Understanding Stage 2 Turbo Kits for the Subaru Forester

The Subaru Forester has earned a reputation as a versatile, reliable crossover, but beneath that practical exterior lies a platform with serious performance potential. For owners seeking to transform their daily driver into a genuinely quick machine, a Stage 2 turbo kit offers one of the most effective paths to substantial horsepower gains. Unlike basic bolt-on modifications, a properly engineered Stage 2 setup can push output well beyond factory levels, often delivering increases of 40 horsepower or more at the wheels. This article provides a comprehensive, technical breakdown of what Stage 2 turbo kits entail, the tangible performance benefits, installation considerations, supporting modifications, and the critical importance of professional tuning.

What Defines a Stage 2 Turbo Kit?

The term "Stage 2" varies between manufacturers, but in the context of Subaru turbocharged engines, it typically refers to a combination of upgraded hardware that significantly increases the engine's ability to flow air and fuel. A complete Stage 2 turbo kit for the Subaru Forester generally includes the following core components:

  • Larger Turbocharger: A compressor wheel and turbine housing designed to move more air volume than the stock unit. Common upgrades include TD04HL-19T, VF series, or modern billet wheel turbos.
  • Upgraded Intercooler: A larger or more efficient air-to-air intercooler (or a front-mount intercooler conversion) that reduces intake air temperatures, thereby increasing air density and reducing the risk of detonation.
  • High-Flow Fuel Injectors: Injectors with increased flow capacity (e.g., 750cc–1000cc) to match the additional air volume, ensuring the air-fuel ratio remains optimal under boost.
  • High-Flow Fuel Pump: An upgraded in-tank or inline fuel pump (such as a Walbro 255 lph or AEM 340 lph) to supply consistent fuel pressure at higher flow rates.
  • Engine Management Tuning: The most critical element — a custom ECU tune or a reprogrammed engine control unit that adjusts ignition timing, boost pressure, fuel maps, and knock control parameters specifically for the new hardware.

Some kits also include upgraded boost control solenoids, intercooler piping, intake systems, and exhaust downpipes. However, the absence of any of the above core components can compromise both performance and reliability.

Performance Gains: More Than Just 40 Horsepower

While the headline figure of "40+ HP" is achievable on many Forester models, the actual gains depend on the vehicle's platform (naturally aspirated vs. turbocharged from the factory), engine condition, and the quality of the tune. On the popular Subaru Forester XT (which comes factory-equipped with a turbocharged EJ or FA engine), a Stage 2 setup can yield the following typical results:

  • Peak horsepower increase: 40–70 whp (wheel horsepower) over stock, depending on turbo selection and fuel type.
  • Torque improvement: A broad, flat torque curve with 50–80 lb-ft additional torque in the mid-range, transforming highway passing and hill climbing.
  • Reduced turbo lag: Many modern Stage 2 turbochargers feature billet compressor wheels and improved journal or ball bearing designs that spool faster than stock, delivering boost earlier in the RPM range.
  • Consistent power delivery: With an upgraded intercooler and properly tuned fuel maps, the engine can maintain higher levels of boost without pulling timing due to heat soak.

Dyno results from Cobb Tuning and other reputable tuners show that a properly calibrated Stage 2 Forester XT can exceed 280 wheel horsepower on 93 octane fuel, a significant jump from the stock 224–250 crank horsepower. For reference, Subaru's official specifications for the current Forester lineup highlight the base turbo engine's output, but aftermarket modifications can unlock hidden potential that transforms the vehicle's character.

Detailed Installation Process: A Step-by-Step Guide

Installing a Stage 2 turbo kit is a moderately complex job that requires mechanical experience, specialized tools, and several days of work for a first-timer. Below is a thorough breakdown of the major stages involved.

Preparation and Safety

  • Disconnect the battery and depressurize the fuel system.
  • Drain engine oil and coolant to prevent spills when removing the oil and coolant lines from the turbocharger.
  • Gather all necessary tools: socket set, torque wrench, screwdrivers, pry bars, and a shop manual specific to your Forester's year and engine code (EJ255, EJ257, FA20F, etc.).

Removal of Stock Components

  • Remove the intake air box, intake duct, and air filter housing.
  • Carefully disconnect the turbocharger's oil feed line, oil return line, coolant lines, and the exhaust downpipe.
  • Unbolt the turbocharger from the exhaust manifold and lift it out. Typically this is done from under the vehicle after removing the skid plate and passenger side axle (on some models).
  • Remove the stock intercooler and its mounting brackets. For front-mount intercooler conversions, the bumper beam and crash bar may need modification.
  • Disconnect fuel injectors and remove the factory fuel rails. Replace injectors with the upgraded units, taking care to lubricate O-rings with clean engine oil.

Installation of Stage 2 Components

  • Install the upgraded fuel pump into the tank according to the manufacturer's instructions. Test the pump before reassembling the rear seat or access panel.
  • Mount the new intercooler (either top-mount or front-mount position) with the provided brackets. Ensure all silicone couplers and T-bolt clamps are tight.
  • Bolt the new turbocharger to the exhaust manifold using new gaskets. Tighten to manufacturer torque specifications.
  • Connect the oil and coolant lines. Use high-temperature thread sealant on fittings. Reconnect the boost control solenoid (if applicable) and vacuum lines.
  • Reattach the downpipe (many Stage 2 kits include a high-flow cat or catless downpipe). Ensure no exhaust leaks.
  • Reinstall the intake system, ensuring the air filter is securely housed and that there are no unmetered air leaks between the MAF sensor and the turbo inlet.

Post-Installation Checks and First Start

  • Triple-check all hose clamps, electrical connectors, and ground straps.
  • Refill engine oil to the correct level and top up coolant.
  • Prime the fuel system by cycling the ignition several times before cranking the engine.
  • Start the engine and inspect for leaks. Allow it to idle until the coolant reaches operating temperature, then turn off and recheck fluid levels.
  • Do not drive the vehicle until a custom tune has been applied. Running a Stage 2 setup on a stock ECU map can cause engine damage within seconds.

Supporting Modifications for Maximum Reliability

A Stage 2 turbo kit places additional stress on the engine, drivetrain, and cooling systems. To ensure long-term durability, several supporting upgrades are strongly recommended:

Exhaust System Upgrade

The factory exhaust on the Forester is restrictive. A full turbo-back exhaust (downpipe plus catback) with a 3-inch diameter reduces backpressure and allows the turbocharger to spool more freely. This often yields an additional 5–10 horsepower on its own and reduces exhaust gas temperatures (EGTs).

Upgraded Engine Oil Cooling

Higher boost levels increase engine heat output. An oil cooler with a thermostatic sandwich plate helps maintain oil temperatures within optimal range, especially during track driving or summer conditions. Mishimoto and other brands offer Forester-specific oil cooler kits.

On older EJ engines, the factory pistons are a known weak point when exceeding 300 wheel horsepower. For those aiming for the upper limits of Stage 2 or planning further upgrades, forged pistons and ARP head studs provide the necessary margin of safety.

Clutch Upgrades

The increased torque from a Stage 2 kit can overwhelm a factory clutch, particularly on manual transmission Foresters. A stage 1 or stage 2 clutch kit from Exedy or ACT can handle the extra power without excessive pedal effort.

Critical Importance of Professional Tuning

No Stage 2 turbo kit can function safely without a proper engine tune. The stock ECU relies on pre-programmed maps that do not account for larger injectors, higher boost pressures, or different airflow characteristics. Attempting to drive with a "base map" provided by some vendors can lead to lean air-fuel ratios, knock, and catastrophic engine failure.

Professional tuning can be performed via:

  • Open-source tuning (Tactrix cable + software): A cost-effective option for experienced tuners, allowing full control over the ECU maps.
  • AccessPort (cobb tuning): The most popular platform for Subaru owners. A custom tune from a reputable Cobb Pro Tuner is recommended over generic "stage 2" off-the-shelf maps, as no two vehicles have identical mechanical tolerances.
  • ECUFlash/ECU reflash: Many performance shops offer socketed ECU services for more advanced tuning features.

A proper tune also dials in the boost control solenoid (often a three-port unit) to achieve consistent boost levels across varying weather conditions. The tuner will also adjust the knock sensor sensitivity and create a failsafe (limp mode) for fuel cuts.

Expected Costs and Budget Planning

Enthusiasts should budget carefully for a Stage 2 conversion. Below are approximate cost ranges (USD) for a Subaru Forester XT:

  • Turbo kit (turbo, intercooler, fuel components): $1,500–$3,500
  • Fuel pump: $100–$200
  • Professional tuning: $400–$900
  • Supporting mods (exhaust, clutch, cooling): $1,000–$2,500
  • Labor (if not DIY): $800–$1,500

Total investment typically falls between $3,000 and $7,000, depending on component quality and whether installation is performed by the owner. While this may seem steep, the improvement in driving dynamics and sheer acceleration often justifies the cost for dedicated enthusiasts.

Reliability Considerations and Common Pitfalls

Even with proper installation and tuning, a Stage 2 Forester requires more attentive maintenance than stock. Key reliability factors include:

  • Oil change intervals: Reduce to 3,000–4,000 miles with high-quality synthetic oil (5W-30 or 5W-40 depending on climate).
  • Spark plug gap: Narrower gap (0.025–0.028 inch) may be needed to prevent misfire under high boost.
  • Fuel quality: Always use premium fuel (93 octane or higher in the US). Even a single tank of low-octane fuel can cause detonation and engine damage.
  • Boost leaks: Regularly check intercooler hoses, couplers, and vacuum lines for cracks or loose clamps.

Common mistakes among first-time modders include using a "one-size-fits-all" off-the-shelf tune, ignoring the importance of a wideband oxygen sensor for monitoring, and failing to upgrade the clutch on manual cars. Additionally, some cheap turbo kits from unknown brands use substandard compressor wheels that fail within a few thousand miles. It is essential to purchase from established performance companies such as Grimmspeed, Perrin Performance, or through Subaru-specific forums like NASIOC.

Conclusion

A Stage 2 turbo kit represents one of the most rewarding modifications available for the Subaru Forester, delivering a genuine 40+ horsepower increase along with dramatically improved torque and responsiveness. However, success depends on careful component selection, meticulous installation, and above all, a professional custom tune. By understanding the full scope of the upgrade — including supporting modifications, potential costs, and reliability requirements — owners can enjoy a reliable, high-performance vehicle that far exceeds factory capabilities. Whether you choose to tackle the installation yourself or entrust it to a specialized shop, the result is a daily driver that feels more like a dedicated sports wagon.