Understanding the Twin-Scroll Advantage for Your Subaru WRX

Upgrading your Subaru WRX with a twin-scroll turbocharger is one of the most effective ways to unlock genuine power gains without sacrificing low-end response. The Mitsubishi TD04L-13T, originally found on certain Subaru models, has become a popular choice for enthusiasts looking to balance quick spool with mid-range punch. Unlike a conventional single-scroll turbo, a twin-scroll design separates exhaust pulses from different cylinders, reducing interference at the turbine wheel. This results in faster spool times, broader torque curves, and improved fuel efficiency under normal driving.

Before diving into the installation, understand that the TD04L-13T is a factory-equipment part that fits directly on many Subaru EJ engines. However, if your WRX originally came with a single-scroll turbo, you will need to swap to a twin-scroll manifold or weld a twin-scroll adapter flange. This guide assumes you already have the correct manifold. The steps below cover a complete, shop-quality installation with an emphasis on safety, torque specifications, and leak prevention.

Tools and Materials Needed

Gather everything before you start to avoid unnecessary trips. Using high-quality tools reduces the risk of rounding bolts or damaging soft metal components.

  • Socket set with extensions – Metric sizes 10mm, 12mm, 14mm, 17mm, and 19mm are most common. A wobble extension is helpful for tight spots.
  • Torque wrench – 1/2-inch drive capable of 30–80 ft-lb for main fasteners; a 3/8-inch drive for smaller bolts.
  • Combination wrenches – Especially 14mm and 17mm for banjo bolts on oil lines.
  • Flathead and Phillips screwdrivers – For hose clamps and small clips.
  • Mitsubishi TD04L-13T turbocharger – Ensure you have a genuine or high-quality rebuild. Verify the wastegate actuator works freely.
  • New gaskets – Manifold-to-turbo gasket, turbo-to-downpipe gasket, oil drain gasket, and copper crush washers for oil and coolant banjo bolts.
  • Oil feed and return lines – OEM or aftermarket braided lines are recommended. The TD04L-13T uses a restrictor in the feed; if yours doesn’t have one, add a 0.035-inch restrictor to prevent oil oversupply.
  • Coolant lines – Two coolant hard lines or flexible hoses; ensure the banjo fittings match the turbo housing.
  • Intercooler piping – You may need a different Y-pipe or silicone couplers if your car used a single-scroll layout. Twin-scroll inlet orientation is unique.
  • Boost controller (optional) – A manual boost controller or electronic boost control solenoid is wise if you plan to run higher boost than stock.
  • Engine coolant and oil – Fresh Subaru coolant (blue) and a high-quality 5W-30 or 5W-40 synthetic oil to refill after draining.
  • Additional items – Thread-locker medium strength, anti-seize (for exhaust studs), penetrating oil, cat-gut scraper for old gasket material, and shop towels.

Preparation Steps for a Clean Installation

Preparation is critical. Working on a hot engine or dirty surfaces will lead to stripped threads or leaks. Follow these steps in order.

Park and Safety

Park the car on level ground and chock the wheels. Disconnect the negative battery cable to avoid shorting the alternator or ECU during the work. Allow the engine to cool completely; the exhaust manifold and turbo will retain heat for hours.

Drain Fluids

Drain the engine oil and coolant. Remove the oil drain plug and let the oil flow into a pan. Next, open the radiator drain cock and remove the radiator cap (after cooling) to drain the coolant. This prevents oil and coolant from spilling when you disconnect the turbo lines. Replace the oil plug and drain plug after draining.

Remove Engine Cover and Heat Shields

Take off the plastic engine cover and any sound-deadening foam. Unbolt the heat shields over the exhaust manifold, up-pipe (if present), and the turbo itself. These shields are often rusted; soak bolts with penetrating oil an hour before. Some shields are held by spring clips; release them carefully.

Removing the Existing Turbocharger

With the old turbo still in place, work methodically to access all fasteners. A twin-scroll setup may have a different flange pattern than a single-scroll, so pay attention to orientation.

Disconnect Intercooler and Intake

Remove the intercooler scoop and any ducting. Unbolt the intercooler from the throttle body and compressor outlet hose. On many Subaru models, the intercooler hinges forward on brackets; you can remove it entirely for better access. Unplug the mass airflow sensor and induction tube.

Remove Downpipe and Exhaust Connections

Unbolt the downpipe from the turbo. The twin-scroll downpipe uses two separate inlet ports; you may need to remove the entire downpipe section. Support the downpipe with a jack stand if it’s heavy. Disconnect the oxygen sensors and unbolt the downpipe from the mid-pipe. Set aside.

Disconnect Oil and Coolant Lines

Using a 14mm wrench, loosen the oil feed banjo bolt at the top of the turbo. Have a shop towel ready to catch residual oil. Similarly, disconnect the oil return line at the bottom of the turbo – usually a rubber hose with a clamp. For coolant, remove the banjo bolts on the turbo coolant inlet and outlet. Cap the lines to keep dirt out.

Unbolt the Turbo from the Manifold

The TD04L-13T mounts to the exhaust manifold with two studs and two bolts (or four studs depending on the manifold). Remove these fasteners carefully. If the turbo is stuck, gently tap it with a rubber mallet. Lift the turbo off the studs and remove it from the engine bay. Inspect the manifold mounting face for cracks or warpage.

Installing the Mitsubishi TD04L-13T Turbocharger

Clean the manifold studs with a wire brush. Apply a small amount of anti-seize to the threads. Position the new gasket on the manifold.

Mount the Turbo

Lower the TD04L-13T onto the manifold studs. The twin-scroll inlet (two separate openings) must align precisely with the divided passages in the manifold. Rotate the turbo until the oil return port faces downward and the coolant ports are on the top side (on most Subaru applications). Hand-tighten the nuts onto the studs. Then torque the nuts to 35–40 ft-lb (consult your specific gasket manufacturer; some require re-torque after a heat cycle).

Connect Oil Feed and Return Lines

Attach the oil feed line to the top of the turbo using the new crush washers. Torque the banjo bolt to 20–25 ft-lb. Do not overtighten; the copper washer deforms for sealing. Reconnect the oil return hose to the bottom of the turbo; use a new clamp. Ensure the hose doesn’t kink.

Install Coolant Lines

Connect the coolant inlet and outlet lines to the turbo. Use the supplied banjo bolts and new crush washers. Torque to 15–20 ft-lb. Many late-model Subarus use a small restrictor in the coolant outlet – ensure it’s installed to control cooling flow.

Attach the Downpipe and Wastegate Linkage

Install a new gasket to the turbo outlet. Attach the downpipe. For twin-scroll downpipes, align the two inlet ports with the outlet divider. Tighten the downpipe bolts gradually; torque to 30–35 ft-lb. Connect the wastegate actuator rod to the flapper arm on the turbo. Adjust the rod length so that the arm just barely holds the flapper closed when at rest. Secure with the roll pin or clip.

Connecting the Intercooler and Intake System

The TD04L-13T compressor outlet is oriented differently than a single-scroll unit. You need to route the intercooler piping accordingly.

Intercooler Y-Pipe or Direct Connection

For a top-mount intercooler, you may need a specific Y-pipe that fits the twin-scroll compressor outlet position. Many aftermarket kits (e.g., Grimmspeed) offer silicone couplers designed for this swap. Attach the Y-pipe to the turbo and intercooler, securing all clamps with a screwdriver or socket. Ensure no hoses are pinched or rubbing against the engine.

Intake Air Duct

Reinstall the air intake duct from the MAF sensor to the turbo inlet. On the TD04L-13T, the inlet is 2.5 inches in diameter. If your intake system used a different size, use a reducer coupler. Tighten the worm-gear clamps snugly to prevent vacuum leaks.

Final Steps: Priming, Leak Check, and First Start

Don’t just start the engine – prime the oil system first to avoid turbo failure.

Prime the Oil System

Disconnect the ignition coils or fuel pump relay to prevent the engine from starting. Crank the engine for 10–15 seconds using the starter. This pushes oil through the turbo’s feed line. Wait 30 seconds for the starter to cool, then crank again. Repeat 2–3 times. Reconnect the coils or relay.

Refill Fluids

Fill the crankcase with fresh oil to the proper level. Fill the cooling system with Subaru coolant mixed 50/50 with distilled water. Bleed the cooling system by running the engine with the radiator cap off until the thermostat opens and air bubbles stop.

Start and Check for Leaks

Start the engine and let it idle. Listen for any unusual noises – a scraping sound indicates contact between the turbine housing and compressor wheel (rare with proper alignment). Inspect all oil and coolant connections for drips. Tighten banjo bolts if needed. After a few minutes, rev the engine gently to 2000 RPM and check again. Shut off the engine and inspect the turbo area with a flashlight.

Test Drive and Boost Check

Take the car on a short drive, keeping boost under 10 psi initially. Monitor the boost gauge to ensure the wastegate manages pressure correctly. If you installed a boost controller, set it to the wastegate spring pressure first. After a 10-minute drive, recheck all fasteners (allow engine to cool). Torque the downpipe bolts and oil line banjo bolts again; they sometimes settle.

Performance Expectations and Tuning Necessity

The TD04L-13T twin-scroll will spool noticeably faster than a stock WRX single-scroll turbo, often reaching full boost by 2800–3000 RPM. However, the factory ECU fuel and ignition maps are not optimal for this turbo. To prevent detonation and maximize power, a professional tune using software like Cobb Accessport or open-source tuning (e.g., RomRaider) is essential. Many enthusiasts report gains of 40–60 whp with proper tuning and supporting mods (bigger injectors, fuel pump).

Double-check that your intercooler piping does not interfere with the hood or other components. If you experience boost creep, a larger wastegate flapper or an aftermarket external wastegate may be needed. For further technical discussions and troubleshooting, the Subaru Motorsports forum on NASIOC is an excellent resource.

Conclusion

Installing a twin-scroll Mitsubishi TD04L-13T on a Subaru WRX is a rewarding upgrade that delivers responsive power and a broader torque curve. By following the step-by-step procedure above – including careful torque specifications, proper gasket replacement, and oil priming – you can ensure a reliable installation. Remember that successful results depend on thorough preparation and post-installation checks. With the engine tuned correctly, your WRX will feel more energetic from low RPM while retaining the daily-driveability that makes these cars so fun.