boost engine will generate significantly more blow-by, especially during aggressive driving or track days. The key is to focus on the cumulative effect over thousands of miles, not the daily volume. Even a thin film of oil constantly reintroduced into your intake will bake onto your valves, causing carbon buildup that robs power and can lead to detonation. By capturing this continuous stream of oil vapor, an AOS prevents that harmful buildup, making it one of the most effective performance parts for ensuring your engine's long-term reliability.
What's Inside an Air Oil Separator?
At its heart, an AOS consists of a canister, typically made from durable aluminum, that houses a series of internal chambers and baffles. These aren't just random plates; they are specifically engineered to maximize the separation of oil from air. The canister has an inlet port where the blow-by gases from the crankcase enter, and an outlet port where the clean air exits to return to the intake manifold. A third port at the bottom serves as a drain, allowing the collected oil to flow back into the oil pan. The quality of an AOS is determined by the efficiency of its internal design and the durability of its materials.
Centrifugal vs. Filter-Based Separation
When it comes to separating oil from air, there are two primary approaches: centrifugal force and physical filters. Filter-based systems, often found in simpler catch cans, use a mesh or fibrous material to trap oil droplets. While effective initially, these filters can become saturated and clogged over time, restricting crankcase ventilation and requiring regular cleaning or replacement. In contrast, a centrifugal AOS uses physics to its advantage. By spinning the incoming gases in a vortex, the heavier oil droplets are thrown against the walls and drain away, while the lighter, clean air passes through freely. This design ensures consistent performance without any filter media to clog, making it a far more reliable and maintenance-free solution for protecting your engine's performance parts long-term.
Your PCV System and an AOS: How They Work Together
An AOS doesn't just supplement your factory PCV system—it replaces and improves it. The AOS integrates directly with your engine's crankcase ventilation ports, intercepting the oily blow-by gases before they ever reach the intake tract. By doing so, it prevents the harmful effects of oil contamination, such as reduced intercooler efficiency and carbon buildup on intake valves. This is especially critical for direct-injection engines. By ensuring only clean air re-enters the combustion chamber, an AOS helps maintain your engine's performance and reliability, which is a core principle of any solid engine tuning strategy. It’s a closed-loop, maintenance-free solution that keeps your engine running cleaner and stronger.
Air Oil Separator vs. Catch Can: Which Is Right for You?
When you're looking to protect your Subaru's engine, you'll often hear about Air Oil Separators (AOS) and catch cans. Both are designed to handle engine blow-by—the mix of oil, fuel, and air that slips past the piston rings—but they go about it in very different ways. A catch can is a simple reservoir that "catches" oil vapor, requiring you to empty it manually. An AOS, on the other hand, is an active system that separates the oil and returns it to the engine. This fundamental difference leads to distinct advantages in performance, maintenance, and long-term engine health, making the AOS a superior choice for most Subaru owners looking for a comprehensive solution.
Why a Self-Draining Design Matters
One of the most significant advantages of an Air Oil Separator is its self-draining design. The system is engineered to capture oil vapor from the crankcase gases and then channel the condensed, liquid oil directly back into the engine's oil pan. This closed-loop process means you never have to worry about manually draining a container. More importantly, it ensures the AOS is always working at peak efficiency to protect your engine. There's no risk of a catch can overfilling and getting sucked into the intake. This automated return system provides consistent, reliable protection for your engine's internal components, making it a truly integrated part of your performance parts ecosystem.
How an AOS Protects Your Engine's Performance
The main job of an AOS is to keep oil vapor out of your engine's intake tract. When blow-by gases are recirculated, they carry oil mist that coats the intake manifold, valves, and intercooler. This buildup can lower the octane of your air/fuel mixture, leading to premature detonation, or "knock," which can cause serious engine damage over time. By installing one of our Subaru Dual Chamber Air Oil Separators, you ensure that only clean air re-enters the combustion chamber. This helps maintain your engine's performance, prevents power-robbing carbon deposits, and contributes to the overall longevity and health of your Subaru.
Why Turbocharged and Direct-Injection Engines Benefit Most
If you drive a modern turbocharged or direct-injection Subaru, like a 2015+ WRX, an AOS is less of an upgrade and more of a necessity. Turbocharged engines generate higher cylinder pressures, which naturally creates more blow-by. At the same time, direct-injection (DI) technology sprays fuel directly into the cylinder instead of the intake port. This means fuel no longer washes over the intake valves, leaving them defenseless against the oily vapor from that increased blow-by. This combination allows oil to bake onto the valves, forming hard carbon deposits that restrict airflow and can lead to engine knock. An AOS is the most effective way to protect these engines by capturing that oil before it ever reaches the intake tract, preserving the performance and reliability of your performance parts.
AOS vs. Catch Can: Which Is Easier to Maintain?
When it comes to maintenance, the difference between an AOS and a catch can is night and day. A catch can needs to be checked and emptied regularly—sometimes as often as every few hundred miles, depending on your driving style and engine condition. Forgetting to do so can cause serious issues. In contrast, an AOS is widely considered a "set it and forget it" modification. Because it automatically drains the collected oil back into the engine, there is no routine service required. This convenience is a major benefit for any driver who wants to spend more time on the road or track and less time under the hood performing routine maintenance tasks.
Comparing Costs: AOS vs. Catch Can
When you're looking at the price tags, a catch can often seems like the more budget-friendly option. Let's be direct: a high-quality Air Oil Separator typically has a higher upfront cost. However, the initial price doesn't tell the whole story. You have to consider the total cost of ownership and the value of what you're protecting. A catch can requires your time and attention for regular draining, and forgetting to do so can lead to bigger problems. The AOS, with its self-draining design, is a one-time investment that protects your engine without demanding ongoing maintenance. Think of it as insurance for your performance parts and engine health, safeguarding you from far more expensive repairs down the road caused by oil contamination and carbon buildup.
Will an Air Oil Separator Affect My Emissions?
For any vehicle that sees street duty, emissions compliance is a key consideration. A properly designed Air Oil Separator works in harmony with your Subaru's factory PCV system. It's a closed-loop system, meaning it manages harmful crankcase vapors without venting them into the atmosphere. By cleaning the blow-by gases and returning only air to the intake, an AOS helps your vehicle run efficiently while remaining compliant with emissions standards. This makes it a responsible and effective upgrade for your Subaru WRX or STI, ensuring you get enhanced engine protection without compromising your car's street-legal status.
How to Choose the Right AOS for Your Subaru
Selecting the right Air Oil Separator is one of the most important decisions you can make for the long-term health of your Subaru’s engine. With different designs and brands on the market, it’s easy to get overwhelmed. The key is to focus on a system that is engineered specifically for the unique demands of a boxer engine, offers proven performance benefits, and is built to last. A quality AOS isn’t just an accessory; it’s a critical component for protecting your investment, whether you’re building a daily driver or a dedicated track car. Let’s walk through what makes an AOS effective and how to find the perfect fit for your vehicle.
Why We Recommend the Crawford V3 AOS Kit
When it comes to protecting your engine, we stand by the Crawford V3 Air Oil Separator because it’s designed to solve the core problem of oil contamination in the intake system. This is essential for preventing issues like oil burning, detonation, and carbon buildup on your valves and pistons, all of which can lead to serious engine damage. The V3 AOS kits are engineered to maintain your engine’s cleanliness and performance over the long haul. By effectively separating and returning oil to the crankcase, it ensures your engine is only burning air and fuel, just as it was designed to. This simple function is crucial for both reliability and power.
Finding the Right Fit for Your WRX, STI, or Crosstrek
Compatibility is key, and you need an AOS that integrates seamlessly with your specific model. The Crawford V3 AOS is engineered for a wide range of Subaru models from 2002 to the present, making it a versatile and reliable choice for nearly any enthusiast. Whether you’re looking for WRX upgrades or enhancing your Forester XT, there’s a kit designed for your vehicle. We offer specific applications for the WRX, STI, BRZ/FRS/86, Crosstrek, and even the Ascent. This broad compatibility ensures that you can get proven engine protection no matter which Subaru you drive, without worrying about custom fabrication or improper fitment.
What Makes a Quality Air Oil Separator?
When you’re comparing options, look for an AOS with a dual-chamber design. This feature is far more effective at capturing oil and other contaminants than simpler catch can systems. For example, the Crawford V3 AOS reduces harmful blow-by—those oil and gas fumes that contaminate your intake—by 78% compared to standard systems. The performance data speaks for itself: engines equipped with our V3 AOS maintain 95% of their original power after 50,000 miles. In contrast, engines without one can lose up to 15% of their power over the same period. These metrics show why investing in a high-quality, well-engineered AOS is one of the smartest performance parts you can add to your Subaru.
The Importance of Heated Units in Colder Climates
If you live in a climate where temperatures drop near or below freezing, a standard Air Oil Separator might not be enough. In cold weather, the condensation that naturally forms inside the AOS and its lines can freeze, creating a blockage. This completely stops the system from working and can lead to a dangerous buildup of crankcase pressure. To combat this, heated AOS units use engine coolant to keep the separator well above 212°F, ensuring everything stays in a liquid state and flows properly. This feature is essential for reliable, year-round engine protection, especially for those who depend on their vehicles in harsh conditions, like in many overlanding solutions.
Choosing the Right Size and Finish
The effectiveness of an AOS comes down to its engineering. A quality unit is defined by the efficiency of its internal design and the durability of its materials. Look for an AOS with a dual-chamber design, which is significantly more effective at separating contaminants than simpler systems. The size of the unit should also be appropriate for your engine's displacement and power output to handle the volume of blow-by correctly. Finally, consider the finish. Anodized aluminum not only looks great in your engine bay but also provides an extra layer of corrosion resistance, ensuring your investment in performance parts lasts for years to come.
How to Install and Maintain Your Air Oil Separator
Properly installing your Air Oil Separator is just as important as choosing the right one. A correct installation ensures the system functions efficiently, protecting your engine for years to come. While the process is manageable for a home mechanic, attention to detail is everything. Taking the time to get it right prevents future headaches and ensures you get the full benefit of your investment. Once installed, a quality AOS requires minimal upkeep, but a few routine checks will keep your Subaru running at its peak. Think of it as simple preventative care for one of your engine’s most important protective systems.
Your Step-by-Step AOS Installation Guide
Before you start turning wrenches, your first step should always be to read the installation manual that came with your AOS kit from front to back. Every kit is slightly different, and the instructions are tailored to your specific Subaru model. Gather all the necessary tools listed in the manual so you aren’t scrambling for a socket mid-install. The general process involves disconnecting the battery, mounting the AOS unit, and carefully routing the new hoses to replace the factory PCV lines. Whether you're working on Subaru WRX upgrades or modifying a Forester, patience is key. Follow the diagrams closely and take your time to ensure every connection is secure.
Tools, Time, and What to Expect
You don't need a professional lift or a full mechanic's workshop to install an AOS. The job can be done in your driveway with a solid set of hand tools, including a socket set, wrenches, and pliers for hose clamps. Plan to set aside a few hours for the project—typically between two and four, depending on your specific Subaru model and your comfort level working on it. This isn't a race; the most important thing is getting it right. A successful installation comes down to careful hose routing and ensuring every connection is tight and secure to prevent leaks. It’s one of those foundational performance parts where attention to detail during the install directly translates to long-term engine protection and peace of mind.
Getting the Mounting and Drainage Just Right
One of the most critical details of the installation is the mounting location. You want to mount the AOS as high as possible on the firewall. This isn't just for looks or convenient placement; it's essential for proper function. Mounting the unit high allows gravity to do its job, ensuring the separated oil drains correctly back into the engine's crankcase. If the AOS is mounted too low, oil can pool inside the separator or its lines instead of draining. This can reduce the system's effectiveness and, in colder climates, potentially lead to blockages. Getting the height right from the start is crucial for a maintenance-free, self-draining design.
How to Connect Your AOS Lines the Right Way
With the AOS mounted, the next vital step is connecting the hoses. This is where you need to pay close attention to your installation manual. Each line has a specific purpose and destination, connecting points on your engine—like the crankcase and cylinder head ports—to the AOS, and then routing clean air back to the turbo inlet. Mixing up these connections will prevent the system from working and could create a vacuum leak or other engine issues. Double-check that each hose is connected to the correct port and that all clamps are tightened securely to prevent leaks. This ensures your engine gets the protection it needs from harmful oil vapors and blow-by.
Your Simple AOS Maintenance Checklist
While a Crawford AOS is designed to be a low-maintenance component, it’s not something you should completely forget about after installation. A few simple checks will ensure it continues to perform flawlessly. After the first few hundred miles, pop the hood and give everything a once-over. Check that all hose clamps are still tight and look for any signs of oil seepage around the fittings. It’s also good practice to periodically inspect the hoses for any cracking or wear, especially if you drive in harsh conditions. These quick checks are a small part of maintaining the overall health of your engine and its collection of performance parts.
Recommended Service Intervals
A quality AOS is a low-maintenance component, but that doesn't mean it's a no-maintenance one. Since it works as a closed-loop system with your factory PCV, it’s smart to give it a quick inspection during your regular oil changes. This isn't a time-consuming task; you're simply ensuring everything is still in top shape. Take a moment to verify that all the hose clamps are secure and that there are no signs of wear or damage to the lines. This simple, routine check ensures your AOS continues to function flawlessly, providing the consistent engine protection your Subaru deserves. Think of it as five minutes of prevention that saves you from potential headaches down the road.
Signs Your Air Oil Separator Needs Attention
Your AOS is a silent guardian, but there are a few tell-tale signs it might need some attention. The most obvious is any visible oil seepage around the unit’s fittings or hose connections. If you see residue, it’s time to tighten the clamps and clean the area. You should also periodically inspect the hoses for cracking, brittleness, or swelling, especially if you live in a climate with extreme temperatures. Finally, pay attention to your engine's performance. If you notice a sudden drop in power, rough idling, or an increase in oil consumption, it could indicate that the AOS is not separating oil effectively. Catching these signs early ensures your engine remains protected.
Common Air Oil Separator Myths, Busted
Air Oil Separators are one of the most important protective modifications you can make for your Subaru’s engine, but there’s a lot of misinformation floating around. An AOS is a smart investment in your vehicle's longevity and performance, yet these myths can keep owners from understanding its true value. Let’s clear up a few common misconceptions so you can make an informed decision for your build.
Myth: An AOS Adds Horsepower
Let’s get this one out of the way first: an Air Oil Separator does not add horsepower. Its job is to protect the power your engine already makes. By preventing oil vapor from contaminating your intake system, an AOS helps stop carbon buildup on your valves and reduces the risk of detonation, or engine knock. This keeps your engine running efficiently and reliably for the long haul. Think of it as a defensive player for your engine, preserving its health so other performance parts can do their job effectively. If you're looking to add power, you should focus on upgrades like a tune or exhaust system.
Myth: It's a Simple Bolt-On
While it might seem like a straightforward addition, installing an AOS is more involved than your average bolt-on part. A proper installation requires carefully rerouting hoses and integrating the unit with your engine’s Positive Crankcase Ventilation (PCV) system. Unlike a strut tower brace or an intake that you can install in under an hour, an AOS requires a bit more planning and patience to ensure all connections are secure and routed correctly. For many Subaru owners, especially those with WRX upgrades, taking the time to install it right—or having a professional do it—is crucial for it to function as designed.
Myth: An AOS and a Catch Can Are the Same
This is a common point of confusion, but an AOS and a catch can are fundamentally different. A catch can is a simple container that traps oil vapor and requires you to manually drain it. An Air Oil Separator is a more sophisticated, closed-loop system. It separates the oil from the crankcase vapor, returns the clean air to the intake, and automatically drains the collected oil back into the engine’s oil pan. This self-draining design makes an AOS a much lower-maintenance and more integrated solution for any performance-oriented Subaru STI build.
Myth: It's a "Set It and Forget It" Mod
While a high-quality AOS is designed to be extremely low-maintenance, no part on a performance vehicle is truly "set it and forget it." It’s always a good practice to periodically check the hoses for any signs of wear or cracking and ensure the connections are still tight. This is especially important for vehicles that see hard use, whether on the track or on an overlanding adventure. A quick inspection during your regular oil changes is all it takes to ensure your AOS continues to protect your engine effectively for years to come.
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Frequently Asked Questions
Do I really need an AOS if my Subaru is mostly stock? While an Air Oil Separator is essential for modified and tuned engines, it's also a very smart investment for a stock Subaru. Modern direct-injection engines are prone to carbon buildup on the intake valves, and an AOS directly addresses this by keeping oil vapor out of the intake tract. Think of it as a long-term preventative measure that helps maintain your engine's health and efficiency from day one.
Is an AOS really that much better than a simple catch can? Yes, the difference is significant. A catch can is a passive container that you must manually empty, and if it overfills, it can send all that collected gunk straight into your engine. An AOS is an active, closed-loop system that separates the oil and automatically drains it back into the engine. This means it provides constant, reliable protection without requiring you to perform routine service.
Will installing an AOS make my car more powerful? An AOS does not add horsepower; its job is to preserve the power your engine already produces. Over time, oil contamination in the intake system can lead to carbon buildup and a higher chance of engine knock, which robs your engine of performance. By keeping the system clean, an AOS ensures your engine runs as efficiently and reliably as it was designed to, protecting your power output for the long haul.
What happens if I install the AOS incorrectly? Proper installation is critical for the system to function. If the unit is mounted too low, it won't be able to drain the collected oil back into the engine via gravity. Likewise, if the hoses are connected to the wrong ports, you could create vacuum leaks or other pressure issues that prevent the system from working and could harm your engine. Following the instructions carefully is key to getting the protection you paid for.
How much maintenance does an AOS actually require? A quality AOS is designed to be a very low-maintenance part, but it's not something you should completely ignore after it's installed. It doesn't require the regular draining that a catch can does, but it's a good idea to visually inspect the hoses and connections during your regular oil changes. Just check to make sure all the clamps are tight and the lines are free of cracks or wear.