The HKS Advantage: Why Upgrade Your GR86 Headers?

The stock exhaust manifold on the GR86 is a cast-iron log-style unit designed for cost-effectiveness and noise suppression. Swapping it for a set of HKS performance headers is widely regarded as the most impactful single modification for the FA24 engine. Replacing the restrictive factory manifold with a properly designed set of equal-length headers reduces backpressure, improves exhaust scavenging, and lowers under-hood temperatures. The result is a measurable increase in horsepower and torque, typically peaking around 15-20 wheel horsepower, alongside a significantly more aggressive and refined exhaust note. Furthermore, the HKS header is substantially lighter than the stock manifold, contributing to a noticeable reduction in front-end weight. If you are looking to unlock the true potential of your GR86, installing an HKS header is an essential first step.

Selecting the Right HKS Header Model for the FA24

HKS offers several header variations for the GR86 and Subaru BRZ. Understanding the differences is key to matching the product to your performance goals and local regulations.

Catless vs. Catted Options

HKS primarily offers catless header options for maximum performance. These headers completely replace the restrictive factory catalytic converters (located in the manifold) with straight-through tubing, resulting in the highest power gains and the most aggressive sound. However, catless headers will trigger a Check Engine Light (CEL) for catalytic converter efficiency (P0420) unless a tune is applied. They also produce a stronger fuel odor and may not pass visual emissions inspections. For track use and dedicated performance builds, catless is the standard. HKS also produces a GT Spec header which features a high-flow metal catalyst, balancing performance improvements with street legality and reducing the likelihood of a CEL without a tune.

4-2-1 (High-Power Spec L) vs. 4-1 (Super Turbo)

  • HKS High-Power Spec L (4-2-1): This design merges the four primary tubes into two intermediate pipes before merging into a single collector. The 4-2-1 configuration is designed to broaden the torque curve across the mid-range, making it an excellent choice for street driving and autocross where response and mid-range punch are prioritized.
  • HKS Super Turbo (4-1): The 4-1 design merges all four primary tubes into a single collector simultaneously. This configuration typically favors top-end horsepower and is often preferred for track driving and high-rpm racing applications. The 4-1 design can sometimes sacrifice a small amount of low-end torque for significant peak power gains.

Both options are CNC-machined and TIG-welded from high-quality stainless steel, ensuring durability and a perfect fit. Carefully consider your driving needs and local regulations when choosing your HKS header. You can explore the specific product details and official documentation on the HKS official website.

Tools and Components Checklist

Before beginning the installation, gather all necessary tools and components. Having everything on hand will streamline the process and prevent frustrating interruptions.

Essential Parts

  • HKS Performance Header (ensure it matches your specific year and model)
  • New Exhaust Gaskets (header-to-head gaskets and header-to-overpipe gasket – some HKS kits include these, but it is wise to have spares)
  • Exhaust Nuts and Studs (M8 and M10; it is highly recommended to purchase new genuine Subaru or high-temperature alloy nuts)
  • Anti-Seize Compound (copper-based for exhaust components)
  • Penetrating Oil (e.g., PB Blaster, Liquid Wrench, or Kroil)

Required Tools

  • Socket Set (3/8" and 1/2" drive, including 10mm, 12mm, 14mm, 17mm, and 22mm sockets)
  • Deep Socket Set (12mm for header stud nuts)
  • Oxygen Sensor Socket (22mm offset or crowfoot wrench)
  • Torque Wrench (able to read in ft-lbs and in-lbs)
  • Breaker Bar (long handle for breaking stubborn bolts)
  • Ratchet Extensions (3", 6", and 12")
  • Wrench Set (combination wrenches in 12mm, 14mm, and 17mm)
  • Jack and Jack Stands (minimum 2-ton capacity)
  • Wheel Chocks
  • Trim Removal Tools (for plastic undertray clips)

Safety Equipment

  • Safety Glasses (to protect from debris and penetrating oil)
  • Mechanic's Gloves (to protect hands from heat and sharp edges)
  • Fire Extinguisher (rated for Class B and C fires)

Preparation and Safety Protocols

Proper preparation is the foundation of a successful header installation. Rushing this phase can lead to personal injury or damage to your vehicle.

  1. Workspace Setup: Park your GR86 on a perfectly level, hard surface. Concrete is ideal. Allow the engine and exhaust system to cool completely. Working on a hot exhaust system is dangerous and can result in severe burns or stripped hardware.
  2. Vehicle Securement: Loosen the rear wheel lug nuts, then jack up the front of the vehicle using the front center jacking point. Place jack stands under the reinforced front pinch weld jacking points. Lower the vehicle onto the stands. Chock the rear wheels securely.
  3. Battery Disconnect: Disconnect the negative terminal of the battery. This is a critical safety step to prevent any electrical shorts while working near the engine bay and O2 sensors. It also resets the ECU, allowing it to relearn fuel trims with the new headers.
  4. Prepare the Hardware: Liberally spray the exhaust header bolts, O2 sensors, and overpipe connection nuts with penetrating oil. Allow the oil to soak for at least 15-20 minutes before attempting to loosen any fasteners. This will drastically reduce the risk of snapping a stud.

Step 1: Removing the Stock Exhaust Manifold

This is often the most time-consuming part of the job due to the tight tolerances and corrosion. Patience and careful technique are essential.

Disconnect the Oxygen Sensors

Locate the primary (Air-Fuel ratio) sensor in the stock manifold and the secondary (Lambda) sensor downstream. Use the 22mm O2 sensor socket and a ratchet or wrench to carefully loosen them. If they are stuck, apply more penetrating oil and use a breaker bar. Unplug them from the engine wiring harness. The connectors are located on the intake manifold and the transmission bell housing. Be gentle with the locking tabs.

Remove the Undertray and Heat Shielding

Using the trim removal tools or a flathead screwdriver, remove the clips securing the engine undertray. Set the undertray aside. You may also need to remove a metal heat shield covering the stock manifold. This usually requires a 10mm or 12mm socket.

Unbolt the Header from the Cylinder Heads

The stock manifold is held to the cylinder heads by 8 nuts (two per cylinder, ranging from 12mm to 14mm depending on the revision). Using your deep socket and breaker bar, break the torque on these nuts. Do NOT remove them completely yet; loosen each one by a few turns in a spiral pattern (outer to inner). This prevents warping the manifold flange if it is under tension. Once all nuts are loose, remove them by hand.

Unbolt and Remove the Stock Header

The stock header connects to the exhaust system via the overpipe. Locate the two spring bolts or standard bolts at this connection. Remove them. The stock manifold is heavy and awkwardly shaped. It will need to be maneuvered out from the bottom of the engine bay, between the subframe and the oil pan. Some gentle wiggling and rotating are required. It helps to have a second set of hands to guide it out without damaging the O2 sensor wiring or the engine bay components. Once free, compare the weight of the stock unit to your new HKS header – the difference is immediately noticeable.

Step 2: Preparing the New HKS Header for Installation

Before lifting the new headers into place, some pre-installation prep work ensures a smooth process and a leak-free seal.

  • Transfer the O2 Sensors: Thread your primary and secondary O2 sensors into the bungs on the HKS header. It is critical to apply a generous amount of anti-seize compound to the threads of the O2 sensors. This will ensure they can be removed easily in the future and helps ensure a proper ground connection. Torque them to spec (typically 30-35 ft-lbs).
  • Inspect the Headers: Hold the HKS header up to the light and look down each primary tube. Ensure there are no obstructions or debris from manufacturing.
  • Install New Gaskets: Place the new multilayered steel (MLS) gaskets onto the exhaust studs on the cylinder heads. Ensure they are oriented correctly (they usually have an "UP" or "FRONT" marking). If the HKS header uses a different gasket style for the collector, install it now.

Step 3: Installing the HKS Header

This step is largely the reverse of removal, but with a much lighter and more precisely manufactured component.

Positioning the Header

Unlike the stock manifold, the HKS header is lightweight and easy to handle. It is typically easier to install from the bottom of the vehicle. Guide the header up into the engine bay, aligning the primary tubes with the exhaust ports on the cylinder heads. Ensure the O2 sensor wiring is routed carefully to avoid contact with the header tubes or the steering shaft.

Securing the Header to the Heads

Start all 8 nuts by hand. It is highly recommended to use new, high-quality flange nuts. Once all nuts are started, use your torque wrench to tighten them in a specific crisscross pattern.

  1. Tighten all nuts to 8 ft-lbs.
  2. Tighten all nuts to 16 ft-lbs.
  3. Final torque all nuts to the factory specification of 26 ft-lbs.

This staged torquing process ensures even compression of the gaskets and prevents leaks. Over-torquing can warp the header flange or damage the studs.

Reconnecting the Overpipe

The HKS header is designed to bolt directly to the stock overpipe. Align the collector flange with the overpipe flange. Install the new gasket (if provided with the header) and secure the connection using the factory spring bolts or standard bolts with the gasket. Ensure it is aligned perfectly before tightening. Torque these bolts to 35-40 ft-lbs.

Step 4: Reassembly and Final Checks

  • Reinstall Heat Shielding: If you removed any engine bay heat shielding, reinstall it now. Some aftermarket headers allow you to reuse the factory shields; some do not. Check fitment carefully to avoid rattles.
  • Reinstall the Undertray: Secure the plastic undertray using the factory clips.
  • Lower the Vehicle: Carefully jack up the front of the car, remove the jack stands, and lower it to the ground.
  • Tighten Lug Nuts: Torque the wheel lug nuts to 89 ft-lbs.
  • Reconnect the Battery: Reconnect the negative battery terminal. This resets the ECU, which is beneficial for the adaptation process we will discuss next. For a detailed visual reference of the installation process, forums like GR86.org provide excellent community-driven walkthroughs.

Step 5: First Startup, Heat Cycle, and Leak Inspection

This is the moment of truth. A careful and methodical first start minimizes risk and identifies potential issues immediately.

Priming and Starting

Turn the ignition key to the "ON" position (without cranking) for about 5 seconds to prime the fuel pump and oil system. Then, start the engine. It may idle higher than normal for the first 30 seconds as the ECU recalibrates its idle air control.

Checking for Exhaust Leaks

Let the engine idle. Listen carefully for any hissing or ticking sounds around the header flanges and the overpipe connection. A small leak will sound like a rapid tick. If you suspect a leak, carefully (the engine is hot!) place your hand near the connection points. Better yet, use a mechanic's stethoscope or a piece of rubber hose to pinpoint the sound. If a leak is detected, shut off the engine, allow it to cool, and re-torque the offending bolts.

The Heat Cycle

Allow the engine to reach full operating temperature (coolant temperature stabilized at ~190°F). Keep the engine under 3000 RPM. Once up to temperature, let the engine idle for another minute, then shut it off. Allow the engine to cool completely. This heat cycle allows the metal components to expand and contract, seating the gaskets thoroughly. After the cooling period, re-check the torque on the header nuts and overpipe bolts. You will often find they require a small additional turn to reach the final spec, accounting for the gasket compression.

ECU Adaptation and Tuning Considerations for the FA24

While the GR86's ECU can adapt to minor changes, installing an HKS header, particularly a catless one, requires attention to software calibration.

OEM ECU Adaptation

The stock ECU will attempt to compensate for the increased airflow and reduced backpressure. It will likely adjust short-term fuel trims (STFT) and long-term fuel trims (LTFT). However, it cannot fully optimize the ignition timing or variable valve timing (VVT) to take advantage of the header's flow characteristics. You will most likely trigger a P0420 code (Catalyst System Efficiency Below Threshold) with a catless header, illuminating the Check Engine Light.

The Case for Professional Tuning

To fully realize the performance potential of your HKS headers and avoid the check engine light, an ECU tune is highly recommended. Platforms like Ecutek or the OpenFlash Tablet (OFT) allow tuners to remap the fuel, ignition, and throttle maps. A proper tune for your HKS headers can yield another 10-15 wheel horsepower over the off-the-shelf gains, optimize the throttle response, and even eliminate the P0420 code. Tuning is the key to transforming a good modification into a great one, ensuring reliability and maximizing your return on investment.

Documenting the Gains: Performance and Sound

With the installation complete and the ECU calibrated, you can now enjoy the fruits of your labor. The difference is dramatic, especially when paired with a cat-back exhaust system.

  • Horsepower and Torque: Expect peak gains of 15-25 wheel horsepower and 12-18 ft-lbs of torque. The torque curve is noticeably flatter and broader, pulling hard from 3500 RPM all the way to redline.
  • Throttle Response: The reduction in backpressure makes the engine feel significantly more responsive. The FA24 feels freer and more willing to rev.
  • Exhaust Note: The HKS header introduces a deeper, more aggressive tone. The distinct Subaru boxer rumble is replaced by a higher-pitched, almost exotic shriek at high RPM, characteristic of equal-length headers. The sound is purposeful without being overwhelmingly loud if the rest of the exhaust remains stock.
  • Weight Reduction: The stock cast-iron manifold weighs approximately 25-30 lbs. The HKS stainless steel header weighs roughly 8-10 lbs, saving over 15 lbs from the front of the car, which improves handling dynamics.

Conclusion

Installing HKS headers on your GR86 is a rewarding project that delivers a tangible and exhilarating performance upgrade. While the installation demands mechanical aptitude and careful attention to detail, the step-by-step process is well within the capabilities of a dedicated DIY enthusiast. The resulting improvements in power, sound, and throttle response transform the character of the car, unlocking the true potential of the FA24 engine. By following this guide, using the correct tools, and respecting the need for proper ECU calibration, you can confidently boost your GR86's performance and enjoy a more engaging driving experience every time you get behind the wheel.