Why Upgrade to Mishimoto Headers?

Mishimoto headers are a popular upgrade for Mazda Miata owners because they reduce exhaust backpressure, lower under-hood temperatures, and sharpen throttle response. The factory exhaust manifold on NA, NB, and NC Miata models is restrictive and heavy. Mishimoto’s stainless steel headers, with mandrel-bent tubes and a smooth collector, allow exhaust gases to exit the engine more efficiently. This typically yields a 5–8 horsepower gain on naturally aspirated engines and more when combined with a full exhaust system. The sound also deepens, with a more aggressive note at high RPMs. While the installation is a moderate DIY job, careful attention to hardware and torque specs ensures a leak-free, long-lasting result.

Tools and Materials

Gather everything before you start to avoid mid-job trips to the hardware store. Having the right sockets and a torque wrench is essential for tightening header bolts to specification.

  • Socket set with 10mm, 12mm, 14mm, and 17mm sockets (deep and shallow)
  • Ratchet with a 3/8” drive and an extension bar (6” and 12”)
  • Torque wrench capable of 20–50 ft-lb range
  • Jack and two jack stands rated for your vehicle weight
  • Penetrating oil (like PB Blaster or WD-40 Specialist) for rusted bolts
  • New header gaskets – Mishimoto usually includes them, but confirm; if not, buy OEM or Fel-Pro quality gaskets
  • Anti-seize compound (copper- or nickel-based)
  • Exhaust gasket for the header-to-midpipe connection
  • Safety glasses and mechanic’s gloves
  • Shop rags and wire brush for cleaning mating surfaces
  • Optional but helpful: oxygen sensor socket, thread chaser set, impact driver for stubborn bolts

Preparation

Proper preparation reduces frustration and risk. Park the Miata on a level concrete floor, let the engine cool completely (at least one hour after driving), and disconnect the negative battery terminal. This prevents accidental shorts and also resets the ECU’s adaptive fuel trims, which helps the engine adjust to the new headers.

Jack up the front of the car and place jack stands under the front subframe lift points (the reinforced sections behind the front wheels). Never rely on the jack alone. For extra clearance, you can also remove the front wheels, but it’s not mandatory. Wear safety glasses when working under the car – rust and dirt will fall into your eyes.

Spray penetrating oil on all exhaust manifold bolts, the EGR tube (if present on earlier models), and the O2 sensor threads. Let it soak for 10–15 minutes while you remove the underbody splash shield. On most Miatas, this is a single plastic panel held by 10mm bolts and some push clips. Set the shield aside.

Removing the Factory Exhaust Manifold

The factory manifold consists of a cast iron exhaust manifold and a heat shield. You’ll remove both.

Remove the Heat Shield

  • Unbolt the stamped metal heat shield using a 10mm socket. It typically has three bolts on top and two on the side near the transmission.
  • If the bolts are rusted, use the penetrating oil and a breaker bar gently – the metal shield can bend easily.
  • Set the shield aside; you may reuse it with the new headers or discard it (Mishimoto headers are often ceramic-coated and don’t need it, but check local laws).

Disconnect the Exhaust System

  • Loosen the two nuts/bolts that connect the downpipe to the catalytic converter or midpipe (usually 14mm).
  • If your car has a front O2 sensor, unplug its connector near the firewall and unscrew the sensor from the manifold using an O2 sensor socket or a 22mm wrench. The sensor may be tight; penetrating oil and a few thermal cycles help.
  • Support the catalytic converter or midpipe with a jack or jack stand so it doesn’t hang from the rubber hangers.

Unbolt the Manifold from the Cylinder Head

  • There are either six or eight manifold nuts holding the iron manifold to the head (depending on year). Use a 12mm or 14mm socket with an extension to reach the rear nuts near the firewall.
  • Break them loose in a crisscross pattern to avoid warping the manifold. If any nut spins but the stud does not, stop and spray penetrating oil. Use vice grips on the stud if needed.
  • Once all nuts are removed, gently pry the manifold away from the head. It may be stuck from years of heat cycling. Tap it with a rubber mallet or use a pry bar on the flange – but be careful not to damage the cylinder head surface.
  • Lower the manifold out from above or below. On most Miatas, it’s easier to drop it out the bottom after removing a motor mount brace (two 14mm bolts).

Clean the Mating Surfaces

Remove all old gasket material from the cylinder head flange and the downpipe flange. Use a plastic razor blade or a wire brush – avoid scratching the aluminum head. A clean surface is critical for a leak-free seal. Wipe down with brake cleaner and a rag. Also inspect the head studs for corrosion; if any are damaged, replace them before installing the new header.

Installing the Mishimoto Headers

Mishimoto headers are designed as a direct replacement, but you will need to transfer any heat shielding, EGR tube, or O2 sensor bungs as applicable.

Pre-Assembly Checks

  • Test-fit the header to the engine bay before applying any sealant or gaskets. Ensure the primary tubes clear the steering shaft, alternator, and brake lines. Minor clearance issues can be solved by loosening and repositioning engine mounts, but this is rarely needed.
  • If your header has two O2 bungs, check that the front bung location matches your car’s sensor. 1.6L and 1.8L Miatas use different positions; Mishimoto usually marks them for your specific model.

Install the Header Gasket

Place the new gasket over the head studs. Use OEM-style multi-layer steel (MLS) gaskets if possible – they seal better than single-layer copper ones. Apply a thin smear of copper spray gasket sealant to the gasket if you want extra insurance, but it’s not required.

Mount the Header

  • Lift the header into position from below. Slide it over the studs, aligning the flanges with the gasket holes. You may need to angle it slightly to clear the subframe.
  • Hand-thread the old manifold nuts or the supplied nuts onto the studs. If the studs are too long, use washers or order shorter studs – Mishimoto often includes them. Never use lock washers on header studs because they can distort the flange.
  • Tighten the nuts in a star pattern: start with the center two, then the outer ones, etc. Use a torque wrench set to 35–40 ft-lb for the stud nuts. Consult the included instruction sheet for the exact spec.

Connect the Downpipe Flange

  • Slide the new gasket between the header collector and the midpipe. Use the supplied bolts or the original hardware.
  • Hand-tighten, then torque to 30–35 ft-lb. Do not overtighten – the flange can warp.
  • If your header has a flex section, ensure it is oriented correctly and not binding against the transmission tunnel.

Reinstall the O2 Sensor

Apply anti-seize to the sensor threads (a tiny amount only on the threads, not the tip) and screw it into the bung. Torque to 30 ft-lb. Reconnect the wiring clip and secure the harness away from moving parts.

Reinstall Any Accessories

If your car has an EGR tube, you may need a flexible adapter if the header lacks a dedicated port. Some Mishimoto headers for NA Miatas require you to block off the EGR port – check the manual. For water-cooled O2 sensors, route the hose carefully.

Reconnecting the Exhaust System

With the header securely bolted to the head, reconnect the rest of the exhaust. If you removed the catalytic converter support bracket, reinstall it. Check that the exhaust hangers are not stretched and that the system is centered in the tunnel – otherwise you’ll get rattles on the driveshaft tunnel.

  • Tighten all midpipe and axle-back connections to the manufacturer’s spec (typically 25–35 ft-lb).
  • Inspect each joint for leaks by looking for soot marks or listening for a ticking sound. A helper can plug the tailpipe with a rag to pressurize the system while you feel for leaks with your hand (engine off!).

Final Steps After Installation

  • Reinstall the underbody splash shield and any other panels you removed.
  • Double-check that all bolts are torqued and no tools are left in the engine bay.
  • Reconnect the battery – positive first, then negative.
  • Start the engine. It may run a bit rough for the first 30 seconds as the ECU adjusts to the decreased backpressure. Rev it to 2000 RPM for a few seconds and listen for exhaust leaks. A faint ticking at cold start is normal – it may be the header contracting as it heats up. If it persists after warm-up, tighten the header bolts slightly (engine cold).
  • Allow the engine to reach operating temperature while monitoring the dashboard for check engine lights. If the CEL illuminates, it’s likely an O2 sensor code (e.g., P0037 or P0135) due to repositioning the sensor. Confirm the sensor is plugged in and not damaged.
  • Take a short test drive (5–10 minutes) under varied throttle. Listen for any rattles between the header and frame or steering shaft. After the test drive, park and re-torque the header nuts once the engine is cool – the heat cycle may loosen them slightly.

Performance Gains and Maintenance Tips

After installation, you should notice a more responsive throttle and a deeper sound. For maximum gains, pair the header with a high-flow catalytic converter and a sport cat-back exhaust. Mishimoto’s header is ceramic-coated on the inside and out, which helps reduce under-hood temperatures – but the engine bay will still get hotter than stock. Consider wrapping the header with DEI titanium wrap if you plan to track the car, or simply keep the factory heatshield as an added barrier.

Check the header bolts after 500 miles and re-torque to spec. Stainless steel gaskets tend to compress over time. Use anti-seize on any joint you may need to disassemble later – especially the O2 sensor.

Additional Resources

For model-specific details, refer to the official Mishimoto website for installation PDFs. The Miata.net forum has thousands of threads on header installations if you encounter unique problems. Torque spec references can be found in the Mellens online manuals for NA, NB, and NC generations.

Conclusion

Installing Mishimoto Miata headers is a straightforward weekend project that delivers noticeable improvements in power delivery and exhaust note. By following the step-by-step process – from soaking rusted bolts to torquing the final nuts – you can avoid common pitfalls like leaks and stripped studs. The result is a freer-breathing engine that pulls harder through the midrange and sings to the redline. Take your time, use the right tools, and you’ll enjoy many miles of spirited driving with your upgraded Miata.