AOS vs Catch Can Explained: Which Is Best for You?

When it comes to engine protection, how much maintenance are you really willing to do? This is the central question you need to answer when choosing between an Air/Oil Separator and a catch can. An AOS is designed to be a nearly "set it and forget it" system that automatically returns oil to your engine. A catch can is a simpler device that effectively filters contaminants but requires you to manually empty it. In this detailed look at aos vs catch can explained, we’ll explore the pros and cons of each approach, helping you decide which one best fits your lifestyle.

Key Takeaways

• Choose based on your maintenance style: An Air/Oil Separator offers a hands-off solution by automatically returning oil to your engine, while a catch can is a more budget-friendly option that requires you to manually empty it.
• Match the part to your driving habits: An AOS is the best choice for high-performance builds, track use, and cold climates due to its robust, self-managing design; a catch can is a practical and effective solution for daily drivers and lightly modified cars.
• Understand that neglect has consequences: Both systems require attention to prevent serious engine damage, as a full catch can or a blocked AOS can create dangerous crankcase pressure, making routine inspections a critical part of protecting your investment.

What Is an Air/Oil Separator (AOS)?

An Air/Oil Separator, or AOS, is a device designed to protect your Subaru’s engine by managing blow-by. Blow-by is a mix of oil vapor, unburnt fuel, and other contaminants that gets past the piston rings and enters the crankcase. In modern engines, especially turbocharged ones, this vapor is recirculated back into the engine's intake to be burned off for emissions reasons. The problem is that this oily mist can coat your intake manifold, valves, and intercooler, leading to carbon buildup, reduced airflow, and lower octane. An AOS is an active system that intercepts these vapors, separates the oil, and returns it to the engine, keeping your intake clean and your engine running efficiently.

How an AOS Works

Think of an AOS as a small refinery for your engine's crankcase vapors. When blow-by gases are pushed from the crankcase, they enter the AOS unit. Inside, a series of baffles and chambers cause the hot oil vapor to cool and condense, turning it back into liquid oil. From there, the system automatically drains the collected oil back into the engine, usually into the oil pan. The remaining air and water vapor, now mostly free of oil, continue to the intake manifold to be burned. This closed-loop process makes it one of the most effective performance parts for maintaining long-term engine health.

Pros of an AOS

The biggest advantage of an AOS is its low-maintenance design. Because it automatically returns the separated oil to the engine, it’s a true "set it and forget it" solution. You don't have to worry about manually emptying a container, which is a major plus for a daily driver or a track car you want to keep ready. By preventing oil from coating your intake system, an AOS helps your engine run cleaner and more efficiently over its lifespan. This is especially beneficial for direct-injection engines, which are prone to carbon buildup on the intake valves. Many owners consider it an essential part of their Subaru WRX upgrades for this reason.

Cons of an AOS

While effective, an AOS isn't without its drawbacks. The primary one is the higher upfront cost and more involved installation compared to simpler systems. Another consideration is that an AOS can return small amounts of water and fuel condensation along with the oil. This can slightly dilute your engine oil over time, so you might want to stick to a more frequent oil change schedule to be safe. In very rare cases, if a line were to become blocked, it could lead to pressure issues within the crankcase. Proper installation and pairing with professional engine tuning services can help ensure everything works together seamlessly.

What Is a Catch Can?

A catch can is a straightforward but effective component designed to protect your Subaru's engine. Think of it as a filter for your crankcase ventilation system. Your engine naturally produces "blow-by," which is a mix of oil vapor, unburnt fuel, and moisture that slips past the piston rings and into the crankcase. To prevent pressure from building up, the factory PCV (Positive Crankcase Ventilation) system vents these gases back into your intake to be burned off. The problem is, that oily mist can coat your intake manifold, ports, and valves with carbon buildup. Over time, this reduces airflow and hurts performance, an issue especially common in direct-injection engines.

A catch can intercepts these blow-by gases before they reach the intake. It’s a small, sealed container that is installed between the crankcase vent and the intake manifold. Its job is to capture the oil and other contaminants, allowing cleaner air to circulate back into your engine. By adding one of these performance parts to your build, you’re taking a proactive step to maintain engine health and efficiency, whether you’re daily driving your Crosstrek or pushing your STI on the track. It’s a popular modification for a reason: it addresses a fundamental issue in modern internal combustion engines.

How a Catch Can Works

A catch can functions on a simple principle: condensation. Hot, oily blow-by gases are routed from your engine’s crankcase into the can. As these gases enter the cooler environment of the can, they expand and slow down. This change in temperature and pressure causes the oil and water vapors to condense into liquid droplets. Most quality catch cans contain internal baffles or a mesh medium that creates more surface area, which helps to separate the oil from the air more effectively. The collected liquid settles at the bottom of the can, while the cleaner, drier air continues its path out of the can and back into the intake manifold.

Pros of a Catch Can

The most significant advantage of a catch can is its ability to keep your engine’s intake system clean. By preventing oil and fuel contaminants from coating your intake valves, it helps prevent carbon buildup. This is especially critical for direct-injection (DI) engines, like those found in the 2015+ Subaru WRX, where fuel is injected directly into the cylinder and doesn't wash over the intake valves. Another major pro is cost. Catch cans are typically more affordable than their AOS counterparts, making them an accessible first step for owners looking to improve their engine's longevity without a huge investment. They effectively do their job with a simple, reliable design.

Cons of a Catch Can

The main drawback of a catch can is that it requires manual maintenance. The collected oil, fuel, and water mixture doesn't go anywhere; it stays in the can until you empty it. Forgetting to drain it can cause serious problems. If the can fills up, the collected gunk can be sucked directly into your intake, defeating the entire purpose of the system. In a worst-case scenario, a full or frozen can could block crankcase ventilation entirely. This can lead to pressure buildup that may damage engine seals or even your turbo, resulting in costly repairs. You have to remember to check and empty it regularly, typically with every oil change.

AOS vs. Catch Can: The Key Differences

While both an Air/Oil Separator and a catch can are designed to protect your Subaru’s engine from harmful blow-by, they handle the task in very different ways. Choosing the right one isn't just about preference; it's about matching the system to your car's setup, your driving habits, and even the climate you live in. Understanding these differences is the first step to making a smart investment in your engine's health and performance.

Let's break down the five key areas where these systems differ, so you can see which one makes the most sense for your build. From maintenance to installation, each system has its own set of pros and cons that can have a real impact on your ownership experience.

Oil Return and Dilution

The most fundamental difference between an AOS and a catch can is what they do with the oil they collect. An AOS is a closed-loop system that separates oil from crankcase vapors and automatically returns the collected oil to your engine's oil pan. This means you never have to empty it. A catch can, on the other hand, simply collects the oil and contaminants in a canister that you must manually drain.

Because an AOS returns everything it separates, a small amount of water and fuel vapor can get mixed back into your oil supply. This can lead to minor oil dilution, so you might want to stick to a slightly shorter oil change interval. With a catch can, those contaminants are completely removed from the engine system, ensuring they never re-enter your oil.

Cold Weather Performance

If you live in a region with freezing temperatures, this is a critical factor. An AOS is the clear winner in cold climates. Most high-quality AOS units, including our own, integrate heated coolant lines. This warmth prevents the collected water vapor from freezing or turning into a thick, milky sludge inside the system, ensuring it functions properly year-round.

A catch can can be problematic in the cold. The collected water can freeze solid, potentially blocking the PCV hose and creating dangerous pressure buildup in your crankcase. This can lead to blown seals and other serious engine issues. If you're committed to a catch can in a cold climate, you'll need to be extremely diligent about emptying it before temperatures drop.

Emissions and Street Legality

Factory PCV systems were designed to meet environmental regulations by routing blow-by gases back into the engine's intake to be burned off. An AOS maintains this closed-loop design, venting the cleaned air back into your intake. This makes it the preferred option for maintaining emissions compliance and keeping your car street-legal.

Catch cans can be set up as closed-loop systems, but some are designed to vent to the atmosphere (VTA). A VTA setup is almost never street-legal and will cause you to fail an emissions test. For any daily driven Subaru WRX or Forester, a closed-loop system is the only way to go.

Installation Complexity

Here, the catch can has the edge in simplicity. A typical catch can installation involves mounting the canister in your engine bay and connecting two hoses to the PCV system. It’s a straightforward process that many DIY enthusiasts can handle in their garage.

An AOS installation is more involved. In addition to connecting the PCV hoses, you also need to tap into your engine's coolant lines to provide heat to the unit. While this is what gives the AOS its superior cold-weather performance, it adds a layer of complexity to the installation that might be intimidating for some. It’s a trade-off: more work upfront for less maintenance later.

Cost

Upfront cost is another area where these two systems diverge. Catch cans are generally the more budget-friendly option, with quality units typically costing between $150 and $300. Their simpler design and construction lead to a lower price point, making them an accessible first mod for many owners.

An Air/Oil Separator is a more significant investment, usually starting around $400 and up. The higher price reflects its more complex engineering, internal baffling, and integrated heating elements. While the initial cost is higher, an AOS offers a "set it and forget it" advantage, saving you the recurring task of emptying a can. You can explore our full range of performance parts to compare options for your specific model.

Which System Is Right for Your Subaru?

Choosing between an Air/Oil Separator and a catch can isn't about which one is universally "better." It's about what’s best for your specific car and how you drive it. Your Subaru's modification level, your driving habits, and even your local climate all play a role in determining the right fit. Let's walk through some common scenarios to help you find the perfect solution for your build.

For High-Performance and Track Builds

If you’re pushing your Subaru to its limits on the track or have a highly modified engine, an AOS is essential. These high-performance builds generate significantly more blow-by. An AOS is designed to handle this increased volume efficiently, separating the oil and returning it directly to the engine. This is a huge advantage during intense driving sessions where you can't just pull over to empty a can. For anyone serious about track days or running aggressive performance parts, the continuous, maintenance-free operation of an AOS provides critical engine protection and peace of mind.

For Daily Drivers and Lightly Modified Cars

For a daily driver or a car with light modifications, a catch can is often the most practical and effective choice. If your Subaru is mostly stock, it produces a lower volume of blow-by, so a catch can is perfectly capable of keeping the intake clean. They are typically more affordable and simpler to install, making them a great entry-level upgrade. You will need to empty the can periodically, but for a car that sees mostly street duty, it won't fill up quickly. This makes it a smart, budget-friendly solution for protecting your Subaru Impreza or Forester.

For Turbocharged and Heavily Modified Engines

If your engine consumes a notable amount of oil, an AOS is the clear winner. A good rule of thumb is if you find yourself adding a quart or more of oil between changes, your engine will benefit from an AOS. Turbocharged and heavily modified engines, like those found in many Subaru WRX builds, often experience higher oil consumption and blow-by. The AOS automatically returns captured oil to the sump, ensuring your engine is properly lubricated while keeping harmful contaminants out of the intake system. This closed-loop system is ideal for maintaining engine health under demanding conditions.

For Cold Climate Driving

Living in a region with cold winters adds another layer to your decision. In this case, an AOS generally performs better. The system uses heated coolant lines to prevent the collected oil and water vapor from turning into a thick, mayonnaise-like sludge, which can happen in freezing temperatures. A catch can, on the other hand, will collect more water condensation in the cold and may require more frequent emptying to prevent the contents from freezing and potentially damaging the can. If you’re building a vehicle with overlanding solutions for all-weather adventures, an AOS is the more reliable choice.

Can You Run Both an AOS and a Catch Can?

It’s a question we hear from dedicated builders: can you run both an Air/Oil Separator and a catch can? The short answer is yes, you absolutely can. While it might seem redundant, combining these two systems offers an extra layer of protection for highly modified or hard-driven Subaru engines. For some enthusiasts, this dual setup provides the ultimate peace of mind.

Think of it as a two-stage filtration process. An AOS is designed to separate oil from crankcase gases and return the liquid oil to the pan, while a catch can simply collects contaminants in a reservoir. When used together, the AOS does the heavy lifting of separating and returning the bulk of the oil. The catch can is then installed "downstream" to capture any fine oil mist or aerosolized gunk that might get past the AOS, ensuring the air re-entering your intake is exceptionally clean.

This dual setup is most common in purpose-built track cars or vehicles running very high cylinder pressures, where blow-by is a significant concern. Some tuners find this combination provides the ultimate defense against intake valve deposits and potential detonation caused by oil contamination. While it is possible you can use a catch can with an AOS, it’s not a simple bolt-on solution. A dual setup adds more hoses, clamps, and complexity to your engine bay.

Proper installation is critical. An incorrect setup can create unwanted restrictions in the crankcase ventilation system, leading to pressure issues and oil leaks. You need to ensure the system is well-planned and follows installation best practices to avoid doing more harm than good. For most Subaru owners, a high-quality AOS is more than sufficient for protecting their engine. However, for those pushing the absolute limits, a dual system can be a worthwhile investment in engine longevity.

What's the Maintenance Like for Each System?

When choosing between an AOS and a catch can, it’s important to be honest about how much hands-on maintenance you’re willing to do. One system is designed to be largely self-sufficient, while the other requires regular manual checks. Neither is a completely "set it and forget it" solution, and neglecting the maintenance on either system can lead to serious issues for your engine. Understanding the upkeep before you buy is key to protecting your investment and keeping your Subaru running at its peak.

Your choice will depend on your driving habits, your climate, and your personal preference for routine checks versus a more integrated system. Let’s break down what you can expect with each.

AOS Maintenance

An Air/Oil Separator is about as close to a hands-off solution as you can get. Its main advantage is that it doesn't need to be manually emptied. The system uses coolant lines from the engine to keep the unit warm, preventing the water and fuel vapors from condensing into the thick, milky sludge that can build up in catch cans, especially in cold weather.

However, it’s not entirely without maintenance considerations. Because the AOS returns separated oil along with small amounts of fuel and water vapor back to your engine, it can slightly dilute your oil over time. For this reason, you might consider more frequent oil changes to ensure your oil maintains its protective properties. We offer a range of performance parts designed to work with these systems for optimal engine health.

Catch Can Maintenance

A catch can requires a more hands-on approach. Its primary maintenance task is simple: you have to empty the can. How often you do this depends on your driving style, your engine's condition, and even the weather. In colder, more humid climates, you'll likely collect more watery sludge and need to empty it more frequently.

The biggest risk with a catch can is forgetting to check it. If the can fills up, the collected oil and gunk can get sucked back into your intake, which is exactly what you’re trying to prevent. A full can may also block crankcase ventilation, creating pressure that can damage engine seals or your turbo. Regular checks are not just recommended; they are essential for protecting your engine. Proper engine tuning services can also help optimize your engine's performance and minimize blow-by.

What Happens If You Neglect Maintenance

Ignoring maintenance on either system can have serious consequences. With a catch can, the danger is immediate and clear. If you forget to empty it and it overfills, the system can no longer do its job. The collected oil can be pulled directly into your engine's intake, or worse, the blocked ventilation can cause a buildup of crankcase pressure. This pressure can force oil past your engine seals or damage your turbocharger, leading to costly repairs.

While an AOS is lower maintenance, it isn't immune to problems from neglect. Though it's rare, the lines on an AOS can become blocked, which also leads to dangerous pressure buildup. It’s a good practice to inspect the hoses during your regular engine checks, especially if you're pushing your car hard on the track or trails with your Subaru WRX upgrades.

4 Common Myths About AOS and Catch Cans

When you start researching engine protection, you’ll quickly run into a lot of strong opinions about Air/Oil Separators and catch cans. It can be tough to sort fact from fiction. Let's clear up a few common misunderstandings so you can make a confident choice for your Subaru.

"These systems are only for race cars"

This is one of the biggest myths out there. While an AOS is absolutely essential for a track car or a heavily modified engine that produces a lot of blow-by, it’s a valuable upgrade for any Subaru. Every direct-injection engine, including the one in your daily driver, creates blow-by that can lead to carbon buildup and reduced performance over time. Installing a catch can or an AOS is a proactive way to protect your engine’s health, whether you’re commuting to work in your Subaru Crosstrek or pushing your STI to its limits on the weekend.

"Catch cans are always the cheaper option"

Looking purely at the upfront cost, a quality catch can is typically less expensive than an AOS. You can find good ones for a few hundred dollars, while a high-performance AOS often costs more. However, that initial price tag doesn't tell the whole story. The higher cost of an AOS reflects its more complex design, which includes features like coolant lines for heating and an automatic oil return system. Think of it as an investment in convenience and a more integrated solution. Both are excellent performance parts, but their pricing reflects different approaches to solving the same problem.

"An AOS is a 'set it and forget it' system"

While an AOS is a wonderfully low-maintenance solution, it’s not entirely "set it and forget it." The main advantage is that you don't have to manually empty collected oil and gunk, because the system automatically drains clean oil back into the engine. However, it still needs to be installed correctly, with coolant lines properly connected to prevent condensation from turning into sludge in cold weather. It’s a self-sufficient system, but it’s still a critical component that relies on other parts of your engine to function. It’s more "install and enjoy" than "install and forget."

"One system is always better than the other"

There is no universal "best" choice between an AOS and a catch can. The right system for you depends entirely on your vehicle, your driving habits, and your goals. Both are effective at capturing oil vapors and protecting your engine from harmful deposits. An AOS is often the preferred choice for high-performance builds and for owners who want a hands-off system. A catch can is a fantastic and more budget-friendly option for daily drivers and enthusiasts who don't mind performing routine maintenance. If you're unsure, discussing your build with an expert can help you choose the perfect setup for your needs.

Making the Right Choice for Your Subaru

Deciding between an Air Oil Separator and a catch can isn’t about which one is universally better; it’s about what’s best for your car and how you drive it. Your engine's modifications, your driving style, and even the climate you live in all play a part in this decision. Think of it as choosing the right tool for a specific job. Both systems protect your engine from harmful blow-by, but they do it in different ways. Let's break down which system fits your specific Subaru build so you can make a choice you feel confident about.

For High-Performance and Track Builds

If you’re pushing your Subaru on the track or have a high-horsepower build, an AOS is your best bet. During aggressive driving, your engine produces significantly more blow-by. An AOS is a closed-loop system that automatically separates oil from vapor and returns it to the engine, ensuring your oil levels remain consistent. The last thing you want is to worry about a catch can overflowing mid-session. An AOS provides a reliable solution that lets you focus on your lap times, making it an essential part of any serious Subaru STI upgrades or track-focused project.

For Daily Drivers and Lightly Modified Cars

For a daily driven Subaru with stock or light modifications, a catch can is a perfectly practical and effective choice. It does the essential job of capturing oil and other contaminants, preventing them from gunking up your intake valves and lowering your fuel's octane rating. While it requires you to manually empty the can every so often, the maintenance is straightforward for a car that isn't constantly under heavy load. It’s an affordable way to give your engine extra protection without the higher upfront cost of an AOS, making it a popular starting point for many Subaru WRX upgrades.

For Overlanding and Off-Road Builds

When your adventures take you off the beaten path, reliability is everything. For overlanding and off-road builds, an AOS is the clear winner. These long trips into remote areas are exactly where you don't want to be performing routine maintenance like emptying a catch can. An AOS provides a more hands-off solution that handles the varied engine loads and dusty conditions of off-roading without a second thought. It ensures your engine stays protected whether you're crawling over rocks or cruising down a fire road. Investing in an AOS is a smart move for any serious overlanding solutions.

For Cold Climate Driving

If you live where temperatures regularly drop below freezing, an AOS is generally the safer option. The mixture of oil, fuel, and water vapor that a catch can collects can freeze into a solid block of sludge in the winter. This can block your PCV system, leading to a dangerous buildup of crankcase pressure that could blow out your engine seals. An AOS avoids this problem by immediately returning separated oil to the warm engine pan, preventing any liquid from accumulating and freezing. It’s a critical piece of protection for keeping your Subaru’s performance parts safe through harsh winters.

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Frequently Asked Questions

Do I really need one of these for my stock daily driver? Yes, it is a valuable upgrade even for a completely stock Subaru. Modern direct-injection engines are prone to carbon buildup on the intake valves from the factory. A catch can or an AOS is a proactive measure that keeps your intake clean and helps your engine run efficiently for its entire life. Think of it as preventative maintenance that protects your investment, whether you drive a Forester to the grocery store or a WRX on your daily commute.

How often do I actually have to empty a catch can? The frequency depends on your engine's condition, your driving habits, and the climate. A good starting point is to check it with every oil change, or about every 3,000 to 5,000 miles. In colder, more humid weather, you will likely find it fills faster with a milky, watery mixture and may need to be checked more often. After a few checks, you will get a good sense of the right interval for your specific car.

Is an AOS really worth the extra cost over a catch can? The higher price of an AOS reflects its advanced design and convenience. It is a self-contained system that automatically returns oil to the engine, so you never have to empty it. It also uses engine coolant to prevent sludge buildup in winter. A catch can is simpler and more affordable, but it requires you to perform regular maintenance. The AOS is worth the cost if you want a hands-off solution, especially for a track car, an off-road build, or if you live in a cold climate.

I live where it freezes. Is a catch can completely out of the question? While an AOS is the safer and more reliable choice for freezing climates, you can use a catch can if you are extremely diligent. The water vapor collected in the can will freeze solid, which can block the system and create dangerous pressure in your engine. If you choose a catch can, you must check and empty it very frequently during the winter, especially before leaving the car parked overnight in freezing temperatures.

Can I install one of these systems myself? Many enthusiasts successfully install these systems at home. A catch can is generally a more straightforward installation, typically involving mounting the can and connecting two hoses. An AOS is more complex because it requires tapping into your engine's coolant lines in addition to the crankcase ventilation hoses. If you are comfortable working on your car and have the right tools, it is a manageable project. If not, having a professional handle the installation is always a great option.