Why Upgrade Your A90 Supra Downpipe

Replacing the factory downpipe on your Toyota GR Supra A90 (or BMW Z4 M40i) is one of the most effective modifications for unlocking the full potential of the B58 engine. The stock downpipe features a restrictive catalytic converter and narrow piping, which chokes exhaust flow and limits both power and response. A high-flow downpipe like the Mishimoto A90 Supra downpipe reduces backpressure, allowing exhaust gases to exit the turbo more freely. The result is quicker spool, increased horsepower (typically 20-30 whp with a tune), and a more aggressive exhaust note. This guide covers the entire installation process from a beginner's perspective, providing the detailed steps, torque specifications, and troubleshooting advice needed to complete the job safely and correctly.

Tools and Materials Needed

Before you begin, gather the following tools and supplies. Having everything within reach will prevent unnecessary trips to the toolbox during the installation.

  • Socket set – 3/8” and 1/2” drive, metric sizes (8mm, 10mm, 12mm, 13mm, 14mm, 15mm, 17mm, 18mm, 21mm for O2 sensors)
  • Wrench set – combination wrenches in the same metric sizes, plus a ratcheting wrench for tight spaces
  • Torque wrench – 1/2” drive, range 20-150 ft-lbs
  • Jack and jack stands – minimum 2-ton capacity; use rubber pad adapters to protect the pinch welds
  • Safety goggles and gloves – hot exhaust components and sharp edges require protection
  • PB Blaster or penetrating oil – to loosen rusted fasteners
  • O2 sensor socket – 22mm or 7/8” crows-foot or slotted socket
  • Mishimoto A90 Supra downpipe – verify it is the correct version (catless or catted) for your local regulations
  • New gaskets – turbo-to-downpipe gasket (copper or multi-layer steel) and downpipe-to-midpipe gasket (often a flat donut gasket); do not reuse old gaskets
  • Anti-seize compound – copper-based for O2 sensor threads and exhaust flange bolts
  • Extension bars – 6”, 12”, and a universal joint for awkward angles
  • Flathead screwdriver or pry bar – to separate stubborn flanges

Preparation Steps

Proper preparation reduces frustration and prevents damage to your vehicle. Follow these steps before touching any exhaust components.

Secure the Vehicle

Park the Supra on a level concrete surface. Place wheel chocks behind the rear wheels. Using a jack, lift the front of the car at the central front jacking point (the aluminum cross brace). Position jack stands under the designated lift points on the front frame rails, then lower the car onto the stands. Shake the car gently to confirm it is stable.

Disconnect the Battery

Disconnect the negative battery terminal (10mm bolt) to prevent any accidental short circuits or ECU issues while unplugging sensors. Wrap the terminal end in a cloth or tape to avoid contact with the battery post.

Allow the Exhaust to Cool

If the car was driven recently, the exhaust system, catalytic converter, and turbo housing will be extremely hot. Wait at least one hour until all components are cool to the touch. Hot exhaust can cause severe burns and make gaskets difficult to remove cleanly.

Apply Penetrating Oil

Spray the bolts securing the factory downpipe to the turbo (four nuts on the turbo flange) and the bolts at the midpipe connection. Also spray the O2 sensors where they thread into the downpipe. Let the oil soak for 10-15 minutes.

Removing the Stock Downpipe

The removal process involves a few tricky steps, especially around the O2 sensors and heat shields. Take your time to avoid stripping bolts or damaging sensor wiring.

Remove the Heatshields

Locate the heatshield above the downpipe (attached to the firewall side) and any heatshields covering the turbo outlet. Use a 10mm socket to remove the bolts. Set the shields aside; you may need to reinstall them depending on your local laws and whether you want to protect surrounding components from heat.

Disconnect the O2 Sensors

There are two oxygen sensors in the factory downpipe: a wideband sensor near the turbo and a secondary sensor farther downstream. Unplug the sensor connectors from the wiring harness (they are located on top of the engine, near the cowl). Carefully feed the sensor wires through the engine bay so they hang free. Use the O2 sensor socket and a breaker bar to loosen each sensor. Turn counterclockwise. If a sensor is stuck, apply more penetrating oil and let it sit. Avoid using excessive force that could damage the sensor threads.

Unbolt the Downpipe from the Turbo

Working from underneath the car, locate the four nuts connecting the downpipe flange to the turbo outlet. Use a deep-well 12mm or 13mm socket (depending on nut size) with an extension and universal joint. Turn each nut counterclockwise to loosen. They may be tight – use a ratcheting wrench if space allows. Once all nuts are removed, gently pry the downpipe flange away from the turbo with a flathead screwdriver if it is stuck.

Unbolt the Downpipe from the Midpipe

At the rear of the downpipe, there are two bolts connecting it to the exhaust midpipe (or front pipe). Use a 14mm or 15mm socket to remove these bolts. Support the downpipe with your hand as you remove the last bolt to prevent it from dropping.

Remove the Downpipe

With all fasteners removed, slide the downpipe out from the engine bay and from under the car. You may need to tilt it or rotate it to clear the subframe and steering shaft. Be careful not to hit the O2 sensors or the turbo inlet pipe. If the downpipe is stuck, check for any missed brackets or heatshield bolts.

Installing the Mishimoto Downpipe

Now that the stock downpipe is out, prepare the new Mishimoto downpipe and install it in the reverse order.

Prepare the Flanges and Gaskets

Clean the turbo outlet flange and the midpipe flange with a wire brush or sandpaper to remove any old gasket material or carbon deposits. Install the new turbo gasket onto the turbo studs. Ensure it is oriented correctly (the raised ring faces the turbo). For the midpipe connection, place the new donut gasket into the flared end of the downpipe or onto the midpipe, depending on design.

Transfer O2 Sensors

If you are reusing the factory O2 sensors, thread them into the bungs on the Mishimoto downpipe. Apply a small amount of anti-seize to the threads of each sensor. Tighten the sensors to 30-35 ft-lbs (torque value from BMW/Mishimoto documentation). Do not overtighten. Route the sensor wires away from moving parts and hot surfaces, securing them with zip ties if necessary.

Position the Downpipe

From underneath the car, guide the Mishimoto downpipe into place. Align the turbo flange with the studs on the turbo. Slide the flange on so that it sits flush. Hand-tighten the four nuts onto the turbo studs. Next, align the rear of the downpipe with the midpipe. Insert the two bolts and hand-tighten them.

Torque All Fasteners

Using a torque wrench, tighten the turbo flange nuts in a cross pattern to the manufacturer’s specification. For the Mishimoto downpipe on the A90 Supra, the recommended torque is 45-50 ft-lbs (confirm with the included instructions). Then tighten the midpipe bolts to 35-40 ft-lbs. Recheck all fasteners after the first heat cycle (after a short test drive) as gaskets compress.

Reinstall Heatshields

If you removed any heatshields, reinstall them now. Some aftermarket downpipes may require modification to heatshields (bending tabs or trimming) to clear the new piping. Use your judgment to ensure no metal-on-metal contact that could cause rattles.

Reconnect the Battery

Reattach the negative battery terminal and tighten the 10mm nut.

Post-Installation Checks and Test Drive

Before starting the engine, double-check that all connections are secure and nothing is interfering with the steering shaft, suspension, or brake lines.

Check for Leaks

Start the engine and listen for any exhaust hissing or ticking sounds. Have a helper rev the engine gently while you feel around the flanges with your hand (be careful of hot surfaces). A leak will present as a puff of exhaust or a distinct sound. If you detect a leak, tighten the relevant bolts slightly (do not exceed torque spec) or inspect the gasket seating.

Inspect for Fault Codes

If you have a code reader, scan for O2 sensor codes or catalyst efficiency codes. The check engine light (CEL) may illuminate immediately if the downpipe is catless. This is normal and will require an ECU tune to disable the P0420/P0430 codes. For catted downpipes, the CEL may appear after a few drive cycles. A tune or O2 sensor spacer may be needed.

Test Drive

Take the car on a short, low-speed drive to allow the exhaust system to heat cycle. Listen for rattles or strange vibrations. Accelerate gently to part throttle, then gradually to full throttle. Notice the improved spool and sound. After the drive, re-torque the turbo flange nuts (when the exhaust is cool enough to touch) to ensure they have not loosened.

Troubleshooting Common Issues

Even with careful installation, you may encounter some of these typical problems. Here are solutions based on real-world experience and forum feedback.

Rattling Noises

If you hear a metallic rattle under acceleration or over bumps, check clearance between the downpipe and the subframe, steering shaft, or front cross brace. The Mishimoto downpipe is designed to fit without major contact, but heat shield brackets or bolt heads may touch. Use a pry bar to slightly bend a heatshield or add a small piece of rubber hose as a buffer.

Exhaust Leaks at the Turbo Flange

The most common leak spot is the turbo-to-downpipe connection. If you hear a high-pitched ticking that gets worse with throttle, the gasket may be misaligned or the nuts may be loose. Always use a new gasket and tighten in a crisscross pattern. If the leak persists, check that the downpipe flange is perfectly flat – rare machining defects can occur; contact Mishimoto for a replacement if needed.

Check Engine Light (Catalyst Efficiency)

As mentioned, a catless downpipe will almost always trigger a P0420 code. A catted downpipe may also trigger it after several hundred miles as the catalyst ages. The only permanent fix is a custom ECU tune that disables the rear O2 sensor monitoring. An O2 sensor spacer (angled or mini-cat) can sometimes delay or mask the code, but results vary. Do not rely on spacers for emissions testing; check local laws.

O2 Sensor Issues

If you encounter a P0135 (heater circuit) or P0171 (lean) code after installation, inspect the O2 sensor wiring for damage or poor connection. Ensure the sensors are plugged into the correct harness connectors. If you swapped the primary and secondary sensor connectors, the car will run poorly. Label them during removal to avoid this.

Loss of Boost or Lag

An improperly installed downpipe can create a boost leak if the gasket is not sealing. This will result in slower spool and lower peak boost. Check all clamps and bolts. Also, verify the wastegate actuator arm is not being interfered with by the downpipe – the Mishimoto design should provide clearance, but aftermarket wastegate brackets may need adjustment.

Performance Gains and Tuning Considerations

Installing a downpipe alone will yield modest gains (5-10 whp on stock tune) because the ECU compensates for the increased flow by adjusting fuel trims. To fully realize the 20-30+ whp potential, an ECU tune is required. Popular tuning platforms for the A90 Supra include BootMod3, MHD, and ECUTEK. These tunes also eliminate the catalyst efficiency CEL and allow you to adjust for ethanol blends or boost increases.

Be aware that removing the catalytic converter (catless downpipe) may violate local emissions laws. Many areas require a functional catalytic converter for road use. A high-flow catted downpipe (like the Mishimoto catted version) retains a smaller catalyst that still provides some filtration while offering improved flow over stock. Check your local regulations before purchasing.

Additionally, a downpipe upgrade will increase exhaust volume – both inside the cabin and outside. The sound becomes deeper and more aggressive, especially with an aftermarket cat-back exhaust. Some drivers find drone at highway speeds; adding sound deadening or a resonator can help.

Final Thoughts

Installing a Mishimoto A90 Supra downpipe is a satisfying afternoon project that delivers real performance and a soundtrack upgrade. By following this detailed guide, even a first-time installer can achieve a leak-free, reliable installation. Always prioritize safety: use quality jack stands, wear gloves and eye protection, and never rush a fastener. With the downpipe in place and a proper tune, your Supra will pull harder and sound fiercer than ever.

For additional resources, refer to the Mishimoto product page for official installation instructions and warranty information. The SupraMKV forum thread contains owner experiences and tips for common installation hurdles. If you plan to tune, reputable sources like BootMod3 provide OTS maps designed for downpipe-equipped Supras.