The Subaru Forester has long been a favorite among drivers who need practicality without sacrificing performance. Under the hood of many models, a turbocharged boxer engine delivers punchy power, but factory compromises often leave enthusiasts wanting more. Two of the most impactful upgrades for turbocharged Subarus are a high-quality blow-off valve (BOV) and an upgraded intercooler. In particular, pairing a Turbosmart BOV with a properly matched intercooler can dramatically improve throttle response, reduce turbo lag, and unlock consistent power gains. This guide offers an in-depth look at how these modifications work, what to consider before installing them, and how to get the most from your Forester.

Why the Subaru Forester Benefits from Turbo Upgrades

The Subaru Forester, especially the XT and later turbocharged variants, shares much of its engine and turbo architecture with the WRX and Legacy GT. While the stock turbo system is reliable, it is tuned conservatively for fuel economy and emissions. The OEM blow-off valve is often a plastic or simple diaphragm unit that can leak under higher boost. The factory top-mount intercooler is adequate for stock power levels but quickly heat-soaks during aggressive driving or in warm weather. Upgrading these components addresses two critical areas: managing boost pressure spikes and lowering intake air temperature. Cooler air means denser air, which allows the engine to burn more fuel and produce more power without increasing boost pressure unsafely.

Moreover, Subaru’s boxer engine design is known for its low center of gravity and robust bottom end. With proper supporting modifications, a Forester can reliably handle 300–350 wheel horsepower. A Turbosmart BOV and an upgraded intercooler form the foundation of a stage 2 or stage 3 setup, whether you are targeting daily drivability or weekend track use. The following sections break down each component in detail.

Turbosmart BOV: Features and Options

Turbosmart is an Australian-based company with decades of experience in boost control systems. Their blow-off valves are engineered for a wide range of turbocharged vehicles, including the Subaru EJ and FA series engines. The key function of a BOV is to release compressed intake air when the throttle plate snaps shut. Without this release, the pressurized air slams back into the compressor wheel, causing surge – a violent flutter that can damage the turbo bearings over time. A quality aftermarket BOV prevents surge and helps the turbo spool faster on the next throttle application.

Plumb‑Back vs. Vent‑to‑Atmosphere

Subarus employ a mass airflow (MAF) sensor system that measures incoming air before the turbo. Venting metered air to the atmosphere (vent-to-atmosphere, or VTA) causes a temporary rich condition because the ECU still injects fuel based on the air it already counted. This can lead to rough idle, stalling, and poor drivability unless the ECU is tuned for it. Many Forester owners prefer a plumb‑back (recirculating) BOV that returns the released air into the intake duct after the MAF sensor, preserving the air‑fuel ratio. Turbosmart offers both plumb‑back, VTA, and dual‑port (adjustable) models. The dual‑port option allows you to route some air back while venting a portion to atmosphere for sound, then later convert to full recirculation if tuning requires it. For most street driven Foresters, a plumb‑back or dual‑port BOV is recommended to avoid drivability issues.

Turbosmart Model Specifics for Forester

  • Kompact EM (Plumb‑Back) Series: Designed for vehicles with a recirculation requirement. It uses a spring‑actuated piston and is available with different spring rates to match boost levels. The Kompact EM provides a subtle sound while maintaining perfect idle and throttle response.
  • VR‑2 (“Vee‑R” II) Universal Series: Suitable for custom installations. It can be configured as VTA or plumb‑back with additional fittings. The VR‑2 is popular among owners who relocate the BOV to a different position on the charge pipe.
  • Supersonic Series: A high‑flow, lightweight BOV primarily for VTA applications. It offers the loudest sound and is best paired with a standalone ECU or aftermarket engine management that can ignore the MAF sensor readings.

For a Forester that retains the factory ECU, the Kompact EM plumb‑back is the simplest and most reliable choice. Installation typically requires a specific adapter flange or a charge pipe that incorporates a BOV mounting boss. Many aftermarket charge pipes for the Forester are already designed to accept a Turbosmart BOV.

Selecting an Upgraded Intercooler

The intercooler sits between the turbo outlet and the throttle body, cooling the compressed air before it enters the engine. On a Subaru Forester, the factory intercooler is a top‑mount (TMIC) located above the engine, cooled by a scoop on the hood. A larger aftermarket intercooler can reduce intake air temperatures (IATs) by 50–80°F under boost, depending on design. Lower IATs prevent detonation and allow the engine to run more advanced timing, which directly translates to power gains.

Top‑Mount vs. Front‑Mount

Upgraded intercoolers come in two primary configurations: larger top‑mount and front‑mount (FMIC). Each has tradeoffs:

  • Upgraded Top‑Mount (TMIC): Retains the stock location, uses the existing hood scoop, and is a direct bolt‑on. Brands like Process West, Mishimoto, and Turboxs offer TMICs with thicker cores and better end tanks. They improve heat dissipation without requiring major front‑end modifications. Best for mild to moderate power levels (up to around 350 whp) and street use where hood scoop airflow is sufficient.
  • Front‑Mount (FMIC): Mounts the core in front of the radiator or AC condenser, fed by ambient air when the car is moving. FMICs offer much greater thermal capacity and are essential for high boost applications (400+ whp). However, they require cutting bumper beams, rerouting piping, and relocating components. The added piping length also introduces a small amount of lag (often negligible with proper tuning). For a daily‑driven Forester aiming for 300–350 whp, a quality upgraded TMIC is usually sufficient and easier to install.

Core Construction and Efficiency

Look for intercoolers with bar‑and‑plate core design, cast aluminum end tanks, and smooth air inlet/outlet transitions. Bar‑and‑plate cores dissipate heat more effectively than tube‑and‑fin designs, especially under sustained boost. Also consider the core thickness and fin density. A core that is too restrictive can cause pressure drop; a core that is too thin may not cool adequately. Reputable manufacturers provide pressure drop and thermal efficiency data. For a Forester, a core around 3–3.5 inches thick with around 300–400 cubic inches of volume works well for stage 2 power levels.

Installation Guide and Common Pitfalls

Installing a blow‑off valve and an upgraded intercooler on a Subaru Forester is a weekend project for experienced DIYers, but certain steps must be done correctly to avoid problems.

BOV Installation Steps

  1. Disconnect battery and relieve fuel pressure (optional but safe).
  2. Remove factory BOV: it is located on the passenger side of the engine, attached to the charge pipe near the throttle body. On later models, the factory unit is a plastic valve integrated into the charge pipe; you may need a replacement charge pipe with a metal bung.
  3. Install adapter or charge pipe with Turbosmart flange. Use thread sealant on any fittings connecting to vacuum lines.
  4. Attach Turbosmart BOV with supplied hardware. For plumb‑back models, connect the recirculation hose back to the intake boot. Ensure no kinks.
  5. Reconnect battery, start engine, and check for vacuum leaks. A common mistake is forgetting to cap the old vacuum port on the intake manifold if the new BOV uses a different signal source.

Intercooler Installation Tips

  1. For an upgraded TMIC: Remove the factory intercooler by disconnecting the Y‑pipe from turbo outlets and throttle body. The Y‑pipe often cracks; replace with a silicone version. Bolt in new intercooler, ensuring the Y‑pipe seals properly. Tighten hose clamps securely.
  2. For FMIC: This is more involved. Remove front bumper, crash beam, and sometimes trim the bumper reinforcement. Mount the core, run piping from turbo to core and core to throttle body. Use silicone couplers and t‑bolt clamps. Ensure the piping clears the radiator fan and AC lines.
  3. Common mistakes: – Reusing old gaskets or hoses. Always replace gaskets on the Y‑pipe and turbo outlets. – Overtightening intercooler bolts can crack the plastic end tanks on some aftermarket units; use a torque wrench. – Routing vacuum lines incorrectly for the BOV, if using a port on the intercooler. Follow manufacturer diagrams.

Tuning Your Forester After Upgrades

A Turbosmart BOV and upgraded intercooler alone can provide minor improvements, but to unlock the full potential, the engine management must be recalibrated. The factory ECU is tuned for the stock intake system. After changing airflow characteristics (more efficient intercooling, different BOV recirculation behavior), the air‑fuel ratio and ignition timing are no longer optimal.

Why Tuning Is Necessary

The intercooler upgrade reduces intake air temperatures, which increases air density. The MAF sensor reads more mass flow, but the fuel maps are set for the colder air of the smaller intercooler. Without tuning, the engine may run lean or experience knock under boost. Similarly, a plumb‑back BOV can affect the air measured by the MAF during throttle closure and reopening – the ECU needs recalibration to maintain smooth transitions. Many owners report that after installing a BOV without tuning, the car hesitates between shifts or stalls at stoplights.

Tuning Options

  • Open‑source tuning with Tactrix cable and software like RomRaider or EcuTek: Requires a laptop, a Tactrix cable, and a willingness to learn. Many Forester forums have base maps for Stage 2 setups (intake, downpipe, intercooler, BOV). Custom tuning by a professional e‑tuner is recommended for safety.
  • AccessPort (COBB Tuning): Offers off‑the‑shelf maps for common modifications, but not specifically for a Turbosmart BOV and aftermarket intercooler combination. However, an AccessPort with a custom protune is very effective. COBB supports Subaru models until 2021 but check compatibility with your Forester year.
  • Standalone ECU (MoTeC, Haltech, Link, etc.): Rarely needed for moderate power levels. Suitable if you are building a high‑boost engine over 400 whp.

Regardless of the route, after installing the intercooler and BOV, get a baseline dyno run or street log. Then tune for an air‑fuel ratio around 11.5:1 under full boost and an ignition timing that avoids knock (usually 7–10 degrees peak torque, rising to 15–18 at redline). A professional tuner can dial in boost targets and wastegate duty cycles to match the new intercooler’s efficiency.

Performance Testing and Expected Gains

Real‑world results vary based on other modifications, but a Forester with a high‑flow downpipe, cat‑back exhaust, a Turbosmart BOV (plumb‑back), and a Process West TMIC typically sees 40–60 hp and 50–70 lb‑ft torque gains at the wheels with a proper tune. The throttle response becomes noticeably sharper, and the engine pulls harder in mid‑range RPM (3,000–5,000). The BOV eliminates the “flutter” sound typical of stock valves under part‑throttle lift, replacing it with a quick “chirp” if vented slightly (on dual‑port) or a muted “whoosh” in full recirculation.

Drag strip results vary, but many Forester XT owners report shaving 0.3–0.5 seconds off their quarter‑mile times (dropping from 14.2 to 13.8 seconds, for example) after these upgrades plus a tune. On the street, the most noticeable improvement is mid‑range torque – the car feels more responsive at highway merging speeds and requires less throttle tip‑in to maintain speed uphill.

Maintenance and Longevity

Aftermarket BOVs and intercoolers generally require less maintenance than factory parts, but periodic checks ensure reliability.

  • BOV maintenance: Turbosmart BOVs are rebuildable. The piston seal and spring can be replaced after 50,000–100,000 miles if the valve starts to leak. Check for oil in the vacuum line – excessive blow‑by can cause the BOV to stick. Clean the diaphragm with a non‑residue cleaner.
  • Intercooler maintenance: Inspect the fins for debris (leaves, bugs). Clean the core surface gently with water and low‑pressure air – do not use a pressure washer that can bend fins. Check couplers and clamps for looseness every 5,000 miles. If you notice a drop in boost, visually inspect for intercooler pipe leaks – common at couplings.
  • Tuning health: Regularly check datalogs for knock correction (feedback knock) on a live gauge. After upgrading the intercooler, you may be able to run higher boost, but only if the fuel system (pump, injectors) can keep up. Many Subarus with stock 565cc injectors max out around 330 whp. If you push beyond, upgrade fuel pump and injectors.

Conclusion

Upgrading your Subaru Forester with a Turbosmart blow‑off valve and a larger intercooler is one of the most effective ways to improve power delivery, reduce heat soak, and enhance overall driving enjoyment. Whether you choose a plumb‑back Kompact EM or a dual‑port for added sound, and whether you stick with a larger top‑mount or go full front‑mount, these modifications work in synergy to create a more responsive, cooler‑running engine. Remember that a professional tune is essential to realize the full gains and maintain engine safety. With the right combination, your Forester can transform from a practical wagon into a confident performance machine that still serves its daily duties.

For further reading and product selection, refer to Turbosmart’s official website for BOV fitment guides, Process West and Mishimoto for intercooler options, and Subaru Forester Owners Forum for community‑tested setups. Always cross‑reference with your Forester’s model year and engine code (EJ255, EJ20X, FA20F) before purchasing components. With careful planning and proper installation, these upgrades will provide years of satisfying performance.