Introduction

Upgrading the intercooler on your Subaru WRX is one of the most effective modifications for reducing intake air temperatures and increasing engine power. The BorgWarner air-to-water intercooler offers a significant advantage over traditional air-to-air designs by using a liquid coolant to absorb heat more efficiently, especially under sustained boost. This guide provides a comprehensive, step-by-step installation process, covering preparation, removal of the factory system, plumbing, and final checks. Whether you're a seasoned DIY enthusiast or a first-time installer, following this detailed walkthrough will help you unlock the full potential of your WRX’s turbocharged engine.

Understanding Air-to-Water Intercooling

Before diving into the installation, it's helpful to understand why an air-to-water intercooler outperforms a standard top-mount or front-mount air-to-air unit. In an air-to-water system, the charge air passes through a compact core that is cooled by a separate liquid loop. This liquid circuit allows much greater thermal transfer due to the high specific heat capacity of water, and it eliminates the need for long charge air paths that create lag. The BorgWarner unit is engineered with a high-density bar-and-plate core and integrated water jackets that maximize heat exchange in a small package. This design keeps intake temperatures closer to ambient even during repeated hard pulls, making it an ideal choice for track days or spirited driving on Subaru WRX platforms.

Tools and Materials Needed

Gathering the right tools and components before you start will save time and frustration. Below is a comprehensive list:

  • BorgWarner air-to-water intercooler kit (including core, brackets, and hardware)
  • 13 mm, 14 mm, 10 mm socket set and wrenches
  • Torque wrench (range 10–50 ft-lb)
  • Flathead and Phillips screwdrivers
  • Trim panel removal tools
  • Drill with ⅛ inch and ¼ inch drill bits (if pump bracket needs mounting)
  • Hose cutters or sharp utility knife
  • Assorted silicone couplers and T-bolt clamps
  • Coolant – pre-mixed 50/50 distilled water and ethylene glycol antifreeze, or a water/Water Wetter blend for track use
  • Coolant funnel and bleed kit
  • Safety glasses and mechanic gloves
  • Rags and a drip pan
  • Jack and jack stands (optional but helpful for bumper removal)

Some kits may include a dedicated pump and reservoir; if not, source a high-quality 12V water pump and a small expansion tank rated for automotive coolant.

Preparation

Park the car on a level surface and allow the engine to cool completely – the coolant system may be hot and the factory intercooler carries hot charge air. Disconnect the negative battery terminal to avoid accidental electrical shorts. Clear the work area around the front bumper and engine bay. A clean, well-lit garage with room to set aside parts will make the installation smoother. If your WRX has a front strut bar, remove it to access the intercooler area.

Step 1: Remove the Front Bumper

The front bumper must come off to gain access to the intercooler mounting points and to route water lines properly. Follow these detailed steps:

  • Working at the wheel wells, remove the push pins and screws securing the fender liner to the bumper.
  • Under the hood, unbolt the top bumper cover fasteners – usually three or four 10 mm bolts along the radiator core support.
  • From underneath the car, remove the lower under-tray screws (if equipped) to release the bumper bottom.
  • Disconnect any electrical connectors for fog lights, parking sensors, and headlight washers. Unclip them carefully using a trim tool.
  • With a helper, pull the bumper forward and off the guide pins. Set it aside on a soft surface to avoid scratching the paint.

This step also provides an opportunity to inspect the front crash bar and clean any debris from the radiator area.

Step 2: Remove the Stock Intercooler

Subaru WRX models (2002–2014 typically) use a top-mount air-to-air intercooler situated above the engine. Later models with a front-mount may require additional disassembly. For a top-mount setup:

  • Remove the factory intercooler shroud and plastic ducting by gently pulling off clips.
  • Loosen the hose clamps on the turbo outlet pipe and throttle body inlet using a flathead screwdriver or nut driver.
  • Pull the intercooler-to-turbo hose off the intercooler outlet. Use a twisting motion if it is stuck.
  • Unbolt the intercooler mounting brackets – usually two 12 mm bolts on each side.
  • Lift the intercooler out of the engine bay, being careful not to damage the fins. Use rags to cover the turbo opening to prevent debris entry.

If your car has a factory bypass valve attached to the intercooler, disconnect its vacuum line before lifting the unit out.

Step 3: Install the BorgWarner Intercooler Core

With the factory unit removed, you can mount the BorgWarner air-to-water core. Most kits are designed to bolt directly to the OEM hardpoints on the radiator support or crash bar.

  • Position the intercooler core into the intended location – usually in front of the radiator or behind the front bumper where airflow is optimal.
  • Attach the supplied brackets using the provided bolts and lock washers. Do not fully tighten until you verify alignment.
  • Check that the inlet and outlet ports (charge air side) are oriented toward the turbo and throttle body respectively. The water inlet/outlet should face toward the reservoir location.
  • Tighten all mounting bolts to the torque specified in the kit instructions – typically 15–20 ft-lb for M8 fasteners.

For vehicles with AC condenser lines in the way, you may need to slightly bend or reroute the lines. Use caution to avoid kinking or puncturing the AC system.

Step 4: Plumbing the Water Lines

The liquid cooling circuit is critical to the intercooler’s performance. You will install a dedicated pump, reservoir, and hoses to circulate coolant through the BorgWarner core.

  • Mount the water pump near the intercooler, using the provided bracket or drilling small holes into a solid chassis member. The pump should sit below the intercooler’s water outlet to allow gravity draining when the engine is off.
  • Install the expansion reservoir in a high point of the system – typically on the strut tower or along the core support. Ensure the cap is accessible for filling.
  • Run ⅝-inch or ¾-inch heater hose from the intercooler water outlet to the pump inlet. Use a hose cutter for clean ends and secure with clamps.
  • Connect the pump outlet to the reservoir inlet, then run a return hose from the reservoir back to the intercooler water inlet.
  • Route the hoses away from hot exhaust manifolds, sharp edges, and moving components. Use zip ties or split loom to keep them tidy.

Wire the pump to a fused 12V source that is active with the ignition (e.g., the fuse box or a switched relay). Many installers use a simple toggle switch to run the pump continuously during track use.

Step 5: Connect Intercooler Piping

With the water circuit in place, reconnect the charge air piping from the turbo to the intercooler, and from the intercooler to the throttle body. Use the supplied silicone couplers and T-bolt clamps for a leak-free seal.

  • Attach a coupler to the BorgWarner core’s charge air inlet and slide it onto the turbo outlet pipe. Tighten clamps evenly to 30–35 inch-pounds (do not overtighten).
  • Attach the outlet coupler to the throttle body elbow. If the kit includes a new aluminum charge pipe, install that now.
  • Ensure all connections are straight and fully seated. Twisted couplers will bulge and leak under boost.
  • Reinstall any factory intake ducting that routes air to the throttle body, such as the snorkel or ram air tubes.

If your WRX has a blow-off valve or bypass valve, mount it in a suitable location on the charge pipe per the BorgWarner kit instructions. Use a vacuum line from the intake manifold to the valve.

Step 6: Fill and Bleed the Coolant System

Proper fluid selection and bleeding are essential for heat transfer and pump longevity. Use a quality coolant mixture: for street use, a 50/50 blend of distilled water and ethylene glycol antifreeze is fine. For track-only cars, consider a mix of distilled water with a corrosion inhibitor like Water Wetter or Evans Waterless Coolant.

  • Open the reservoir cap and fill with coolant until the level reaches the “Full” mark.
  • Activate the water pump (by turning the ignition on or jumping the relay) while slowly filling – this helps purge air pockets.
  • Tap the intercooler core and hoses lightly to dislodge trapped air.
  • Continue filling until a steady stream returns to the reservoir from the return hose. Top off and close the cap.
  • Start the engine and let it idle for a few minutes while monitoring the coolant level in the reservoir. Add more coolant as needed.

Check for leaks at every hose connection, clamp, and the pump housing. Tighten any weeping clamps.

Step 7: Reinstall the Front Bumper

After verifying the coolant system is sealed and the intercooler piping is secure, reattach the front bumper.

  • Align the bumper mounting tabs with the guide pins on the crash bar.
  • Reconnect all electrical connectors – fog lights, turn signals, and any sensors. Ensure the harnesses are not pinched.
  • Press the bumper into place and reinsert the screws and push pins along the top, wheel wells, and bottom.
  • Reinstall the under-tray and fender liner fasteners.

Double-check that the bumper sits flush and all clips are fully engaged. A loose bumper can cause wind noise and vibration.

Final Checks and Test Drive

Before heading out on the road, perform these final inspections:

  • Boost leak test: Pressurize the charge air system to 20 psi using a boost leak tester. Listen for hissing at Coupler connections, throttle body gasket, and bypass valve. Fix any leaks immediately.
  • Coolant level recheck: Top off the intercooler coolant reservoir after the engine reaches operating temperature and the thermostat opens.
  • Electrical verification: Confirm the water pump runs with ignition on. Some installers wire it to a dash switch or a thermostatic controller.
  • Road test: Drive the car gently for the first few miles, watching the coolant gauge and intake air temperature (if you have a scan tool or aftermarket gauge). Monitor for abnormal noises or coolant loss.

After a short drive, park and re-inspect all hoses and clamps for leaks while the system is hot and pressurized.

Conclusion

The BorgWarner air-to-water intercooler dramatically improves charge air cooling on your Subaru WRX, allowing higher boost levels and more consistent power delivery. By following this step-by-step guide, you have replaced the restrictive factory intercooler with a superior liquid-cooled system. For optimal performance gains, pair this intercooler with a quality tune from a reputable tuner – read our guide on tuning your WRX after an intercooler upgrade to maximize safe horsepower. Additionally, refer to BorgWarner’s official product page for torque specifications and kit-specific updates. With the installation complete, your WRX will stay cooler under pressure and deliver the thrilling driving experience you built it for.