The MHI 20G Turbo Upgrade: A Game-Changer for Your 2G Eclipse

The Mitsubishi Eclipse, particularly the second-generation (2G) model produced from 1995 to 1999, remains a beloved platform in the tuning community. With its robust 4G63 engine, the 2G Eclipse GSX and GST offer exceptional potential for performance upgrades. Among the most popular and effective modifications is the MHI 20G turbocharger upgrade. This article provides a comprehensive, authoritative guide to understanding the power gains, installation requirements, and tuning considerations associated with swapping to an MHI 20G turbo on a 2G Eclipse, focusing on the impressive 70+ horsepower increase and significantly improved boost response that enthusiasts consistently report.

Understanding the MHI 20G Turbocharger

The MHI 20G turbocharger is manufactured by Mitsubishi Heavy Industries, the same company that produced the factory turbo units for the Eclipse. This turbo sits in a sweet spot in the upgrade path, offering substantially more airflow than the stock T25 or 14B units without the lag associated with larger turbos like the TD05 16G or 18G variants. The "20G" designation refers to the compressor wheel size, which is larger and more efficient at moving air into the engine.

Key characteristics of the MHI 20G turbo include:

  • Larger compressor wheel: Flows more air at higher boost levels, typically supporting up to 400-450 horsepower with proper supporting mods.
  • Improved compressor housing: Designed for better airflow dynamics, reducing backpressure and improving volumetric efficiency.
  • Journal bearing vs. ball bearing: The MHI 20G uses a journal bearing design, which is durable and cost-effective. While ball bearing turbos spool faster, the 20G's journal bearing design offers excellent reliability and is well-suited for street and track use.
  • Direct bolt-on compatibility: With the right install kit, the MHI 20G is a direct replacement for the stock turbo on the 2G Eclipse, minimizing fabrication work.

The MHI 20G is widely recognized for its ability to deliver strong mid-range and top-end power while maintaining reasonable spool characteristics. This makes it an ideal upgrade for drivers who want aggressive acceleration without sacrificing daily drivability.

Power Gains: How Much Horsepower Can You Expect?

The headline figure of 70+ horsepower is not just marketing hype. Numerous dyno-proven examples from the 2G Eclipse community confirm that an MHI 20G upgrade, when paired with proper tuning and supporting modifications, can yield gains of 70 to 100 wheel horsepower over a stock or mildly modified engine.

The actual horsepower increase depends heavily on your starting point. A bone-stock 2G Eclipse GST produces approximately 210 horsepower at the crank (around 170-180 at the wheels). With an MHI 20G turbo, a well-tuned setup on pump gas can achieve 300-350 wheel horsepower. That represents a gain of 120-170 wheel horsepower, significantly exceeding the 70 HP baseline mentioned in the original content. Even on a relatively stock engine with only basic bolt-ons (exhaust, intake, boost controller), a gain of 70-90 wheel horsepower is realistic.

Factors That Influence Final Power Output

To achieve the maximum power potential from the MHI 20G, several variables must be optimized:

  • Engine condition: A healthy engine with good compression and no vacuum leaks will respond better to increased boost. Low mileage engines or those with recent rebuilds will produce more power safely.
  • Fuel quality and octane: Using high-octane fuel (93 AKI or higher) is essential for preventing knock. For more aggressive tunes, consider ethanol blends like E85, which allow higher boost levels and timing advance.
  • Boost pressure: The MHI 20G can safely run 18-22 psi on a good tune with pump gas. Higher boost requires race fuel or ethanol. Every 1 psi of boost increase typically adds 10-15 horsepower, up to the turbo's efficiency limit.
  • Supporting modifications: The turbo is only one piece of the puzzle. Exhaust flow, intake restriction, and fuel delivery all must be addressed to realize the full potential.
  • ECU tuning quality: A poorly tuned engine can leave power on the table or cause dangerous knock. Professional tuning is strongly recommended.

Real-World Dyno Results

Many 2G Eclipse owners have documented their MHI 20G builds on forums like DSMTuners and EclipseTalk. A common configuration includes the 20G turbo, a 3-inch turbo-back exhaust, a front-mount intercooler (FMIC), upgraded fuel injectors (650-850 cc/min), and a Walbro 255 lph fuel pump. On pump gas at 20 psi, these setups consistently produce 320-350 wheel horsepower. With E85 and 25 psi, 380-410 wheel horsepower is achievable. This represents a gain of 150-200 horsepower over stock, far exceeding the 70 HP figure from the original article.

It is important to note that these numbers are at the wheels. Crank horsepower estimates add drivetrain loss (typically 15-20% for an AWD GSX and 10-15% for a FWD GST). A 350 wheel horsepower GST would be around 400 crank horsepower, a substantial jump from the factory rating.

The Benefits of Better Boost Response

Beyond the headline horsepower gains, one of the most appreciated advantages of the MHI 20G is its improved boost response. "Boost response" refers to how quickly the turbo builds positive pressure (boost) when you press the throttle. A turbo that spools faster delivers power sooner, eliminating lag and making the car feel more responsive.

Compared to the stock T25 turbo (which is small and spools quickly but runs out of breath at higher RPM), the MHI 20G offers a broader powerband. It spools only slightly later than the T25 but continues to pull hard all the way to redline. This eliminates the power drop-off that stock turbo cars experience above 5500 RPM.

How Boost Response Transforms Driving Feel

Improved boost response manifests in several tangible ways that enhance daily driving and track performance:

  • Instantaneous acceleration from a stop: The turbo builds boost quickly from low RPM, reducing the wait time when launching from a standstill. This is particularly beneficial for street driving and autocross.
  • Strong mid-range punch: The 20G's power delivery is linear and aggressive. From 3000 to 7000 RPM, the engine pulls hard without significant drop-off, making overtaking on highways effortless.
  • Better corner exit speed: On a road course, a turbo that spools quickly allows you to carry more speed out of corners. You can get on the throttle earlier without waiting for boost to build, improving lap times significantly.
  • Reduced turbo lag in daily driving: Stop-and-go traffic becomes more enjoyable because the turbo doesn't require high RPM to produce usable power. The 20G is responsive enough to make the car feel lively even at part throttle.
  • Enhanced engine braking: Because the turbo spools faster, it also provides engine braking more effectively when you lift off the throttle, giving the driver more control.

Drivers who upgrade from a stock T25 or a worn 14B frequently describe the MHI 20G as "transforming" the car's character. The engine feels more willing to rev, and the power delivery is smooth yet forceful. This combination of quick spool and strong top-end is what makes the 20G a favorite among enthusiasts who want a versatile, high-performance turbo.

Installation Considerations and Requirements

Installing the MHI 20G turbocharger on a 2G Eclipse requires careful planning. While the turbo itself is a direct bolt-on with the correct install kit, there are several critical supporting modifications and steps that must not be overlooked.

Minimum Required Supporting Modifications

The MHI 20G moves significantly more air than the stock turbo. Without proper supporting components, you risk running lean, detonating, or damaging the engine. The minimum supporting modifications include:

  • Upgraded fuel injectors: The stock 450 cc/min injectors cannot supply enough fuel for the increased airflow. Upgrade to at least 550-650 cc/min injectors. For E85, 750-850 cc/min injectors are recommended.
  • High-flow fuel pump: A Walbro 255 lph or 400 lph fuel pump is essential to maintain fuel pressure at high boost. Without it, the stock pump will drop pressure, leading to lean conditions and potential engine failure.
  • AFPR (Adjustable Fuel Pressure Regulator): When using a high-flow pump, an AFPR is needed to correctly set base fuel pressure. The stock FPR may not be able to handle the extra flow, causing overly rich idle conditions.
  • ECU tuning solution: You cannot simply bolt on the turbo and drive. You need a means to tune the engine. Options include EPROM chip tuning, standalone ECUs (like AEM, Haltech, or Link), or piggyback systems like the Greddy e-Manage or AEM FIC. Professional dyno tuning is strongly advised.
  • Upgraded intercooler (FMIC or SMIC): The stock side-mount intercooler will become a restriction at higher boost levels and can cause high intake air temperatures (IATs). A front-mount intercooler is the most common and effective upgrade. A high-quality side-mount intercooler from a Supra or an aftermarket SMIC can also work.
  • Performance exhaust system: A 3-inch turbo-back exhaust (downpipe, catalytic delete or high-flow cat, cat-back) is critical to reduce backpressure and allow the turbo to spool efficiently. The stock exhaust is too restrictive for the 20G's airflow.
  • Intake system: A high-flow intake pipe and filter (preferably with a heat shield) are needed to reduce inlet restriction. The stock airbox is restrictive and should be replaced.
  • Boost controller: A manual or electronic boost controller allows you to set boost pressure to a safe level. A quality unit (like a Hallman Pro or a Greddy Profec) is recommended.

While not strictly required, the following upgrades will improve reliability and performance:

  • Upgraded clutch: The stock clutch will slip under the increased torque. A stage 1 or stage 2 clutch from ACT, South Bend, or Competition Clutch is recommended for daily-driven cars.
  • Reinforced drivetrain: AWD GSX models may benefit from upgraded axles and a stronger transfer case. FWD models may experience wheelspin issues, requiring better tires or a limited-slip differential.
  • Upgraded cooling: A larger radiator and high-flow water pump help manage increased heat. A lower temperature thermostat and a proper coolant mix are also beneficial.
  • Upgraded PCV and breather system: Under boost, crankcase pressure builds. A catch can setup or a properly vented PCV system prevents oil from being forced into the intake tract.
  • Upgraded engine mounts: Stiffer motor mounts reduce engine movement, improving shift feel and preventing the exhaust from hitting the chassis under acceleration.

Installation Process Overview

The process involves removing the stock turbo and manifold, installing the 20G turbo with the appropriate install kit (which includes new oil feed and drain lines, gaskets, and possibly a new O2 housing), and reconnecting the intercooler piping. The O2 housing may need to be modified or replaced to fit the 20G's larger compressor housing. It is recommended to use new copper or aluminum crush washers on oil fittings to prevent leaks. Torque all fasteners to spec and double-check all connections before starting the engine.

Tuning Your 2G Eclipse After the MHI 20G Install

Proper tuning is the single most important factor in achieving safe and reliable power from your MHI 20G upgrade. Even with all the best parts, a poor tune can lead to detonation, high exhaust gas temperatures (EGTs), and engine failure.

What Tuning Involves

Modern tuning involves modifying the engine control unit (ECU) software to adjust air-fuel ratio (AFR), ignition timing, and boost control. For a 2G Eclipse, the most common tuning paths include:

  • ECMLink (DSMLink): This is the gold standard for DSM tuning. It allows full control over the ECU, including real-time data logging, fuel maps, timing maps, and boost control. ECMLink is user-friendly and has a large community of experienced tuners.
  • Standalone ECU: Systems like AEM Infinity, Haltech Elite, or Link G4+ offer even more control and flexibility. They are more expensive and require more expertise to tune but are ideal for extreme builds.
  • Piggyback systems: Devices like the AEM FIC or Greddy e-Manage intercept sensor signals and modify them to achieve the desired tune. These are less capable than full ECU solutions but can work for moderate power levels.
  • Chip tuning: For those with a stock ECU, a custom EPROM chip can be written with new fuel and timing maps. This is a "set it and forget it" solution but lacks flexibility for future changes.

Critical Tuning Parameters

When tuning an MHI 20G-equipped 2G Eclipse, pay attention to the following:

  • Air-Fuel Ratio (AFR): Target 11.5-12.0:1 under full boost for pump gas. For E85, target 11.0-11.5:1. Rich conditions can cause misfires and waste fuel; lean conditions can cause knock and piston damage.
  • Ignition timing: Advance timing until you reach the knock threshold, then pull back 1-2 degrees for safety. Stock timing maps are overly conservative; a proper tune can increase power safely.
  • Boost pressure: Start with 12-14 psi and slowly increase, logging data to ensure the engine is not knocking. Most MHI 20G setups are safe at 18-20 psi on 93 octane. For E85, 22-25 psi is common.
  • Knock control: Listen for knock via knock sensor feedback. The 2G ECU has a knock sensor; use it to detect detonation and adjust the tune accordingly. Any knock is bad; aim for 0 counts of knock during normal driving.
  • Engine temperature: Monitor coolant and oil temperatures. Under high boost, heat builds quickly. If temperatures exceed 210°F (100°C) coolant or 250°F (121°C) oil, consider upgrading the cooling system.

Choosing a Tuner

If you are not comfortable tuning the car yourself, seek out a professional tuner with DSM experience. Look for someone who has tuned MHI 20G or similar setups on 4G63 engines. A good tuner will not only maximize power but also check for potential issues like knock, high EGTs, and fuel pressure problems. Expect to pay $300-$600 for a dyno tune, depending on your location and the complexity of the setup.

If you choose to tune yourself, invest in a wideband oxygen sensor and a data logging tool (like ECMLink's included logging). Spend time learning the fundamentals of tuning before making changes. The DSM tuning community is active and helpful; many experienced tuners share maps and advice on forums.

Maintenance After Tuning

After the initial tune, keep a close eye on the engine for the first few hundred miles. Watch for:

  • Oil leaks around the turbo and oil lines
  • Coolant leaks from hoses or the radiator
  • Boost pressure creep (boost rising uncontrollably at high RPM)
  • Any unusual noises from the turbo or engine

Regular oil changes (every 3,000-5,000 miles) with a high-quality synthetic oil are critical for turbocharger longevity. The MHI 20G's journal bearing design relies on consistent oil pressure and flow.

Real-World Driving Experience: What to Expect

Driving a 2G Eclipse with an MHI 20G turbo is a memorable experience. The car transforms from a peppy, somewhat peaky platform to a linear, relentless powerhouse. With the proper supporting mods, the car feels stock-like in daily driving at part throttle, but the moment you tip in, the power builds smoothly and strongly.

Enthusiasts often report that the car becomes more fun to drive not just in a straight line but also on winding roads. The improved boost response allows the driver to carry momentum through corners and accelerate out with authority. For those who have experienced a poorly tuned or laggy big turbo setup, the MHI 20G is a refreshing departure. It delivers the power where you can actually use it, without the frustration of waiting for boost to build.

One common sentiment among owners is that the MHI 20G makes the car feel "complete." The powerband matches the car's chassis and suspension capabilities, making it an ideal performance upgrade for both daily drivers and weekend warriors. It is a proven, reliable upgrade that has stood the test of time in the DSM community.

Common Myths and Misconceptions

There are several persistent myths about the MHI 20G turbo upgrade that deserve clarification:

  • Myth: The 20G is too laggy for street use. Reality: While it spools slightly later than the T25 (by about 300-500 RPM), the 20G's powerband is broad and linear. With proper tuning, it is very streetable. Many daily-driven 2G Eclipses use this turbo without complaint.
  • Myth: You need forged internals for the 20G. Reality: The stock 4G63 rods and pistons are strong up to around 400-450 wheel horsepower. A well-tuned MHI 20G setup with 350 wheel horsepower is safe on a stock bottom end. Forged rods and pistons are only necessary if you plan to push beyond that level or run high boost on E85.
  • Myth: The 20G is a direct drop-in with no other changes. Reality: While the turbo bolts to the stock manifold and O2 housing with the correct adapter, the supporting modifications listed above are essential. Running a 20G without injectors, pump, and tuning is a recipe for engine failure.
  • Myth: A boost controller alone will give you 70 HP. Reality: Increasing boost on a stock turbo yields small gains. The MHI 20G's larger compressor wheel is what enables the significant power increase. A boost controller should be used to dial in the boost level, but it is the turbo itself that provides the airflow advantage.

Comparing the MHI 20G to Other Turbo Upgrades

The MHI 20G sits between the popular 16G and the larger 18G or 20G-LT turbos. Here is a quick comparison for context:

  • 16G (EVO III 16G): Spools slightly faster than the 20G and is a popular upgrade for those wanting a responsive, street-friendly turbo. It flows slightly less air and tops out around 350-380 wheel horsepower on pump gas.
  • MHI 20G (standard): Offers a higher power ceiling (400+ wheel horsepower) with only a small spool penalty. It is ideal for those who want strong top-end power without going too large.
  • 18G or 20G-LT: These are larger compressor wheels that require more extensive modifications, including custom intercooler piping and sometimes a different exhaust manifold. They spool later and are best suited for high-horsepower builds (450-500+ wheel horsepower).
  • BorgWarner S200SX-E or Garrett GT3076R: These are modern ball-bearing turbos that spool faster and offer even higher efficiency. They are more expensive and require custom fabrication but represent the next tier of performance.

For most 2G Eclipse owners looking for a significant power upgrade without sacrificing daily driveability, the MHI 20G is the sweet spot. It offers a proven power increase, broad community support, and long-term reliability.

Final Verdict: Is the MHI 20G Worth It?

The MHI 20G turbocharger upgrade for the 2G Eclipse delivers a substantial and repeatable power increase of 70-100+ horsepower, along with noticeably improved boost response that transforms the driving experience. When properly installed, supported, and tuned, this turbo offers a balance of performance, reliability, and cost that few other upgrades can match.

The key to a successful build lies in the details: using the correct supporting modifications, investing in professional tuning or learning to tune yourself, and maintaining the car meticulously after the upgrade. The MHI 20G is not a magic bullet; it requires a holistic approach to the entire engine system. However, for those willing to put in the effort, the reward is a car that feels significantly faster, more responsive, and more engaging than its stock counterpart.

Whether you are chasing lap times at the track, looking for thrilling street performance, or simply want to get the most out of your 2G Eclipse, the MHI 20G is a proven, time-tested upgrade that continues to deliver results. With the right plan and execution, your Eclipse will gain over 70 horsepower, better boost response, and a new lease on life.