Introduction: Unleashing the Potential of Your Subaru Legacy

Upgrading the turbocharger is one of the most effective ways to transform a Subaru Legacy from a reliable daily driver into a thrilling performance machine. Among the popular aftermarket options, the Mitsubishi TD05-20G has earned a strong reputation for striking an excellent balance between quick spool and top-end horsepower. This turbocharger is a favorite among Legacy enthusiasts, particularly those running the EJ20 or EJ25 engines, because it delivers a noticeable increase in power without requiring a complete engine rebuild. In this expanded guide, we’ll dive deep into the TD05-20G—its design, the real-world power gains you can expect, essential supporting modifications, a detailed installation walkthrough, common challenges, and the critical role of tuning. Whether you’re planning a weekend project or a fully built race setup, understanding every aspect of this upgrade will help you achieve reliable performance that lasts.

The TD05-20G Turbo: Specifications and Design

The TD05-20G is part of Mitsubishi’s Heavy Industries turbocharger lineup, originally used on models like the Mitsubishi Lancer Evolution and Eclipse. It features a twin-scroll (or single-scroll) configuration, a 20G compressor (with inlet diameter around 59–60 mm), and a TD05H turbine housing. The “20G” designation refers to the compressor wheel size, which delivers more airflow than the smaller 16G or 18G variants used on many Subarus. Here are key specifications that make this turbo appealing:

  • Compressor wheel: Typically 59.5 mm in diameter, with a larger inducer than stock units, enabling higher boost flow without excessive intake heating.
  • Turbine wheel: A high-flow 57.5 mm wheel often paired with a ported shroud to reduce backpressure and improve exhaust gas flow.
  • Housing: Available in 7 cm², 8 cm², or 10 cm² turbine housing sizes. For a street-driven Legacy, a 7–8 cm² housing is common, providing faster spool while still supporting 350–400 whp.
  • Bearing system: Most TD05-20G units use a journal bearing design, which is durable and cost-effective. Ball-bearing options (like the TD05 20G from Blouch Turbo) offer slightly quicker spool and improved transient response but at a higher price.
  • Boost range: Capable of supporting 18–25 psi on pump gas with proper tuning, and up to 30 psi on race fuel or water-methanol injection.

Compared to the stock VF40 or VF41 turbo on the Legacy GT, the TD05-20G flows nearly 40–50% more air at the same boost level. This means more power potential, but also requires a careful review of supporting components to avoid engine knock or fuel starvation. The turbo is typically mounted using a custom up-pipe or a rotated mount kit—direct bolt-on solutions exist for 2005–2009 Legacy GT models, but earlier models often require adapter flanges or fabrication.

Performance Gains: What to Expect

Adding a TD05-20G to your Legacy can transform its personality. With the right supporting modifications and a professional tune, here are the realistic gains you can anticipate:

  • Horsepower increase: Most well-built Legacy tunes yield 280–350 whp on 93 octane, up from the stock 210–230 whp. Some setups pushing 21–22 psi with forged internals and E85 have achieved over 400 whp.
  • Torque improvement: Peak torque often jumps from ~220 lb-ft to the 300–350 lb-ft range, arriving earlier in the RPM band thanks to the 20G’s strong mid-range boost response.
  • Spool characteristics: The TD05-20G spools slightly later than the stock turbo—reach full boost around 3300–4000 RPM depending on the turbine housing and engine displacement. On a 2.5L Legacy, this lag is moderate; on a 2.0L, it’s more noticeable but still livable.
  • Boost response: Once spooled, the boost ramps aggressively, delivering a strong pull to redline. Twin-scroll versions (if matched with a twin-scroll up-pipe) can improve spool by 200–400 RPM, making the power delivery feel more linear.

It’s important to note that these numbers assume you’ve addressed fuel delivery, intake restrictions, and exhaust backpressure. Running a TD05-20G on a stock Legacy without any tuning will not only fail to produce power gains but can also cause serious engine damage due to lean conditions or boost spikes.

Supporting Modifications for a Successful Upgrade

A turbocharger is only one part of the system. To safely and effectively use the TD05-20G’s extra airflow, you must upgrade several surrounding components. Let’s break them down by subsystem.

Fuel System Upgrades

Your Legacy’s stock fuel system will quickly become the bottleneck. The TD05-20G can move enough air that the factory injectors and pump cannot keep up. Plan on:

  • High-flow fuel injectors: 650–850 cc/min injectors are common for 350–400 whp. For pump gas, 750 cc injectors provide a good margin. For E85, move to 1000–1200 cc.
  • Aftermarket fuel pump: A Walbro 255 LPH or AEM 320 LPH pump is the standard. It replaces the in-tank unit and supplies consistent flow at higher rail pressures.
  • Fuel pressure regulator: An aftermarket adjustable regulator (like Aeromotive) helps maintain steady pressure if you modify the fuel lines or run a return-style system.
  • Fuel lines and rails: For high-horsepower builds (over 400 whp), consider upgrading to stainless steel braided lines and a larger diameter fuel rail.

Intake and Exhaust System

The TD05-20G requires airflow that’s two to three times the stock unit. Restricting it with a small intake or a crushed exhaust will kill power and increase turbo lag.

  • Upgraded intercooler: A front-mount intercooler (FMIC) is highly recommended. It drops intake air temperatures significantly and handles the higher boost volumes without heat soak. A stepped-core design (e.g., 3.5” core) works well for street-driven Legacys.
  • Cold air intake: A 3.0” or larger intake with a high-flow filter (K&N, AEM) reduces restriction before the compressor. Avoid oiled filters if you run a blow-through MAF setup—some tuners prefer speed-density.
  • Downpipe and exhaust: A 3” diameter downpipe (catless or with a high-flow cat) is essential. Pair it with a 3” cat-back exhaust to minimize backpressure. Keep the exhaust mandrel-bent and as straight as possible.
  • Boost control: An electronic boost control solenoid (e.g., Cobb, Grimmspeed) allows precise boost management and can help reduce boost creep.

Engine Management and Tuning

This is non-negotiable. The factory ECU will not recognize the TD05-20G’s airflow curve, so you must use a reflash, a piggyback system, or a standalone ECU. Common options:

  • COBB Accessport: With a custom pro-tune, the Accessport is one of the most user-friendly solutions for 2005+ Legacys. It adjusts fuel maps, timing, and boost targets.
  • Open-source tuning (RomRaider/ECUFlash): Free but requires a wideband oxygen sensor and a Tactrix cable. Ideal for DIYers who understand tuning.
  • Standalone ECUs: Haltech, Link, or Motec for fully custom setups. Expensive but offers unlimited control.

Installation Guide for the TD05-20G on a Subaru Legacy

Installing a TD05-20G is a moderate-to-difficult job that takes one to two full days. You’ll need a lift or jack stands, metric sockets (10mm, 12mm, 14mm, 17mm), a torque wrench, and a set of combination wrenches. A bottle of PB Blaster for rusted bolts is also wise. Below is a step-by-step outline to help you get it right.

Pre-Installation Checks

  • Inspect your existing oil supply and return lines. The TD05-20G often requires a -4 AN oil feed line and a -6 AN oil return line. Many kits come with these, but verify compatibility with your Legacy’s engine block.
  • Check the coolant lines if your new turbo uses water cooling (most TD05-20G units do). You may need to adapt or re-route the coolant hoses.
  • Test-fit the turbo on a spare up-pipe or manifold if you’re using an adapter. Mark any clearance issues with the strut tower or frame rail.

Removing the Stock Turbo

  1. Disconnect the battery and drain the engine oil and coolant partially to avoid messes.
  2. Remove the air intake duct, intercooler pipes, and the factory intercooler.
  3. Unbolt the downpipe from the turbo and exhaust manifold. Use new gaskets on reassembly.
  4. Unbolt the oil and coolant lines from the turbo. Be ready for some fluid spillage.
  5. Remove the up-pipe bolts at the manifold (and at the turbo if it’s an integrated unit).
  6. Lift out the stock turbo and up-pipe assembly. On some Legacy models, you may need to remove the passenger-side engine mount or lift the engine slightly to clear the chassis.

Installing the TD05-20G

  1. Attach the new up-pipe to the turbo (use a new gasket and copper anti-seize on the studs).
  2. Mount the up-pipe to the exhaust manifold. Tighten to factory torque specifications (usually 30-35 ft-lb for the up-pipe studs).
  3. Connect the oil feed line (typically from the oil pressure sending unit location on the passenger side of the block) to the turbo bearing housing. Use a restrictor (0.060” orifice) if the OEM line is too large—ball-bearing turbos often need less oil flow.
  4. Attach the oil return line to the block. Ensure it slopes downward without kinks. If the return port on the new turbo is in a different location, you may need to reposition the drain tube.
  5. Connect the coolant lines (if applicable) using a barbed T-fitting or quick-disconnects provided in the kit.
  6. Install the downpipe with a new gasket and torque to 35–40 ft-lb.
  7. Reinstall the intercooler with a Y-pipe that fits the TD05’s compressor outlet (often 2.0” diameter). You might need a silicone reducer.
  8. Reattach the battery, fill the engine with fresh oil and coolant, and check all fasteners.

Post-Installation Checks

  • Prime the turbo: Disconnect the ignition coil or fuel pump relay, then crank the engine for 10–15 seconds to circulate oil through the turbo before actual startup.
  • Check for leaks: Use a boost leak tester (or a homemade adapter) at the compressor inlet. Pressurize to 20 psi and listen for hisses. Common leak points are the intercooler hoses, throttle body gasket, and intake couplers.
  • Perform a test run: Start the engine and let it idle for 2 minutes. Listen for unusual noises (whining metal-on-metal indicates oil starvation). Check fluid levels again.
  • Data log the first drive: Monitor boost pressure, wideband AFR, and intake air temps. Stop immediately if you see knock or extremely lean mixtures (AFR above 12.5 under boost).

Common Challenges and How to Overcome Them

Every custom turbo installation has its quirks. The TD05-20G on a Legacy is no exception. Here are the most frequent issues and solutions:

  • Fitment issues: The TD05-20G is physically larger than the stock VF40. In many Legacy GT engine bays, the compressor housing can interfere with the strut tower, brake master cylinder, or alternator. Solutions include using a rotated mount kit (like those from Forced Performance) or shimming the motor mounts. Some builders grind the strut tower slightly for clearance.
  • Boost creep: This occurs when the wastegate cannot bypass enough exhaust gas to control boost. It’s common with larger turbos on free-flowing exhausts. Fixes include porting the wastegate passage, using a stronger wastegate actuator (e.g., 14 psi spring), or adding an external wastegate via a divorced downpipe.
  • Oil drain problems: If the TD05-20G’s oil outlet sits lower than the block drain, oil can pool and cause smoke or seal failure. Raise the turbo by using a taller up-pipe, or extend the oil return line with a larger diameter tube and ensure it descends continuously.
  • Heat management: More heat is the price of more power. Consider wrapping the downpipe and up-pipe with titanium heat wrap, installing a turbo blanket, and adding a hood scoop or vent to expel hot air.

Tuning and Final Adjustments

After completing the mechanical installation, a proper tune is what turns a noisy car into a fast one. You have two paths: a dyno tune by a professional or a street tune with an experienced tuner. For most Legacy owners, a dyno tune is recommended because it allows precise load-based tuning and validation of boost control. Expect to pay $350–$600 for a custom map.

During the tune, the technician will:

  • Set base ignition timing and fuel maps for your injectors and turbo.
  • Dial in boost targets for your wastegate setup. For a TD05-20G on pump gas, 18–20 psi is a safe sweet spot.
  • Enable data logging of knock correction, AFR, and boost. Aim for AFR of 11.0–11.5 under full boost to keep exhaust gas temperatures safe.
  • Test for boost creep by doing a third or fourth gear pull. If boost climbs above target after redline, adjust the wastegate duty or change the wastegate spring.

If you’re using a Cobb Accessport, you can purchase a base map from a tuner like Torqued Performance or M-Festuned for a starting point. Still, a custom tune tailored to your specific fuel and parts yields the best results and reliability.

Conclusion

Upgrading to a TD05-20G turbo on your Subaru Legacy is a rewarding project that can elevate the car’s performance from mundane to exhilarating. With the right supporting modifications—fuel system, intercooler, exhaust, and a quality tune—you can expect 280–350 whp on pump gas, with a powerband that pulls hard from 3500 RPM to redline. The installation is not a plug-and-play affair; it requires careful attention to fitment, oil routing, and boost control. But for those willing to put in the work, the TD05-20G offers excellent value and proven results. Remember to always tune your vehicle properly, invest in quality parts, and consult community resources like LegacyGT.com for model-specific advice. Take your time during the install, double-check every connection, and you’ll be rewarded with a Legacy that truly comes alive at the touch of the throttle.