The Subaru Impreza WRX Turbo Legacy

The Subaru Impreza WRX has built a reputation as one of the most accessible turbocharged sport compacts on the market. Its balanced chassis, all-wheel drive, and the characteristic rumble of its horizontally opposed engine create a platform that responds well to power modifications. Among the most effective bolt-on upgrades for the WRX is the Mitsubishi TD04HL-15T mini turbocharger. This unit offers a significant step up from the factory turbo without requiring a full engine build. While the stock TD04-13T turbo serves the WRX well for daily driving and moderate boost, the TD04HL-15T unlocks a broader powerband and supports higher horsepower targets. This article examines the power gains, installation requirements, supporting modifications, and tuning considerations for this popular upgrade.

Origins of the TD04HL-15T Turbocharger

The TD04HL-15T is a product of Mitsubishi Heavy Industries, originally developed for the Mitsubishi Lancer Evolution III and later adopted by other turbocharged vehicles. The "TD04" designation refers to the turbo family (Turbocharger, 4-inch turbine wheel), while "HL" indicates a larger turbine housing compared to earlier variants. The "15T" compressor cover provides a 15 cm² inlet area, allowing higher flow than the stock 13T unit. This turbo is a direct bolt-on for many Subaru WRX and Forester XT models, making it a favorite among enthusiasts looking for a cost-effective power increase.

The TD04HL-15T features a twin-scroll turbine housing that pairs well with Subaru’s equal-length headers or unequal-length manifolds. This design helps improve exhaust gas utilization, reducing turbo lag and promoting quicker spool. The compressor wheel is slightly larger than the TD04-13T, delivering more airflow without sacrificing transient response drastically. Many owners find the TD04HL-15T to be the ideal balance for street-driven WRXs targeting 270–320 wheel horsepower.

Comparing the TD04HL-15T to the Stock TD04-13T

Understanding the differences between the stock and upgraded turbo helps clarify expected gains. The table below summarizes key specs:

  • Compressor Inducer Diameter: TD04-13T – 46.5 mm; TD04HL-15T – 51.3 mm
  • Turbine Exducer Diameter: Both share a similar 45.0 mm turbine wheel, but the HL housing has a larger volute
  • A/R Ratio (Compressor): 0.60 vs. 0.65; higher ratio supports higher boost and flow
  • Flow Capacity: TD04-13T ~ 360 CFM (approx. 300 hp); TD04HL-15T ~ 420 CFM (approx. 350 hp)
  • Spool Characteristics: The 13T reaches full boost around 2800–3000 rpm; the 15T hits full boost around 3000–3300 rpm, with a small increase in lag offset by a stronger mid-range

The increased compressor flow allows the 15T to deliver more air at higher boost levels, resulting in a broader torque plateau. On a dyno, a stage 2 WRX with the stock 13T typically makes 230–250 whp at 16 psi. With the TD04HL-15T and same supporting mods, power climbs to 280–310 whp at 18 psi. With higher octane fuel and aggressive tuning, some owners reach 330 whp. The peak torque also moves up by 500–1000 rpm, reducing the "falling off" feeling after 5500 rpm common with the 13T.

Power Gains: Dyno Results and Driving Experience

Quantified gains depend on the specific WRX model, modifications, and tuning. Based on numerous forum reports and dynojet pull data, here are typical outcomes:

  • Low Boost (14–16 psi): 260–280 whp, 280–300 lb-ft torque. Nearly identical spool to 13T at this level, but holds power to redline better.
  • Medium Boost (17–19 psi): 290–315 whp, 310–330 lb-ft torque. Noticeable increase in mid-range pull; turbo spools by 3200 rpm.
  • High Boost (20–22 psi) with supporting mods: 320–340 whp, 340–360 lb-ft torque. Requires upgraded fuel system and intercooler. Slight lag increase, but strong top-end.

Drivers report that the TD04HL-15T transforms the WRX’s character. Instead of a sharp boost spike tapering off, the power delivery feels linear and sustained. This makes the car more enjoyable on track days and during daily driving, as the engine pulls hard from 3500 to 6500 rpm. The turbo also handles higher boost pressures more reliably than the 13T, which tends to become a heat pump beyond 18 psi.

Installation Considerations and Step-by-Step Guide

Installing the TD04HL-15T is a straightforward process for anyone with mechanical experience. However, the job requires careful attention to gaskets, oil lines, and coolant lines to avoid leaks. Below is a detailed procedure:

Required Tools and Parts

  • Socket set, torque wrench, gasket scraper
  • New turbo gaskets (inlet, outlet, oil feed, oil drain, coolant)
  • New oil feed line (if upgrading to braided line)
  • Copper spray (for gaskets) or replacement gaskets
  • Anti-seize compound and thread locker

Step 1: Disconnect Battery and Drain Fluids

Disconnect the negative battery terminal. Drain engine oil and coolant level below the turbo. Removing the intercooler and intake piping is required to access the turbo.

Step 2: Remove OEM Turbo

Unbolt the downpipe from the turbo. Disconnect oil feed line (banjo bolt) and drain line (usually 2 bolts). Disconnect coolant lines. Remove the turbo from the exhaust manifold (4 studs), taking care not to damage threads. Some models require lifting the engine slightly to clear the subframe.

Step 3: Prepare the New Turbo

Place the TD04HL-15T on a clean surface. Install the oil restrictor if using a larger feed line (Mitsubishi turbos typically do not require restrictors, but verify with your vendor). Attach new oil drain gasket. Prime the turbo by pouring a small amount of clean oil into the feed port and rotating the shaft by hand.

Step 4: Install New Turbo

Bolt the turbo to the exhaust manifold with new gaskets. Torque to spec: typically 35–40 ft-lb for manifold studs, 30–35 ft-lb for turbo-to-dump pipe. Connect oil drain line and coolant lines. Use new copper washers on banjo bolts. Reconnect downpipe.

Step 5: Reassemble Intake and Intercooler

Install the turbo inlet hose and intercooler. Check all clamps and ensure no boost leaks. Refill engine oil and coolant to proper levels.

Professional installation is recommended if you lack experience with turbo systems. Improper torquing can lead to exhaust leaks or oil starvation.

Supporting Modifications for Maximum Benefit

The TD04HL-15T can be used on an otherwise stock WRX, but to realize its full potential, supporting modifications are necessary. A stage 2 package paired with this turbo is the sweet spot.

Intercooler Upgrade

The stock top-mount intercooler (TMIC) becomes a bottleneck above 18 psi. Consider a larger TMIC (e.g., STI spec or aftermarket) or a front-mount intercooler (FMIC). FMIC offers better heat rejection and consistent power on hot days but adds more piping volume. For a street car, a quality TMIC is sufficient and maintains quick spool.

Fuel System Upgrades

Stock WRX fuel injectors (440cc or 560cc depending on year) will max out around 280 whp. Upgrade to 650–800cc injectors (e.g., STI pinks, ID1000s). A high-flow fuel pump (255 lph or higher) is strongly recommended. Pair with a fuel pressure regulator if going above 60 psi base pressure.

Exhaust System

A free-flowing downpipe (catted or catless) and cat-back exhaust reduce back pressure and allow the turbo to spool efficiently. A 3-inch downpipe with a divorced wastegate design helps prevent boost creep. Avoid restrictive stock exhausts.

Intake and Boost Control

A short ram intake or cold-air intake with a heat shield improves airflow. The stock boost control solenoid (BCS) is adequate up to about 18 psi; for higher boost, upgrade to a three-port boost control solenoid for more precise control. Electronic boost controllers also work well.

Tuning for Optimal Performance

Without proper tuning, the TD04HL-15T can cause knock, detonation, or lean conditions. The WRX's ECU must be reprogrammed to match the turbo's flow characteristics.

ECU Options

  • OpenECU (opensource tuning) – Free software with learning curve. Allows full control of maps. Requires a Tactrix cable.
  • COBB AccessPort – Plug-and-play tuning with off-the-shelf maps for common mod combos. Custom tuning by a professional is recommended.
  • Standalone ECUs – For extreme builds (e.g., Syvecs, Haltech, Link). Overkill for a mini turbo street car.

Base Tune Strategy

Start with a safe ignition timing map, retard slightly compared to stock in the high-load areas. Set target Air-Fuel Ratio (AFR) to 11.5:1 under boost. Aim for a boost curve that hits peak by 3500 rpm and holds to redline. Use the wastegate duty cycle table to adjust spool. Dyno tuning is best because it allows logging and real-time adjustments.

Common Tuning Pitfalls

  • Boost creep: The 15T turbine housing may not flow enough exhaust gas at high rpm, causing overboost. Solution: port the wastegate hole or use an external wastegate.
  • Knock detection: Subaru knock sensors are sensitive. Use a knock light or listen for detonation. Reduce timing if knock occurs.
  • Fuel pressure drop: High-flow fuel pumps can overpower the stock FPR. Return-style systems benefit from an aftermarket regulator.

Common Issues and Troubleshooting

Boost Leaks

With increased boost pressure, weak hoses and clamps can pop off. Inspect silicone couplers, intercooler end tanks, and throttle body gasket. A boost leak test a few days after installation saves headaches.

Oil Supply Problems

Some TD04HL-15T units have a .060" oil restrictor built into the feed banjo bolt. If not present, install a restrictor to prevent oil from blowing past seals. The stock oil line often works, but braided lines reduce failure risk. Always use OEM or high-quality gaskets.

Exhaust Back Pressure

A restrictive exhaust (stock downpipe or too small diameter) raises back pressure and slows spool. Also, boost creep worsens. Ensure the downpipe is at least 3 inches.

Real-World Owner Experiences

Subaru WRX forums reveal consistent satisfaction with the TD04HL-15T upgrade. Owners of 2002–2007 WRX models note that the car becomes more rewarding to drive daily, with stronger mid-range passing power. Several users report that the turbo holds up well even after 30,000 miles with proper maintenance. A few negatives: the turbo may require a thicker oil (10W-40 in hot climates) to prevent shaft play, and some units develop boost creep when paired with catless downpipes, resolved by porting the wastegate.

One owner documented a dyno result of 303 whp and 328 lb-ft at 18 psi on a 2004 WRX with injectors, pump, and FMIC. The car drove fine on 93 octane and saw 22 mpg highway. Another built a 2.5L WRX with the TD04HL-15T making 350 whp at 20 psi, but noted it needed E85 fuel to avoid knock. These examples show the turbo's versatility.

Maintenance and Longevity

The TD04HL-15T is a robust unit, but like all turbos, it requires care. Follow these tips:

  • Change oil and filter every 3,000–5,000 miles using a high-quality synthetic oil (5W-30 or 10W-40).
  • Let the engine idle for 30–60 seconds after hard driving to cool the turbo before shutdown.
  • Check for shaft play during oil changes. Slight axial play is normal; radial play beyond 0.005" indicates wear.
  • Inspect the compressor wheel for damage from ingested debris; a good air filter is essential.

Is the TD04HL-15T Worth It?

For a WRX owner seeking a noticeable power increase without the cost and complexity of a larger turbo (like a VF series or Garrett GT), the TD04HL-15T is an excellent choice. It’s a direct bolt-on, spools quickly, and can push power into the 300 whp range with moderate supporting mods. It bridges the gap between mild stage 2 and a full rotated turbo setup. However, if your goal exceeds 350 whp or you want to race on a big track, a larger turbo with better top-end flow will be needed. For the street and autocross, the TD04HL-15T hits the sweet spot of response and output.

External resources for further reading:

With careful planning, proper installation, and a safe tune, the TD04HL-15T can transform a WRX into a far more capable and enjoyable machine.