Introduction

Upgrading the headers on your Toyota GR86 (or Subaru BRZ) is one of the most effective modifications for unlocking both performance and a more aggressive exhaust note. The factory exhaust manifold is a restrictive, cast-iron unit designed for emissions and quiet operation. By swapping to a set of aftermarket headers, such as those offered by Mishimoto, you significantly improve exhaust flow, reduce backpressure, and allow the FA24 engine to breathe more freely. This translates to noticeable gains in horsepower and torque, particularly in the mid-to-upper RPM range, along with a deeper, more refined exhaust tone.

Mishimoto offers two primary header options for the GR86: a catted version and an uncatted (catless) version. The catted version retains a high-flow catalytic converter, keeping you legal for emissions testing in most regions while still delivering a solid power increase. The catless version eliminates the catalyst entirely, maximizing flow and providing the loudest, most aggressive sound, but requires a tune to avoid a check engine light and is not street-legal in many areas. Both options are constructed from mandrel-bent T304 stainless steel for durability and thermal efficiency, with thick flanges to resist warping under heat cycles.

In this comprehensive guide, we will walk you through every step of installing Mishimoto headers on your GR86. From gathering the correct tools and preparing the vehicle, to removing the stock manifold, fitting the new headers, and performing final checks, this guide covers all the details needed to complete the job safely and correctly. Whether you are a seasoned DIY mechanic or new to working on your car, following these instructions will help you achieve a professional-quality installation.

Tools and Materials Needed

Having the right tools on hand before you start will save time and frustration. Here is a complete list of what you will need for the installation:

Essential Tools

  • Socket set – 10mm, 12mm, 14mm, and 17mm sockets (both shallow and deep preferred). A 3/8″ drive ratchet with extensions (6″, 12″) is ideal.
  • Torque wrench – 1/2″ drive, capable of reading in ft-lb and in-lb. You will need precise torque values for header bolts and O2 sensors.
  • Combination wrenches – 12mm, 14mm, 17mm for tight spaces where a socket won’t fit.
  • Breaker bar – 1/2″ drive, 18″ long minimum, for loosening stubborn bolts.
  • Jack and jack stands – A low-profile floor jack and two sturdy jack stands rated for at least 3 tons. Never rely on a jack alone.
  • Universal O2 sensor socket – A 22mm crowsfoot or slotted socket makes removing the oxygen sensors much easier.
  • Pick tool set – Useful for unclipping wiring harnesses and prying off rubber hangers.
  • Safety glasses and gloves – Always protect your eyes and hands from debris and sharp edges.

Consumables and Replacement Parts

  • Header gaskets – Mishimoto headers come with new gaskets, but it is wise to purchase an extra set (OEM or high-quality aftermarket) in case of damage during fitting.
  • New exhaust manifold bolts and nuts – While the factory bolts can be reused, they are prone to corrosion and stretching. A fresh set of grade-8 hardware with copper anti-seize is recommended.
  • Copper anti-seize compound – Apply to all bolt threads to prevent galling and make future removal easier.
  • Penetrating oil – Products like PB Blaster or Kroil are essential for loosening rusted fasteners, especially if your car has seen winter driving.
  • Shop towels and degreaser – For cleaning gasket surfaces and any oil leaks before reassembly.

Optional but Helpful Tools

  • Transmission jack or helper – The headers are bulky and awkward to maneuver alone.
  • Heated work lights or propane torch – Expanding the flange bolts with heat can break them loose if they are severely corroded.
  • Exhaust hanger removal tool – A silicone spray and a small pry bar can substitute, but a dedicated tool saves knuckles.
  • Vacuum gauge or smoke tester – For verifying no leaks after installation.

Preparation Steps

Proper preparation is critical for a smooth installation. Rushing this stage often leads to lost hardware, stripped bolts, or injuries. Follow these steps methodically:

Safety First

  • Park the vehicle on a flat, level concrete surface. Asphalt can settle under the jack stands.
  • Disconnect the negative terminal of the battery and wrap it in a shop towel to prevent accidental contact. This avoids short circuits and resets the ECU’s learned fuel trims, which is beneficial when bolting on new headers.
  • Allow the engine to cool completely – ideally overnight. Exhaust components reach high temperatures, and working on them hot increases burn risk and causes warping of flanges if you apply torque unevenly.

Raising the Vehicle

  1. Locate the front-center jack point on the front subframe (a reinforced flat area behind the oil pan). Place the jack pad carefully to avoid the oil pan or aluminum crossmember.
  2. Raise the front of the car until the jack stands can be positioned under the designated lift points – the pinch welds just behind the front wheels. Use rubber-padded jack stand adapters to prevent damaging the pinch welds.
  3. Lower the car onto the stands and verify stability by giving the vehicle a firm shake. The front wheels should be several inches off the ground to allow comfortable access underneath.
  4. If your GR86 is lowered, you may need to use ramps or a low-profile jack to get the jack under the front bumper.

Removing Underbody Panels

Subaru/Toyota engineers installed several plastic and metal underbody panels for aerodynamics and cooling. To access the headers, you need to remove the following:

  • Front undercover – The large plastic tray held by 10mm bolts and push clips. A few clips are fragile; use a trim removal tool to pop them without breaking.
  • Lower engine splash shield – Located directly under the oil pan, secured with 10mm bolts and several push pins.
  • Side undercovers (driver and passenger) – Small plastic panels near the front wheel well openings. Removing these gives additional clearance for header removal and sensor access.

Set all fasteners aside in a labeled bag or magnetic tray. Note the positions of any longer bolts – using them in wrong locations will strip threads.

Disconnect Necessary Components

  • Intake system – Remove the factory intake duct from the airbox to the throttle body (held by hose clamps and a plastic shroud). This is not strictly required for header removal, but it provides more elbow room for accessing the rear header bolts, especially on the passenger side.
  • Oxygen sensors – There are two primary O2 sensors (one on each header, before the catalytic converter) and two secondary sensors (after the converters). You must unplug the connectors from the harness and then unscrew the sensors from the stock headers. The connectors are located on the engine harness near the intake manifold. Use the 22mm O2 sensor socket and plenty of penetrating oil. Be extremely careful – O2 sensor threads are fine and prone to stripping. Do not use impact tools.
  • Engine wire harness brackets – Several wiring clips are attached to the factory heat shielding. Pry them off gently using the pick tool to avoid breaking the plastic tabs.
  • Heated oxygen sensor wiring – Route the wires out of the way, ensuring they won’t be pinched or melted by the new headers.

Removing the Stock Headers

With access clear and everything disconnected, you can begin removing the factory exhaust manifold. This is often the most time-consuming part due to corroded fasteners and tight quarters. Work slowly and methodically.

Loosening the Exhaust Manifold Bolts

  1. Start from the top: use a 14mm socket with an extension to reach the eight bolts connecting the exhaust manifold to the cylinder heads. There are four bolts per side (two per cylinder, upper and lower).
  2. Apply penetrating oil to each bolt 30 minutes before attempting to loosen. If bolts feel stubborn, apply heat with a propane torch to the area around the bolt head (not the bolt itself) for 10-15 seconds.
  3. Use a breaker bar to break each bolt loose. Turn slowly and steadily – sudden jerks can snap a rusty bolt head. If a bolt rounds off, immediately stop and try a six-point socket or a bolt extractor set.
  4. Once loose, remove the bolts by hand. Keep them in order; you may need to reuse some as donators if the Mishimoto kit includes new hardware (most kits include new bolts).

Detaching the Exhaust System

  • Remove the header-to-midpipe connection – Located just below the oil pan, a two-bolt flange joins each header to the front pipe (also called the overpipe or catalytic converter section). Use a 14mm socket for the nuts. They are often very tight; again, use penetrating oil and heat as necessary. You may need to support the midpipe with a jack or a helper to avoid stressing the remaining mounts.
  • Unbolt the midpipe hangers – There are two rubber hangers near the catalytic converters. Lubricate them with silicone spray and pry them off using a hanger removal tool or a large flathead screwdriver (wrap the tip with tape to protect the rubber). Completely free the midpipe from the car and lower it out of the way.
  • Remove the heat shields – The stock headers are covered with stamped metal heat shields. These are bolted to the manifold with 10mm bolts. Removing them is optional, but often necessary to access the O2 sensors and to free the manifold. The heat shields are heavy and awkward; have a helper support them while you unbolt the last fastener.

Extracting the Stock Manifold

With all bolts removed and sensors disconnected, the stock headers should be free. However, they may be stuck due to gasket adhesion. Gently rock the manifold side to side while pulling downward. If it does not move, check for any missed bolts – there is often a hidden bolt near the alternator bracket on the passenger side. Also verify that the O2 sensor wires are fully unclipped. Once free, maneuver the header out from underneath the car. On the driver side, it sometimes helps to guide it out rearwards near the steering shaft. On the passenger side, tilting the header towards the firewall may provide more clearance. Be careful not to damage the coolant lines or the power steering pump.

Cleaning the Engine Block Surface

Thoroughly scrape off all old gasket material from the cylinder head flanges using a plastic razor blade or a gasket scraper. Do not use metal scrapers that could gouge the aluminum. Finish with a rag soaked in brake cleaner. Inspect the head surface for any cracks or warpage – unlikely on a modern engine, but worth verifying. Also clean the mating surface on the midpipe flange.

Installing Mishimoto Headers

Now the rewarding part begins. Take your time during installation to ensure a leak-free seal and proper component alignment.

Prepping the New Headers

  • Remove the Mishimoto headers from the packaging and inspect them for any shipping damage. Check that flanges are flat and the mandrel bends are smooth inside.
  • Test fit the new gaskets on the studs/bolt holes. Mishimoto includes high-quality multilayer steel (MLS) gaskets. Ensure they are oriented correctly – the embossed side usually faces the header flange. If in doubt, consult the included instructions.
  • Apply a thin film of copper anti-seize to the threads of the header bolts and to the mounting studs (if using studs) to prevent seizing.

Bolting on the Heads

  1. Position the new gasket onto the engine block studs (or hold it in place with a dab of grease). Then lift the Mishimoto header into position. This is easier with a helper; the header is heavy and must be aligned without disturbing the gasket.
  2. Start all bolts by hand – both for the cylinder head flange and the midpipe flange. Hand-tighten them until the flanges are seated flush against the gaskets.
  3. Tightening sequence – Use a crisscross pattern (star pattern) for the head flange to ensure even clamping force. For the typical four-bolt-per-side design, tighten in a cross: top left → bottom right → top right → bottom left. Gradually increase torque in two stages:
    • First pass: 15 ft-lb with a torque wrench.
    • Second pass: final torque of 25-28 ft-lb for the head bolts (check Mishimoto instructions; some kits may spec 30 ft-lb).
  4. After torquing the head flange, tighten the midpipe flange bolts. Use a fresh set of nuts (usually 14mm) and torque to 30-35 ft-lb. Apply anti-seize to the threads to prevent future rusting.

Reinstalling Sensors and Harnesses

  • Oxygen sensors – Install the primary O2 sensors into the bungs on the Mishimoto headers. Apply a small amount of anti-seize to the sensor threads (do not get any on the sensor tip). Torque to 25-30 ft-lb – do not overtighten, as sensor housings are delicate. Route the wires through the heat-resistant sleeves provided in the kit, and secure them with zip ties away from the header tubes.
  • Secondary (downstream) O2 sensors – These install near the midpipe connection. Follow the same procedure.
  • Heat shield attachment – Mishimoto headers often come with a heat shield bracket kit. Install the supplied heat shield (if any) using the included hardware. This protects nearby components from radiant heat. If no heat shield is provided, consider aftermarket wrap or ceramic coating – but be aware that wrap can trap moisture and accelerate corrosion on stainless steel.
  • Engine harness brackets – Reattach all wiring clips to the new header or to nearby chassis points using zip ties. Ensure that no wiring contacts the header tubes directly; use high-temp split loom or silicone heat tape for added protection.

Reassembling the Vehicle

With the headers securely installed, it is time to put everything back together. Work in reverse order of disassembly, but pay attention to a few critical details.

Reinstalling the Intake and Underbody Panels

  • Reinstall the intake tube and airbox. Route the O2 sensor wiring carefully to avoid interference with the intake duct.
  • Install the lower splash shield and undercover panels. Use the fastener layout you noted earlier. Overtightening plastic push pins can break them; seat them by hand until they click.
  • Check that all bracket bolts are tight and no tools are left under the hood.

Reconnecting the Battery and Clearing Codes

  • Reconnect the negative battery terminal. Torque the clamp to 6-8 ft-lb.
  • If you are installing a catless header, you will eventually need a tune or an O2 sensor spacer (defouler) to prevent a P0420 code. Have a plan for that; ideally, arrange for a custom ECU flash or use an off-the-shelf tune from a reputable tuner like OpenFlash or ECUTek.
  • Reset the ECU by disconnecting the battery for 10 minutes (if you already did so, no need to repeat). Some prefer to disconnect after installation as well – this forces the ECU to relearn fuel trims with the new airflow.

Lowering the Vehicle

Using the jack, lift the car slightly off the jack stands, remove the stands, and carefully lower the car to the ground. Do not fully lower until you are sure the exhaust does not contact the transmission tunnel or crossmember. If you hear dragging, raise the car again and adjust.

Post-Installation Checks and Testing

Before you take your first spirited drive, perform these checks to ensure everything is safe and functioning.

Leak Detection

  • Start the engine and listen near the header flanges. A hissing sound indicates a leak. Use a stethoscope or a length of hose to pinpoint the location.
  • If you suspect a leak, spray soapy water (or use a smoke machine) around the head flange and midpipe connections. Bubbles will reveal the leak. Immediately shut off the engine if you see a major leak – driving with an exhaust leak can cause poor performance and even melt nearby wiring.
  • Snug up any loose bolts (only after the engine has cooled). Never attempt to tighten a header bolt while the engine is hot – the aluminum threads expand and you risk stripping.

Sensor Function Verification

  • Start the engine and let it idle until the cooling fans cycle on. Watch the dashboard for any warning lights – the Check Engine Light (CEL) for O2 sensor codes, or the traction control light if a sensor is unplugged.
  • If the CEL comes on, read the codes with an OBD2 scanner. Common codes after a header swap: P0420 (catalyst efficiency below threshold) for catless headers, or P0135/P0155 (heater circuit malfunction) if the O2 sensor was damaged during removal.
  • For catless headers, a P0420 is expected without a tune. You can temporarily clear the code, but it will reappear. A tune or a mechanical O2 sensor spacer (such as discussed on FT86Club) can delay or eliminate it.

Test Drive

  • Take the car on a short drive, keeping the RPMs moderate for the first 10 minutes to allow the headers to heat-cycle evenly. Accelerate gradually and listen for any rattles or odd vibrations.
  • Check the sound: you should notice a deeper tone, more pronounced at full throttle. If there is a metallic rattle, the header may be touching the crossmember or the steering shaft – inspect clearances and adjust with pry bars or reposition hangers.
  • After the test drive, park on level ground and allow the engine to cool. Re-torque the head flange bolts to the final spec (25-28 ft-lb) – this accounts for gasket crush. Most manufacturers recommend a re-torque after one heat cycle.

Performance Expectations and Tuning Considerations

Mishimoto headers, whether catted or catless, provide substantial gains over the factory manifold. With a proper tune, you can expect 15-25 whp and 10-20 lb-ft of torque, mostly in the 4000-6500 RPM range. Even without a tune, the headers will improve throttle response and sound, but the full potential is unlocked with ECU calibration.

Catted vs. Catless – Choosing What’s Right

  • Mishimoto Catted Headers – Retain a high-flow catalytic converter, making them street-legal in most states (check local laws). They produce a moderate volume increase and still require a tune to avoid a P0420 code, though some owners find the code does not appear immediately. They are ideal for daily drivers who want performance without the hassle of yearly emissions testing.
  • Mishimoto Catless Headers – Maximum flow and power. The exhaust note is significantly louder, with a raspier edge at high RPM. A tune is mandatory to keep the ECU from going into limp mode due to missing catalytic converter signals. Not legal for road use in many jurisdictions, but popular on track cars.

Tuning Options for the GR86

The FA24 engine in the GR86 responds well to tuning. Popular solutions include:

  • ECUTek – A full ECU flash platform with custom mapping. Many professional tuners offer remote tuning services. A must-have for catless headers.
  • OpenFlash Tablet (OFT) – A more affordable plug-and-play solution with off-the-shelf maps for header upgrades. Requires some user interaction but works well for mild builds.
  • G4+ Plug-in ECU (Link ECU) – For enthusiasts building a heavily modified engine, a standalone ECU provides full control but is overkill for a header-only build.

Investing in a tune not only extracts power but also smooths out the air-fuel ratio curve, preventing lean conditions that could harm the engine. A good tune will also eliminate the cold-start roughness some headers cause.

Common Issues and Troubleshooting

Stubborn Bolts

If a bolt snaps during removal, drill it out using a left-hand drill bit set, or use a bolt extractor. For severely stuck bolts, it may be faster to remove the entire cylinder head, but that’s rarely needed. Use plenty of heat and penetrating oil before attempting to remove any fastener.

O2 Sensor Ceiling

If you break an O2 sensor during removal, carefully drill out the remnants using a reverse-drill extractor kit. Replacing a damaged sensor with a universal Bosch unit is a cost-effective fix, but ensure the connector matches.

Rattling or Banging

After installation, the new headers may contact the engine crossmember or steering intermediate shaft. Loosen the midpipe flange bolts and shift the header slightly while tightening again. If the header touches the frame, a small notch can be made (carefully) in the heat shield or crossmember – but consult a professional if unsure.

Check Engine Light with Catted Headers

Even with high-flow cats, some ECU calibrations trigger a P0420 code after a few hundred miles. Installing an O2 sensor spacer (angled mini-cat) on the secondary sensor can often suppress the code. Alternatively, a custom tune can disable the catalyst efficiency monitor.

Conclusion

Installing Mishimoto headers on your GR86 is a project that yields immediate and satisfying results: sharper throttle response, a more engaging exhaust note, and measurable horsepower gains. While the process requires patience, careful technique, and the right tools, it is well within the capabilities of a home mechanic with some experience. Whether you choose the catted option for daily-driver compliance or the catless version for maximum performance at the track, following the steps outlined in this guide will help you achieve a leak-free, reliable installation.

Remember to always consult the specific instructions provided by Mishimoto for your exact header model, as torque specs and hardware may vary slightly. After the install, consider a professional tune to fully realize the potential of your new headers. For further reading and community experiences, check out the Mishimoto product page or the FT86Club forum, where owners share detailed guides and Dyno charts. Happy wrenching!