Why Upgrade to the APS Cold Air Intercooler?

The factory intercooler on your Subaru WRX is a competent piece of engineering, but it quickly becomes a bottleneck when you push the engine beyond stock power levels. The APS Cold Air Intercooler is designed to dramatically reduce intake air temperatures (IATs), allowing your turbocharger to deliver denser, oxygen-rich air to the combustion chamber. This translates directly into more consistent horsepower, reduced knock risk, and improved throttle response—especially during spirited driving or in warmer climates.

Unlike many "bolt-on" coolers that merely swap cores, the APS kit replaces the entire intercooler assembly, including redesigned end tanks and optimized flow paths. The result is a pressure drop that stays low even at high boost, while heat soak resistance is significantly improved. For WRX owners seeking a reliable, track-ready intercooler that supports stage 2 and beyond, the APS unit is a proven choice. If you want to understand intercooler theory in more depth, Engine Basics offers a solid technical reference.

Tools and Materials Required

Before you begin, gather all tools and supplies. Having everything on hand prevents mid-job delays and ensures a smooth installation.

  • APS Cold Air Intercooler Kit – Verify contents include the core, silicone couplers, T-bolt clamps, and any specific brackets or hardware for your WRX generation.
  • Socket set – Metric sockets from 8mm to 14mm, plus a 10mm deep socket for hard-to-reach bolts.
  • Wrench set – Combination wrenches (10mm, 12mm, 14mm) for nuts and tight spaces.
  • Screwdrivers – Flathead and Phillips; a long-reach flathead helps with hose clamps.
  • Torque wrench – For properly tightening bolts without stripping threads or cracking plastic parts.
  • Air intake cleaner – Non-residue cleaner suitable for silicone and aluminum surfaces.
  • Protective gloves – Mechanic’s gloves to protect hands from sharp edges and hot components.
  • Safety goggles – Essential when cleaning or working under the hood.
  • Optional but recommended – Trim removal tool (plastic pry bar), thread locker (Loctite), and a boost leak tester for post-installation verification.

Preparation Steps

Proper preparation reduces the risk of mistakes and ensures your WRX is ready for the upgrade. Follow these steps closely.

Vehicle Positioning and Safety

Park the car on a flat, level surface in a well-ventilated area. Engage the parking brake and chock the rear wheels. If you lack overhead clearance, consider using ramps or jack stands, though the intercooler installation is largely performed from above.

Battery Disconnection

Disconnect the negative terminal of the battery using a 10mm wrench. This prevents any accidental short circuits and resets the ECU, which will adapt to the improved airflow once the engine is started. Wait at least five minutes after disconnection to allow capacitors to discharge.

Engine Cooling

Intercooler piping and the turbocharger retain heat long after the engine is turned off. Let the engine cool completely—ideally overnight or for several hours. Working on a hot intercooler or turbo can cause burns and may damage silicone couplers during installation.

Removing Obstructions

For some WRX model years, you may need to remove the radiator fan shroud or the air intake duct to access the stock intercooler. Refer to your APS manual for specific vehicle-year notes. If you have an aftermarket intake system, you might need to reposition or remove it temporarily.

Removing the Stock Intercooler

Taking your time during removal prevents damage to surrounding components and makes the installation of the APS unit easier.

Step 1: Locate the Stock Intercooler

The factory top-mount intercooler (TMIC) sits directly on top of the engine, below the hood scoop. It is secured by four bolts (two on each side) and has two main hoses: the turbo outlet hose (driver’s side) and the throttle body inlet hose (passenger’s side).

Step 2: Remove the Bolts

Using a 12mm socket, remove the four bolts that hold the intercooler brackets to the intake manifold. Keep the bolts and brackets organized—you will reuse some of them with the APS kit. If the bolts are rusted, apply penetrating oil and let it sit for a few minutes.

Step 3: Disconnect the Hoses

Loosen the clamps on the turbo outlet hose and the throttle body hose. Use a flathead screwdriver or a 8mm socket (depending on clamp type). Carefully slide the clamps back and pull the hoses off. If they are stuck, gently twist them with a pliers while pulling. Note the orientation and length of each hose for reinstallation reference.

Step 4: Lift Out the Stock Intercooler

Grasp the intercooler firmly and lift it straight up. It may be heavier than expected, especially if it is filled with oil residue. Set it aside on a clean surface—you may want to keep it as a spare or sell it. Inspect the stock unit for cracks or debris; a damaged intercooler could have been causing performance loss.

Step 5: Prepare the Mounting Surface

Clean the top of the intake manifold and the turbo outlet flange with a lint-free rag and air intake cleaner. Remove any oil, dirt, or gasket remnants. This ensures a proper seal for the APS unit.

Installing the APS Cold Air Intercooler

Now the exciting part: fitting the APS cooler. Follow the kit’s instructions closely, but use these expanded steps as a guide.

Step 1: Position the APS Core

Place the new intercooler onto the mounting brackets. Depending on your WRX generation, you may need to install supplied spacer washers or reposition the brackets. Align the core so that the inlet and outlet ports face the correct direction—turbo outlet to the driver’s side, throttle body to the passenger’s side.

Step 2: Reconnect the Hoses

Slide the provided silicone couplers and T-bolt clamps onto the intercooler ports. Connect the turbo outlet hose first, then the throttle body hose. Tighten the clamps evenly—do not overtighten, as this can cut into the silicone. Use a torque screwdriver if available, or hand-tighten until snug plus a quarter turn.

Step 3: Secure the Intercooler

Bolt the intercooler down using the original bolts or those supplied in the APS kit. Apply a drop of blue thread locker to each bolt to prevent loosening from vibration. Torque to the specifications in the APS manual (typically 12–15 ft-lb). Avoid overtightening the brackets, which could crack the plastic end tanks on some versions.

Step 4: Route and Connect Any Accessory Hoses

If your APS kit includes a recirculation valve or boost reference line, connect them now. Ensure no hoses are kinked or pinched. Use zip ties to secure hoses away from hot surfaces like the turbo heat shield.

Step 5: Verify Clearances

Spin the engine by hand (using a socket on the crank pulley) to confirm the intercooler does not contact the fan or hood. Close the hood slowly and check for interference. Some WRX models require trimming the hood scoop duct or removing a foam seal; the APS kit usually includes a replacement duct.

Finalizing the Installation

Once the APS intercooler is physically installed, you must complete a few critical steps before starting the engine.

Reconnect the Battery

Reattach the negative battery terminal and tighten it securely. Reset the ECU by turning the ignition key to the "on" position (without starting) for 10 seconds, then off. This helps the ECU learn the new airflow parameters faster.

Initial Leak Check

Start the engine and let it idle. Listen for hissing sounds that indicate a boost leak. Spray a soapy water solution (1 part dish soap to 3 parts water) around all couplers and connections—if you see bubbles, you have a leak. Tighten the clamps carefully. A boost leak tester (available at auto parts stores) is a worthwhile investment; this guide explains how to build and use one.

Engine Warm-Up and Second Check

Let the engine reach operating temperature (normal coolant temp gauge position). Watch the boost gauge during a gentle first drive. If you have an aftermarket boost gauge, verify that peak boost is consistent and not lower than before—this indicates a possible leak. If boost is higher, the cooler’s lower pressure drop may cause overboost; a custom tune is recommended (see tuning section below).

Monitor Engine Performance

Check for any dashboard warning lights. The ECU may throw a code for a small boost leak or for unmetered air if the MAF sensor was disturbed. Clear codes with an OBD-II scanner if necessary. For detailed trouble codes, consult Subaru’s official support resources.

Testing and Performance Evaluation

A proper test drive is essential to validate the upgrade.

Road Test Procedure

Find a safe, empty road. Accelerate gently through the gears, then perform a medium-throttle pull from 2500 to 5500 rpm. Listen for detonation or pinging—if present, you may need a tune (see below). Record intake air temperatures using an OBD-II app; you should see lower IATs compared to before, especially after repeated pulls.

Boost and Temperature Logging

If you have an AccessPort or similar tuning tool, log boost pressure, IAT, and knock correction. Ideally, peak boost should remain the same or slightly increase (by 0.5–1 psi) after the intercooler upgrade due to reduced restriction. If boost drops significantly, check for leaks. IATs should drop by 15–30°F under sustained load compared to the stock intercooler.

Subjective Feel

Many drivers notice sharper throttle response and less power fade during aggressive driving. The engine feels more willing to rev and holds power longer. If you experience the opposite, revisit the installation—particularly the clamp torque and hose routing.

Maintenance Tips for Longevity

Keeping your APS intercooler in top condition ensures years of reliable performance.

Regular Visual Inspections

Every oil change, inspect the couplers for cracks, the clamps for corrosion, and the core for debris or oil residue. Silicone couplers degrade over time; replace them if they feel sticky or show abrasions.

Cleaning the Core

Use a gentle foam cleaner designed for aluminum radiators and intercoolers. Rinse with low-pressure water (avoid pressure washers that can bend fins). Compressed air can help dislodge embedded bugs or leaves. Never use harsh chemicals that can corrode the aluminum.

Check for Boost Leaks

Over time, clamps can loosen due to thermal cycles. Re-torque all T-bolt clamps once a year or after any maintenance that involves removing the intercooler. A boost leak test every 12 months is cheap insurance against hidden leaks.

Tuning Considerations After Intercooler Upgrade

While the APS cold air intercooler is a direct bolt-on, the increased cooling efficiency and reduced pressure drop can alter your engine’s air/fuel and timing requirements. For stock or stage 1 WRXs, the ECU may adapt, but for stage 2 or higher setups, a custom tune is highly recommended.

Why Tune?

The colder, denser air can increase effective boost pressure by 1–2 psi even if the wastegate duty cycle remains the same. This may push the engine beyond the safe limits of the stock fuel system or knock threshold. A professional tuner can adjust boost targets, fuel maps, and ignition timing to maximize power safely.

Ethanol Blends

With the APS cooler, running E30 or E85 becomes even more effective. Lower IATs combined with ethanol’s cooling effect yield significant knock resistance. However, ensure your fuel system and injectors are up to the task. Resources like Tuning Tech FS offer guides on ethanol tuning for Subarus.

If you are not comfortable self-tuning, seek a reputable Subaru specialist. Look for dyno-proven results and customer reviews. A good tune not only unlocks horsepower but also provides drivability improvements and reliability.

Common Mistakes and How to Avoid Them

Even experienced DIYers can fall into these traps.

  • Overtightening clamps – T-bolt clamps can easily crush thin silicone walls. Use a torque wrench set to 3–4 Nm (25–35 in-lb) for 2-inch couplers.
  • Forgetting to connect the boost reference line – If your APS kit includes a line for the blow-off valve or boost controller, missing it will cause erratic boost.
  • Not trimming the hood duct – Fitting a larger core often requires the hood duct to be cut or replaced. Follow APS’s template precisely; use a rotary tool or sharp knife.
  • Ignoring the need for a tune – Even if the car runs, you risk knocking and limiting power. At a minimum, data-log the first few drives.
  • Using generic hose clamps – The worm-gear clamps found at hardware stores cannot handle boost pressure and will slip. Always use T-bolt clamps rated for silicone hoses.

Conclusion

Installing the APS Cold Air Intercooler on your WRX is a rewarding upgrade that delivers measurable performance gains: lower intake temperatures, reduced pressure drop, and consistent power even in demanding conditions. By following this step-by-step guide—paying careful attention to preparation, torque specs, and leak checking—you can successfully complete the installation in a single afternoon. The investment in a quality intercooler, combined with proper maintenance and a well-calibrated tune, will transform your WRX’s driving character. Enjoy the cooler intake air, the sharper throttle response, and the confidence that your turbocharger is running efficiently. For further reading on intercooler performance comparisons, NASIOC’s intercooler comparison thread (2023) is an excellent community resource.