04 WRX Cold Air Intake Filter: A Comprehensive Guide

​Upgrading your 2004 Subaru WRX with a cold air intake filter is one of the most effective modifications for unlocking hidden performance potential.​​ This bolt-on enhancement can yield notable gains in horsepower and torque, improve throttle response, and produce a more aggressive engine sound. For owners of the iconic 2004 Subaru WRX, selecting the right cold air intake system is crucial for ensuring both performance and engine safety. This guide will provide all the necessary information to make an informed decision, covering top brands, installation procedures, and key benefits.

Why Upgrade Your 2004 WRX's Air Intake?

The factory air intake system is designed with cost and noise reduction in mind, often resulting in a restrictive airflow path. A cold air intake addresses this limitation by replacing the stock airbox and filter with a high-flow air filter and a less restrictive intake tube. The primary goal is to allow your engine to "breathe" more easily. Cooler, denser air contains more oxygen, which leads to more efficient combustion and increased power. ​The main advantages of this upgrade include measurable horsepower and torque gains, improved throttle response, and enhanced engine sound.​​ Additionally, many modern intake systems feature reusable air filters, which can be cleaned and re-oiled, offering long-term cost savings over disposable OEM paper filters 1,9.

Many enthusiasts worry about whether an aftermarket intake will void their vehicle's warranty. Reputable manufacturers design their systems to be safe for the engine. Brands like Injen emphasize that their intakes incorporate existing sensors to prevent check engine lights and are engineered to maintain safe air/fuel ratios, often making them acceptable within the boundaries of factory warranties 1. Furthermore, for owners in states with strict emissions standards, several options are available with California Air Resources Board (CARB) Executive Order (EO) numbers, making them 50-state legal for street use 1,12.

Top Cold Air Intake Options for the 2004 Subaru WRX

Several reputable manufacturers offer cold air intake systems specifically engineered for the 2004-2007 Subaru WRX and 2004-2007 STI models. Here are some of the leading choices:

​1. Injen Cold Air Intake (SP1202P / SP1202BLK)​​

Injen is a leader in air intake technology, known for its patented MegaRam (MR) Technology. This technology uses a stepped intake tube design that strategically increases or decreases the tube's diameter to control airflow for optimal performance. The Injen system for the 2004 WRX relocates the high-flow air filter to a position behind the bumper and inner wheel well, where it can draw in cooler air from outside the hot engine bay. ​A key advantage of the Injen system is that it is designed to work safely with the factory tune, preventing check engine lights and maintaining safe air/fuel ratios without requiring an immediate ECU tune.​​ The intake is constructed from 6061-T6 aluminum and includes a cleanable and reusable air filter. It also carries a CARB EO number (D-476-5), making it street legal in all 50 states 1.

​2. K&N Typhoon Intake Series​

K&N is a household name in performance filtration. The Typhoon cold air intake system is designed to dramatically reduce airflow restriction by smoothing and straightening the path of incoming air. It replaces the restrictive factory air filter and housing with a mandrel-bent aluminum intake tube and a large, conical, high-flow air filter. ​The K&N Typhoon intake is known for its significant power gains, with some estimates showing increases of around 15.85 horsepower and 12.2 lb-ft of torque, and its filter is serviceable for up to 100,000 miles before cleaning is needed.​​ The system accommodates the factory mass air flow sensor and includes a water-repellent DryCharger filter wrap for protection against moisture. Potential buyers should verify the specific model's CARB compliance status, as it can vary 2,12.

​3. AEM Cold Air Intake System (21-474B)​​

AEM offers a performance cold air intake system specifically for the 2002-2005 Subaru WRX. This system features a mandrel-bent aluminum tube with a durable powder-coated finish and an oversized AEM DryFlow synthetic air filter. ​A standout feature of the AEM system is its dry air filter, which requires no oiling, eliminating the risk of over-oiling which can contaminate the MAF sensor.​​ The filter is washable and reusable and can last up to 100,000 miles under normal driving conditions. The AEM system is designed to increase horsepower and torque and is backed by a lifetime limited warranty. Like other options, users should confirm the CARB status of the specific part number for their location 8.

Key Considerations Before You Buy

Before purchasing a cold air intake, it's essential to consider a few critical factors to ensure compatibility and performance.

• ​Tuning Requirements:​​ While some intakes, like the Injen SP series, are marketed as not requiring an immediate tune, any modification that significantly alters airflow can affect the engine's air/fuel ratio. For optimal performance and safety, especially if you plan on further modifications, a professional ECU tune is highly recommended. This ensures your engine runs optimally and avoids potential lean conditions that could cause damage.

• ​Dry vs. Oiled Filters:​​ Cold air intakes typically come with one of two filter types: oiled or dry. ​Oiled filters​ (like those from K&N) use a cotton gauze media treated with a special oil to trap contaminants. They offer excellent airflow but can potentially allow excess oil to contaminate the mass airflow (MAF) sensor if over-oiled during cleaning. ​Dry filters​ (like AEM's DryFlow) use a dry synthetic material that captures dirt without oil. They are generally considered safer for MAF sensors and are easier to maintain, though they may have a slightly different filtration profile 8,12.

• ​Quality and Warranty:​​ Invest in a quality product from a reputable brand. Look for intakes made from durable materials like 6061-T6 aluminum and backed by strong warranties, such as the lifetime warranties offered by Injen, AEM, and Mishimoto. This protects your investment and indicates the manufacturer's confidence in their product 1,7,8.

How to Install a Cold Air Intake on Your 04 WRX

Installing a cold air intake is a straightforward DIY project that can typically be completed in 60 to 90 minutes with basic hand tools 9,12. Here is a general overview of the process. Always refer to the specific instructions provided with your intake kit.

​Tools Needed:​​

• Screwdriver (flathead and Phillips)
• Pliers
• Ratchet and socket set
• Allen wrench set 9

​Installation Steps:​​

1. ​Safety First:​​ Park the vehicle on a level surface and ensure the engine is completely cool. Disconnect the negative terminal from the battery to prevent any electrical shorts or accidents 9.

2. ​Remove the Old Airbox:​​ Open the hood and locate the factory airbox. Carefully disconnect the intake air temperature sensor or mass airflow (MAF) sensor from the stock assembly. Loosen the hose clamp connecting the airbox to the intake throttle body. Remove any screws or bolts securing the airbox in place, then lift the entire assembly out of the engine bay 9.

3. ​Install the New Intake:​​ Following the manufacturer's instructions, assemble the new intake tube and filter. Reinstall the MAF sensor into its new location on the cold air intake pipe, ensuring the arrow on the sensor points toward the engine to indicate proper airflow direction. Position the new intake system in the engine bay, connecting it to the throttle body with the supplied couplers and clamps. Secure the intake pipe to the chassis using any provided brackets to prevent movement 9.

4. ​Reconnect and Test:​​ Once everything is tightly secured, reconnect the MAF sensor and any other electrical connectors. Reattach the negative battery terminal. Start the engine and listen for any unusual sounds or air leaks. Let the car idle for a few minutes, then check for any warning lights on the dashboard. A quick test drive will allow you to experience the improved throttle response and new engine sound 9.

Maintenance and Care for Your Cold Air Intake

To ensure your cold air intake continues to perform optimally, regular maintenance is required. The primary task is cleaning the air filter. For oiled filters, this involves a specific cleaning process using a specialized kit that includes a cleaner and a recharge oil. The filter is sprayed with the cleaner, rinsed, dried, and then lightly re-oiled. ​It is crucial to avoid over-oiling the filter, as excess oil can be drawn onto the MAF sensor, causing inaccurate readings and potential engine issues.​​ Dry filters are simpler to maintain, typically requiring only a gentle cleaning with soap and water or a specific dry filter cleaner, and no oiling is needed 8,9,12. Always check the manufacturer's recommendations for the specific cleaning interval, but a general guideline is every 25,000 to 50,000 miles, depending on driving conditions.

Conclusion

Upgrading to a cold air intake filter is a smart and effective first modification for any 2004 Subaru WRX owner. The benefits are tangible: more power, sharper throttle response, a more aggressive engine note, and the potential for improved fuel economy under light throttle conditions. By choosing a high-quality system from a reputable brand like Injen, K&N, or AEM, you can ensure a safe and reliable upgrade that respects your engine's needs. With the installation being a manageable task for most DIY enthusiasts, this mod offers a high reward for a relatively low investment of time and money, truly unleashing the breathing potential of your turbocharged boxer engine.