Upgrading your Subaru WRX with a TD05H-20G turbocharger is one of the most rewarding modifications you can make. This turbo represents a sweet spot between quick spool and substantial top-end power, making it a favorite among enthusiasts who want a streetable yet aggressive setup. In this guide, we will walk through the entire process — from understanding the turbo's design and expected power gains to a detailed installation procedure and the supporting modifications you need for a reliable build.

Understanding the TD05H-20G Turbocharger

The TD05H-20G is a Mitsubishi Heavy Industries turbocharger that originally found its home on Mitsubishi Evolutions and various performance applications, but it has become a legendary upgrade for the Subaru EJ platform. The “TD05H” refers to the turbine housing and wheel family, while “20G” indicates the compressor wheel size and trim. This combination delivers a significant step up in airflow capacity compared to the stock VF-series turbos found on WRX models.

Origins and Design Philosophy

Mitsubishi designed the TD05H family to be compact, durable, and capable of sustaining high boost levels. The 20G compressor wheel features an extended-tip design that moves more air without sacrificing too much spool speed. For a WRX owner, this means you get a turbo that can push past 400 wheel horsepower on pump gas with the right supporting mods, while still being responsive enough for daily driving. The TD05H-20G is often described as a “big turbo that doesn’t completely kill low-end torque” — it retains enough bottom-end punch to keep the car fun off boost.

Key Specifications and Flow Characteristics

The TD05H-20G typically features a 68mm compressor inducer and a 56.5mm turbine inducer, with a 10cm² turbine housing being the most common size for WRX applications. This configuration flows approximately 520–550 CFM at 15–20 psi, which is roughly a 30–40% increase over a stock VF39 or VF48 turbo. The 20G is capable of supporting up to 50 lb/min of airflow, making it suitable for power targets between 350 and 450 wheel horsepower. Spool characteristics vary depending on engine displacement, header design, and tune, but most setups see full boost by 3500–3800 RPM.

How It Compares to Stock and Other Upgrades

Compared to the stock VF-series turbos, the TD05H-20G offers significantly more headroom. A stock VF39 might run out of breath around 300–320 whp, whereas the 20G keeps pulling hard past that point. Relative to larger turbos like the Garrett GT3076R or a BorgWarner EFR 6758, the 20G is easier to tune, simpler to install, and more forgiving on stock engine internals if boost and timing are kept sensible. It is a proven, time-tested upgrade that doesn't require a standalone engine management system — a quality AccessPORT or open-source tune will do just fine. For a thorough understanding of turbo sizing and matching to the EJ engine, resources like the NASIOC turbo FAQ provide excellent background reading (NASIOC Turbo FAQ).

Power Gains and Performance Expectations

Moving from a stock WRX turbo to a TD05H-20G is not a subtle change. The power delivery transforms from a sharp, peaky hit to a broader, more sustained surge that pulls hard well past redline. The gains are substantial, but they also demand respect for the increased stress on the drivetrain and engine.

Horsepower and Torque Curves

On a well-tuned 2.0-liter EJ205 with supporting mods, a TD05H-20G typically produces 330–380 whp on 93-octane pump gas. On a 2.5-liter EJ255 or EJ257, those numbers climb to 370–430 whp. With ethanol blends like E85 and a conservative tune, 450+ whp is achievable. Torque peaks around 370–400 lb-ft on 2.5-liter engines, often requiring careful boost management to avoid bending rods. The torque curve is broad and flat, with peak torque arriving by 3800 RPM and holding strong to 6500 RPM. This makes the car feel extremely responsive in the mid-range, far more so than a stock turbo that peaks early and fades.

Spool Characteristics and Boost Response

The 20G spools more slowly than the stock turbo — expect full boost around 3500–3700 RPM on a 2.5-liter, and closer to 3800–4000 on a 2.0-liter. This is a noticeable change from the VF-series turbos that hit hard by 3000 RPM. However, once the 20G is on boost, it pulls harder and continues pulling all the way to 7000+ RPM. The trade-off is a small amount of low-rpm lag in exchange for a massive top-end gain. Antilag or a good boost controller strategy can help, but most drivers adapt quickly and appreciate the broader powerband.

Real-World Driving Improvements

Beyond dyno numbers, the 20G transforms how the WRX drives. Highway passing becomes effortless — a quick downshift from 60 mph sends the car surging past traffic. On track days, the extended powerband means fewer shifts and faster lap times. The turbo also responds well to higher boost levels, so if you ever want to turn up the wick with ethanol or race gas, the hardware can handle it. Just be prepared for increased tire spin and a shorter lifespan on stock clutch and transmission components.

Supporting Modifications for a Successful Build

Dropping a TD05H-20G onto a stock WRX without any other changes is a recipe for disappointment — and potentially a blown engine. This turbo needs fuel, cooling, and proper engine management to operate safely and deliver its full potential.

Fuel System Upgrades

The stock fuel injectors on a 2002–2005 WRX (440cc) are grossly inadequate for a 20G. Even on low boost, they will hit 100% duty cycle quickly, leading to lean conditions and knock. You need at least 750cc–850cc injectors for pump gas setups targeting 350–400 whp, and 1000cc–1300cc for E85. A high-flow fuel pump — such as a Walbro 255 or AEM 340 — is mandatory. A fuel pressure regulator is also a good idea if you are pushing high boost, as it helps maintain stable delivery. Cobb Tuning offers a comprehensive guide to fuel system planning for turbo upgrades (Cobb Tech Articles).

Intercooling and Intake Improvements

The stock top-mount intercooler (TMIC) will become a heat soak liability with a 20G pushing more boost and sustained high RPM runs. A larger TMIC from Spearco, Process West, or a quality air-to-air front-mount intercooler (FMIC) is strongly recommended. For power above 400 whp, most builders go with a FMIC to keep charge air temperatures under control. The intake system also needs attention — the stock airbox restricts flow. A short-ram intake with a quality filter and a larger MAF housing (or a speed-density tune) lets the turbo breathe freely. An aftermarket blow-off valve or recirculating bypass valve rated for higher boost is also a wise addition to prevent compressor surge.

Exhaust System Considerations

To let the 20G exhale properly, you need at least a full turbo-back exhaust. A 3-inch downpipe with a high-flow catalytic converter (or catless for maximum flow) is the minimum. The stock mid-pipe and axle-back are too restrictive and will choke the turbo's top-end. A equal-length header can also improve spool and reduce engine knock risk by balancing exhaust pulses. However, unequal-length headers retain the classic Subaru rumble, so this is a personal choice.

Engine Management and Tuning

You cannot simply bolt on a 20G and drive away. The engine management system must be reprogrammed to account for the increased airflow and fuel requirements. An AccessPORT with a professional protune, a Tactrix cable and open-source tuning software, or a standalone ECU like Haltech or Link are all viable options. A proper tune includes adjustments to fuel maps, ignition timing, boost control, and knock detection. Do not trust “off-the-shelf” maps for a 20G — every setup is different, and a custom tune is the only way to ensure safety and performance. The Subaru performance tuning community on forums like IWSTI offers extensive data on 20G tunes and common pitfalls (IWSTI Forums).

Step-by-Step Installation Guide

Installing a TD05H-20G on a WRX is a job that can be completed in a weekend with basic mechanical skills and common tools. This guide assumes you have the new turbo, gaskets, oil lines, and all necessary hardware. Work on a cold engine and disconnect the battery before starting.

Preparation and Tools Required

You will need a socket set (10mm, 12mm, 14mm, 17mm), combination wrenches, a torque wrench, flathead and Phillips screwdrivers, pliers, a gasket scraper, and a drain pan. New gaskets are essential — do not reuse old ones. You also need a replacement oil feed line (the TD05H uses a different fitting than stock) and a new oil return line gasket. Antiseize on exhaust bolts is recommended. Having a second person helps with positioning the turbo, which is heavy and awkward.

Removing the Stock Turbo Assembly

Start by disconnecting the negative battery terminal. Drain the engine oil and coolant if you plan to service the lines. Remove the intake snorkel, airbox, and MAF sensor. Unbolt the turbo heat shield and the downpipe from the turbo exhaust housing. Disconnect the intercooler hoses and remove the TMIC. Unplug the turbo oil feed line from the engine block and the oil return line from the oil pan. Unbolt the turbo from the uppipe (or header) and lift it out. Take care not to drop debris into the open oil lines. Clean the mating surfaces thoroughly.

Installing the TD05H-20G Turbo

Attach the new oil feed line and oil return line to the TD05H-20G before mounting it — this is much easier with the turbo out of the car. Apply a thin layer of antisieze to the bolt threads. Position the turbo onto the uppipe studs and hand-tighten the nuts. Then torque them to spec (typically 30–35 ft-lbs). Reattach the oil return line to the oil pan using a new gasket. Connect the oil feed line to the engine block. Reinstall the downpipe with a new gasket and torque the bolts evenly. Reconnect the intercooler hoses and the TMIC. Reattach the intake system, ensuring all couplers are tight and no loose clamps exist.

Reconnecting Systems and Final Checks

Reconnect the battery. Refill engine oil to the correct level and top off coolant if you drained it. Check all hose connections, especially the boost reference line to the wastegate actuator and the recirculating bypass valve hose. Inspect the oil and coolant lines for any signs of leakage before starting the engine.

First Start and Break-In Procedure

Prime the turbo by pulling the fuel pump relay and cranking the engine for 10 seconds — this pushes oil through the new turbo before it spins. Reconnect the relay and start the engine. Let it idle for 5 minutes, monitoring for oil leaks, coolant leaks, and unusual sounds. Check oil pressure on your gauge. After idling, take the car on a gentle drive, staying under 3000 RPM and low boost for the first 50 miles. This seats the turbo bearings and ensures everything is settled. After a few heat cycles, you can begin to gradually increase boost as the tune is dialed in.

Potential Challenges and How to Avoid Them

Even a well-planned installation can present obstacles. Being aware of common issues beforehand saves time and frustration.

Oil and Coolant Line Fitment Issues

The TD05H-20G uses a different oil feed thread pitch and banjo bolt size than the stock Subaru turbo. You may need an adapter fitting or a custom braided oil line. Similarly, the coolant lines may not align perfectly with the 20G's housing. Measure twice and order the correct lines in advance. Many aftermarket turbo kits come with the proper lines, but if you are piecing a setup together, consult a supplier like TurboSource for application-specific line kits.

Boost Control and Wastegate Setup

Most TD05H-20G turbos come with an internal wastegate, but the spring pressure may not match your boost target. You might need an aftermarket boost controller (electronic or manual) and possibly a larger wastegate actuator if you experience boost creep. Porting the wastegate passage on the turbine housing can also help regulate boost. Without proper boost control, you risk overboosting and detonation.

Engine Reliability and Longevity

With a 20G pushing more air and fuel, the stock engine internals — especially pistons and rods on the EJ205 — become the weak link. Keeping boost under 20 psi on pump gas and running a conservative tune with a safe air/fuel ratio is critical. Regularly inspect oil quality, watch for knock correction on your tuner's software, and consider a forged piston upgrade if you plan to track the car or run high boost on ethanol. The TD05H-20G is robust, but the engine it sits on must be prepared for the extra load.

Conclusion

Upgrading to a TD05H-20G turbocharger on your WRX delivers a transformative increase in power and driving enjoyment. With proper planning, supporting modifications, and a professional tune, this turbo can push your car well into the 350–450 whp range while remaining streetable and reliable. The installation is within reach of a skilled home mechanic, and the aftermarket knowledge base is vast. Whether you use your WRX as a daily driver, weekend canyon carver, or track machine, the 20G is a proven, time-tested upgrade that will make every throttle input feel like an event. Do your research, invest in quality parts, and enjoy the build process — your WRX will thank you with every pull.