Introduction: Why AEM WRX Headers?

Upgrading the exhaust headers on your Subaru WRX is one of the most effective bolt-on modifications for unlocking genuine horsepower and torque. The factory headers are designed for cost, emissions, and noise compliance, often using restrictive cast iron or thin-walled manifolds that choke airflow. AEM’s aftermarket headers replace this bottleneck with equal-length, mandrel-bent stainless steel tubing that improves exhaust scavenging, reduces backpressure, and can add 15–25 wheel horsepower with a proper tune. This guide provides a detailed, shop-tested procedure for installing AEM WRX headers safely and correctly, so you see maximum gains without leaks or fitment issues.

The installation is considered moderate to advanced – expect to spend 4–6 hours for your first time. You’ll need basic mechanical skills, patience, and the correct tools. This expanded guide covers every step you’ll encounter, from snapping stubborn bolts to torque sequences. We’ve incorporated tips from professional Subaru technicians and online communities to help you avoid common pitfalls. Let’s get started.

Tools, Materials, and Safety Gear

Having everything prepared before you lift the car saves frustration and prevents mid-job trips to the auto parts store. Below is a comprehensive list based on a typical 2002–2014 WRX (GD/GR chassis); adjustments may be needed for newer FA20 models.

Required Tools

  • Socket set: 10mm, 12mm, 14mm, 17mm deep and standard
  • Torque wrench (3/8″ and 1/2″ drive) – range 10–80 ft-lbs
  • Swivel or universal joint for socket extensions
  • O2 sensor socket (22mm) or flare nut wrench
  • Breaker bar (long handle) for stubborn bolts
  • Penetrating oil (e.g., PB Blaster or Kroil)
  • Gasket scraper or razor blade
  • Wire brush
  • Jack and two jack stands (minimum 3-ton rating)
  • Safety glasses and mechanic gloves
  • Shop towels and a drop pan for coolant/oil if unanticipated

Materials and Parts

  • AEM WRX headers – confirm part number for your chassis (e.g., AEM 28-28800 for 02-07 WRX)
  • New exhaust gaskets – manifold-to-engine, manifold-to-up-pipe, and any cross-pipe gaskets
  • New exhaust donut gasket if applicable
  • High-temperature thread sealant (for O2 sensor threads and studs)
  • Anti-seize compound (copper-based for exhaust bolts)
  • Replacement exhaust studs (if any break during removal)
  • New spring bolts for up-pipe connection (often included with AEM kit)

Tip: Purchase a small assortment of replacement M8 and M10 class 10.9 bolts – Subaru exhaust bolts are prone to rust and snapping, and having spares on hand saves hours.

Preparation Steps – Set Yourself Up for Success

1. Choose the Right Workspace and Safety Setup

Park the WRX on a level, concrete or asphalt surface. Allow the engine to cool completely – header and catalytic converter temperatures can exceed 500°F, risking severe burns. Disconnect the negative battery terminal to prevent accidental fuel pump activation and protect the ECU during sensor disconnection.

2. Raise the Vehicle Securely

Using a jack, lift the front of the car and place jack stands under the factory reinforced jack points (or under the front subframe). For additional clearance, you can also elevate the rear slightly, but the front must be high enough to slide under comfortably. Always rock the car after setting stands to confirm stability.

Safety Note: Never work under a vehicle supported only by a hydraulic jack. Use jack stands rated for your car’s weight (WRX curb weight ~3,200 lbs).

3. Protect Yourself and Your Paint

Wear safety glasses and gloves. Lay a drop cloth or cardboard under the exhaust area to catch dripping fluids or dropped hardware. Remove or cover any plastic undertrays – they often have clips that can be brittle in cold weather.

Removing the Stock Headers – Step-by-Step

4. Remove the Under Engine Cover and Heat Shields

Start by removing the plastic undertray using a 10mm socket (or flathead for push pins). You’ll see the lower portion of the exhaust manifolds. Next, unbolt the heat shields covering the headers. These are usually attached by 10mm or 12mm bolts; some may have spring clips. Work carefully to avoid cracking brittle heat shield metal. Set the heat shields aside – they may be reusable, but many aftermarket headers do not use them, or you can trim them for clearance.

5. Disconnect Oxygen Sensors

Locate the front O2 sensor (in the passenger side downpipe) and the rear O2 sensor (in the up-pipe or third catalytic converter). Using the O2 sensor socket or a correctly sized wrench, break the sensors loose while the exhaust is still relatively solid – the leverage helps more than when the manifold is loose. Clean threads with a wire brush and apply anti-seize before reinstallation. Place the sensors safely out of the way to avoid damaging the wires.

Pro Tip: If an O2 sensor is seized, apply penetrating oil around the threads and let it soak 15–20 minutes. Use a quality 6-point socket to avoid rounding. If it still won’t budge, gentle heat from a propane torch (300-400°F) on the bung can free it.

6. Unbolt the Factory Headers

Working from underneath, you’ll see the header bolts connecting each manifold to the engine block. On a WRX, each side typically has 4 or 5 bolts (M10 x 1.25). Use a 14mm socket with a short extension and a swivel joint for the lower, awkward bolts. Loosen in sequence from the outer bolts inward to reduce warping stress on the flanges. Most bolts come out with a breaker bar, but if any are seized, soak them again and use slow, steady pressure – avoid impact guns that can snap studs.

7. Remove the Up-Pipe Connection

The factory header merges into the up-pipe (the pipe connecting to the turbo). There will be a spring-bolt flange or a standard gasket joint. Remove the nuts or spring bolts. The up-pipe may drop slightly; support it with a jack or a wire hanger to prevent stress on the turbo flange. You may need to disconnect the up-pipe from the turbo as well for clearance – this requires an additional gasket and care to not damage the turbo studs.

8. Drop the Stock Headers

With all bolts removed, gently wiggle each manifold free. They often come out in two pieces (left and right) on older WRXs, or a single piece on later models. Be careful not to damage the A/C lines or steering rack boots. Once out, inspect the engine block flanges – use a gasket scraper to remove all old gasket material. A perfectly clean surface is critical for a leak-free seal.

Installing the AEM WRX Headers

9. Prepare the New Gaskets and Headers

Compare the supplied AEM gaskets with the engine ports. Subaru engines have oval exhaust ports; ensure the gasket matches perfectly. Do not use any sealant on the exhaust gaskets unless specified by AEM – most are coated metal gaskets that rely on compression. For the O2 sensor bungs on the AEM headers (usually one or two), apply high-temperature thread sealant to the sensor threads to prevent exhaust leaks.

10. Install the Driver and Passenger Side Headers

Position the new gasket on the engine studs (or bolt holes). AEM headers typically use studs and nuts rather than bolts. Hand-start the nuts on all studs first, ensuring the header flange sits flush. Tighten in a criss-cross pattern progressively, using your torque wrench set to approximately 60–70% of final spec, then do final pass. Torque specs for Subaru exhaust manifold nuts are typically 28–35 ft-lbs (check your specific year; never exceed 40 ft-lbs on M10 studs). Over-torquing can warp flanges or snap studs.

For the up-pipe connection, use the supplied spring bolts or new hardware. Tighten evenly so the flange sits parallel. Many AEM headers include a flex section to reduce stress; make sure it has room to move without contacting the subframe.

11. Reconnect Oxygen Sensors

Thread the O2 sensors into the bungs on the AEM headers. Use anti-seize sparingly on the threads (only on the sensor thread portion, not the tip). Torque to 30–35 ft-lbs (or per sensor manufacturer spec). Route the sensor wires away from hot surfaces and secure them with zip ties to stock brackets.

12. Reinstall Heat Shields (If Applicable)

Some AEM headers come with their own heat shields; if not, you can often reinstall the factory heat shields by bending them slightly or using spacers. Heat wrap is also an option for track use, but be aware that moisture trapped under wrap can accelerate rust on stainless steel.

Reassembly – Putting It Back Together

13. Reattach Removed Components

Reinstall the up-pipe if you disconnected it from the turbo – use a new gasket and torque the turbo flange nuts to 25–30 ft-lbs. Reconnect any exhaust brackets, hangers, and the downpipe. Ensure the entire exhaust system is aligned and not contacting the chassis (listen for knocking when you shake the system).

14. Check Oil and Coolant Levels

While you were under the car, you may have inadvertently loosened an oil drain plug or coolant hose; double-check all fluid levels. Top off as needed. This is also a good time to inspect the radiator hoses for wear and replace if necessary.

15. Lower the Vehicle and Tighten Final Bolts

Remove jack stands and lower the car. With the suspension loaded, go back under (or access from top) and torque the header-to-block nuts to final spec (again, 28–35 ft-lbs). Reinstall the plastic undertray.

First Start, Testing, and Tuning Considerations

16. Start Engine and Inspect for Leaks

Reconnect the battery negative terminal. Start the engine and let it idle. Listen for ticking or hissing sounds at the header joints. With the engine running, carefully feel around each flange (using your hand about 4–6 inches away – do not touch directly) for hot exhaust gas. Alternatively, use a smoke machine. If a leak is detected, shut off and retighten the bolts after the exhaust cools.

17. ECU Reset and Tuning Requirements

After changing headers, the engine’s air-fuel mixture will be different due to improved exhaust flow. The stock ECU may compensate within its fuel trims, but you will not achieve full power without a tune. Perform an ECU reset (disconnect battery for 30 minutes, then drive through a learn cycle) or better, visit a professional tuner with a dyno. For maximum gains and to prevent lean conditions, we strongly recommend custom tuning via AccessPort or open-source software. See our tuning guide for step-by-step.

18. Test Drive and Monitor

Take the WRX for an easy drive – avoid full throttle until the exhaust system has gone through a few heat cycles, which allows gaskets to seat. Listen for any rattles (header hitting crossmember or steering shaft) and watch for abnormal smoke. If all is well, enjoy the improved throttle response and deeper exhaust note.

Expected Performance Gains and Sound Change

With a proper tune, AEM equal-length headers typically show 15–20 whp and 15–20 wtq increase on a 2.0L or 2.5L WRX, with the torque peak shifting slightly higher in the RPM range. Throttle response sharpens, and the iconic Subaru boxer rumble becomes a more aggressive, slightly higher-pitched tone (equal-length headers reduce the distinct burble due to firing order changes). Some enthusiasts prefer the unequal-length headers for sound, but equal-length like AEM maximize top-end power for forced induction builds.

Conclusion

Installing AEM WRX headers is a rewarding upgrade that delivers measurable performance and an unforgettable driving experience. By following this detailed procedure – paying attention to torque specs, gasket surface preparation, and tuning – you will achieve a leak-free, durable installation that passes the test of time. For additional support, refer to the AEM official product page for your specific kit, and check community resources like NASIOC forums for chassis-specific tips. Always use up-to-date torque specifications for your model year; Boosted Forum’s torque database is a good reference. For tuning requirements, Cobb Tuning AccessPort guides offer excellent base maps. Enjoy your faster, more responsive WRX.