Why You Need an IAG Air/Oil Separator (AOS) for Your Subaru

What Is an Air/Oil Separator and Why Does Your Subaru Need One?

Every internal combustion engine produces what's known as blow-by—a mixture of combustion gases and oil vapor that escapes past the piston rings and into the crankcase. Your Subaru's factory PCV (Positive Crankcase Ventilation) system is designed to route these gases back into the intake to be burned off. The problem? It brings oil vapor along for the ride.

The Blow-By Problem in Subaru Boxer Engines

Subaru's horizontally-opposed boxer engines are particularly susceptible to blow-by issues. The horizontal cylinder orientation means oil can more easily find its way into the PCV system. As your engine ages, accumulates miles, or gets pushed harder through modifications, the amount of blow-by increases significantly.

When oil vapor enters your intake tract, it coats everything in its path:

• Intercooler: Oil reduces cooling efficiency and degrades rubber/silicone couplers over time
• Intake manifold: Oil buildup restricts airflow and creates carbon deposits
• Intake valves: Carbon accumulation causes misfires, rough idle, and reduced performance
• Combustion chamber: Oil in the combustion process can trigger detonation, pre-ignition, and knock

What Can Go Wrong Without an AOS?

The consequences of unchecked oil vapor range from annoying to catastrophic:

• Excessive smoke from the exhaust
• Power loss and reduced fuel efficiency
• Increased emissions and failed inspections
• Premature wear on turbocharger seals
• Detonation and pre-ignition leading to ringland failure
• Complete engine failure in severe cases

Why the Factory PCV System Isn't Enough

Subaru's OEM PCV system was engineered for stock power levels and normal driving conditions. It simply routes crankcase gases back to the intake without any meaningful oil separation. For a bone-stock commuter car driven gently, this may be acceptable. But the moment you add modifications, drive spiritedly, or put your Subaru through track days or autocross, the factory system becomes overwhelmed.

An Air/Oil Separator intercepts these PCV gases before they reach your intake, separates the oil from the air, and returns the clean air to the intake while draining the captured oil back to your crankcase. It's a simple concept with profound benefits for engine longevity and performance.

Key Benefits of the IAG AOS: More Than Just a Catch Can

If you've researched oil separation solutions, you've probably come across basic catch cans. While they're better than nothing, the IAG Air/Oil Separator is engineered to a completely different standard. Here's what sets it apart and why it's become the gold standard for Subaru enthusiasts.

Maintenance-Free Operation

Traditional catch cans require regular draining—sometimes as often as every oil change or even more frequently on heavily modified cars. Forget to drain it, and you risk overfilling the can and pushing oil right back into your intake, defeating the entire purpose.

The IAG AOS eliminates this hassle entirely. Separated oil is automatically returned to your crankcase via a dedicated drain line. Install it and forget it—there's no maintenance schedule to remember, no messy draining sessions, and no risk of overfilling.

Heated Design Prevents Sludge Buildup

One of the biggest problems with catch cans and poorly designed separators is sludge. When hot oil vapor meets a cold canister, condensation forms and mixes with the oil, creating a thick, mayonnaise-like sludge that clogs the system and reduces effectiveness.

IAG solved this with an integrated coolant-heated design. By routing engine coolant through the AOS housing, the unit stays warm during operation. This prevents condensation, keeps oil flowing freely, and ensures the separator works at peak efficiency regardless of ambient temperature or climate.

Advanced Baffle System for Superior Separation

Inside the IAG AOS is a precision-engineered multi-stage baffle system. As blow-by gases enter the unit, they're forced through multiple directional changes and expansion chambers. This process causes oil droplets to fall out of suspension and collect at the bottom of the housing while clean air exits to the intake.

The result is dramatically cleaner air entering your engine compared to a simple catch can with a filter element or basic baffle. IAG's design has been refined over years of real-world testing on everything from daily drivers to 1,000+ horsepower race builds.

Compatible with All Fuel Types Including Ethanol

Running E85 or a flex-fuel setup? The IAG AOS is fully compatible. In fact, it's even more beneficial for ethanol users. E85 can contribute to fuel dilution—where unburned fuel washes past the piston rings and contaminates your oil. By separating these contaminants and returning them to the crankcase (where they eventually burn off), the AOS helps protect your oil quality and extend oil change intervals.

Lightweight Cast Aluminum Construction

Built from high-quality cast aluminum, the IAG AOS is both durable and lightweight. The housing is designed to withstand the heat and vibration of the engine bay while adding minimal weight to your build. A precision-machined finish and quality anodizing ensure long-term corrosion resistance.

IAG AOS vs. Basic Catch Cans: A Quick Comparison

When you compare the IAG AOS to budget catch cans, the difference is clear. It's not just an upgrade—it's a fundamentally better approach to oil separation that's purpose-built for Subaru engines.

Who Should Buy an IAG AOS? Stock Engines vs. Built Engines

One of the most common questions we hear at SubiMods is: "Do I really need an AOS if my car is stock?" The short answer is yes—but let's break down exactly who benefits most and why the IAG AOS makes sense at every power level.

Stock Engine Owners: Cheap Insurance for Your Investment

Think of the IAG AOS as an insurance policy for your engine. Even a completely stock WRX or STI produces blow-by from day one. As miles accumulate, piston ring seal degrades slightly, and blow-by increases. By the time you hit 60,000–80,000 miles, a stock Subaru turbo engine is producing significantly more blow-by than when it left the factory.

Here's why stock engine owners should seriously consider an AOS:

• Preventative protection: Stop oil contamination before it causes damage, not after
• Preserve resale value: A cleaner engine with documented preventative maintenance commands higher prices
• Extend engine life: Reducing carbon buildup and detonation risk helps your engine last longer
• Maintain performance: Keep your intake tract clean for consistent power delivery
• Peace of mind: Know that your engine is protected during spirited driving

Modified Stage 1 and Stage 2 Builds: A Must-Have Upgrade

Once you start adding power modifications—intake, downpipe, exhaust, and a tune—an AOS transitions from "nice to have" to essential. Here's why:

• Increased cylinder pressures: More boost means more stress on piston rings, which means more blow-by
• Higher operating temperatures: Performance driving generates more heat, accelerating oil vapor production
• Tuning sensitivity: Oil in the intake can cause knock events that force your tune to pull timing, costing you power
• Intercooler efficiency: Oil coating your intercooler reduces its ability to cool charge air effectively

At the Stage 2 level (roughly 300–350 WHP on most platforms), the IAG AOS should be considered mandatory. The relatively small investment protects thousands of dollars in performance parts and tuning—not to mention your engine itself.

Built Engines and High-Horsepower Builds: Absolutely Critical

If you've invested in a built engine—forged internals, upgraded turbo, fuel system, and a professional tune—skipping an AOS would be a serious oversight. At power levels above 400 WHP, blow-by increases dramatically, and the consequences of oil contamination become even more severe.

Built engine owners benefit from the IAG AOS in several key ways:

• Protect your investment: Built engines cost $5,000–$15,000+. An AOS is a fraction of that cost.
• Consistent tune stability: Clean air means predictable combustion and reliable power delivery
• Track and competition reliability: Extended high-RPM operation produces maximum blow-by—exactly when you need protection most
• Flex fuel compatibility: Many high-power builds run E85; the AOS helps combat fuel dilution

Daily Drivers, Weekend Warriors, and Track Cars

Your driving style also factors into the equation:

The Bottom Line: It's Worth It at Any Level

Whether you're running a stock daily driver or a 600+ horsepower track monster, the IAG AOS provides measurable benefits. The cost of an AOS is insignificant compared to an intercooler replacement, intake cleaning, or—worst case—an engine rebuild. It's one of the few modifications that pays for itself by preventing problems rather than chasing performance numbers.

If you plan to keep your Subaru long-term, drive it hard even occasionally, or have any power modifications, the IAG AOS belongs on your car.

Exclusive SubiMods Edition Colors: Stand Out Under the Hood

Performance matters—but so does presentation. When you pop the hood at a car meet, cars and coffee, or the track paddock, your engine bay tells a story. That's why SubiMods partnered with IAG Performance to offer exclusive color options you won't find anywhere else.

Why Color Matching Matters

The Subaru community takes engine bay aesthetics seriously. Whether you're building a clean OEM+ look, a full color-matched show car, or a purpose-built track weapon with a cohesive theme, every component contributes to the overall presentation. The IAG AOS sits prominently in the engine bay—right where everyone looks first.

Instead of settling for a generic finish that clashes with your build, SubiMods exclusive editions let you:

• Match your exterior color: Coordinate with WR Blue, Pure Red, or other Subaru colors
• Complement your mod theme: Match other anodized components like oil caps, dipsticks, and dress-up hardware
• Stand out from the crowd: Own a rare, limited-availability color that sets your build apart
• Show attention to detail: Demonstrate that every component was chosen intentionally

SubiMods Exclusive Color Options

While standard IAG AOS units come in black or silver finishes, SubiMods offers exclusive anodized colors that are only available through us. These limited-edition finishes feature the same high-quality anodizing process as the standard units—durable, corrosion-resistant, and built to withstand the harsh engine bay environment.

Available exclusive colors include:

• SubiMods Blue: A perfect complement to WR Blue Pearl and other blue Subarus
• SubiMods Red: Ideal for Pure Red, Lightning Red, or red accent builds
• SubiMods Purple: A bold choice for unique builds and Plasma Blue Pearl exteriors
• SubiMods Pink: Stand out with a head-turning finish perfect for themed builds

Same IAG Quality, Exclusive Finish

It's important to understand that SubiMods exclusive colors are not aftermarket knock-offs or cosmetic covers. These are genuine IAG Performance Air/Oil Separators with all the same features and quality you'd expect:

• Coolant-heated housing to prevent sludge
• Advanced multi-stage internal baffle system
• Automatic oil return to crankcase
• Direct bolt-on fitment for your specific Subaru platform
• Full IAG warranty and support

The only difference is the exterior anodized finish—everything under the surface is 100% IAG Performance engineering.

Completing the Look: Matching Accessories

Once you've chosen your AOS color, consider coordinating other engine bay components. SubiMods carries a full range of dress-up accessories in matching colors:

• Anodized oil filler caps
• Billet dipstick handles
• Colored silicone vacuum lines
• Anodized battery tie-downs and dress-up hardware
• Radiator shrouds and coolant expansion tanks

Building a cohesive engine bay is all about the details. Starting with a statement piece like a color-matched IAG AOS sets the foundation for a build that looks as good as it performs.

Final Thoughts: Protect Your Engine the Right Way

The Subaru EJ and FA engines are legendary for their performance potential—but they're also known for their sensitivity to oil contamination in the intake tract. Whether you're running a stock daily driver, a tastefully modified Stage 2 build, or a full-tilt track monster, the IAG Air/Oil Separator is one of the smartest investments you can make.

Unlike basic catch cans that require constant maintenance and offer questionable effectiveness, the IAG AOS delivers:

• True maintenance-free operation with automatic oil return
• Heated design that prevents sludge in any climate
• Superior separation efficiency from an advanced baffle system
• Direct bolt-on fitment engineered specifically for your Subaru

And with SubiMods exclusive colors, you don't have to sacrifice style for function. Protect your engine while making a statement under the hood with a finish that matches your build perfectly.

At the end of the day, an AOS costs a fraction of what you'd spend on a single engine repair caused by oil contamination. It's not a question of if you need one—it's a question of how long you want to wait before giving your engine the protection it deserves.