Upgrading your Subaru Forester Turbo with a Cobb SF Intake and an HKS Blow-Off Valve (BOV) is one of the most popular first steps for enthusiasts looking to unlock more power and sharper throttle response. These two modifications work together to improve airflow into the engine and reduce compressor surge during shifts, respectively. While the installation is straightforward for a competent DIYer, careful attention to detail is essential—especially when it comes to the blow-off valve, as Subaru’s MAF sensor can be sensitive to unmetered air. This expanded guide will walk you through every step, from gathering the right tools to performing a final test drive, while also covering important tuning considerations and common pitfalls.

Why Upgrade the Intake and BOV?

The factory intake system on the Forester Turbo is designed for quiet operation and emissions compliance, not for maximum airflow. Restrictive airboxes, convoluted ducting, and a paper air filter all choke the turbocharger, especially as boost increases. The Cobb SF Intake replaces the entire tract with a smooth aluminum tube and a high-flow silicone intake duct, allowing the turbo to ingest cooler, denser air with minimal restriction. In addition, the intake relocates the MAF sensor into a larger housing, which helps reduce turbulence and provides more accurate airflow readings when properly tuned.

The HKS Blow-Off Valve replaces the factory recirculating bypass valve. The factory valve dumps excess boost pressure back into the intake system, which can cause flutter and slow turbo spool. An aftermarket BOV like the HKS SSQV vents this pressure to atmosphere, producing the iconic “psshh” sound and reducing stress on the compressor wheel. On Subaru MAF-based systems, a vent-to-atmosphere BOV requires either a tune or a recirculation setup to avoid rich fuel spikes. This guide assumes you will be using the HKS adapter that allows recirculation, or you will plan for proper engine management.

Tools, Materials, and Parts

Before you start, gather everything you need. Having all components on hand will prevent frustrating mid-job trips to the hardware store.

Required Parts

  • Cobb SF Intake Kit – Includes intake tube, MAF coupler, silicone intake duct, clamps, air filter, and hardware. Ensure the kit is specific to your Forester generation (SG or SF).
  • HKS Blow-Off Valve Kit – Typically the HKS SSQV (Super Sequential Blow-Off Valve). May come with mounting flange, gaskets, and vacuum fitting.
  • Recirculation adapter (optional but recommended) – If you want to keep the BOV fully recirculating to avoid tuning issues, order the HKS recirculation kit (part number 41004-AT003).
  • New vacuum hose – 3/16” or 4mm silicone vacuum line for the BOV signal.
  • Zip ties and heat shrink – For securing wiring harnesses away from heat sources.

Tools

  • Socket set (metric: 10mm, 12mm, 14mm)
  • Flathead and Phillips screwdrivers
  • Torque wrench (for clamp bolts – 7-9 ft-lbs)
  • Pliers (for vacuum hose removal)
  • Pick tool (for stubborn hose clips)
  • Rags and shop vacuum (for cleaning the engine bay)

Safety Gear

  • Safety glasses
  • Mechanics gloves
  • Fire extinguisher (always good practice when working near fuel and electrical systems)

Preparation and Safety

Park the Forester on a level surface and allow the engine to cool completely. Disconnect the negative battery terminal and isolate it to prevent accidental contact. This avoids any electrical shorts and resets the ECU’s learned parameters, which will help the car adapt to the new intake and BOV during the first drive cycle. If your car has an aftermarket alarm or immobilizer, have the key fob handy to reset it after reconnection.

Work in a well-ventilated area. The intake system may contain oil residue from the PCV system, so avoid smoking or open flames. Also, note that the throttle body opening is exposed once the intake is removed—cover it with a clean rag to prevent debris from entering the engine.

Removing the Factory Intake System

Removing the stock intake is a simple process, but patience is needed with the tight spaces around the turbo inlet and the MAF sensor wiring.

Step 1: Loosen the Intake Duct Clamps

Using a flathead screwdriver or a 7mm socket, loosen the two clamps that secure the intake snorkel to the airbox and the throttle body elbow. On most Forester Turbos, the snorkel runs from the front grille area to the airbox.

Step 2: Disconnect MAF Sensor and Harness

Unplug the MAF sensor connector by pressing the tab and pulling straight back. Be careful not to damage the pins. Then, unclip the harness from the airbox lid. The MAF sensor itself is held into the airbox by two Phillips screws. Remove them and set the sensor aside in a clean, dry place.

Step 3: Remove the Stock Airbox and Inlet

Remove the two 10mm bolts that hold the airbox to the chassis (one at the bottom, one near the fender). Lift the airbox with the filter still inside, then pull it out of the engine bay. Next, remove the rubber intake hose that goes from the airbox to the turbo compressor inlet. This hose is usually held on with spring clamps. Compress the clamps with pliers and slide the hose off. Some models also have a resonator box attached to the fender that may be removed (optional).

Step 4: Clean Up the Engine Bay

Use a shop vacuum to remove any leaves, dirt, or debris that may have collected around the intake opening. Inspect the turbo inlet for any cracks or deformation. If the factory inlet is worn, it’s a good time to upgrade to a silicone turbo inlet, which pairs well with the Cobb intake.

Installing the Cobb SF Intake

With the stock intake removed, you’re ready to install the Cobb system. Follow these steps in order for a proper fit.

Step 1: Prepare the Intake Tube

The Cobb SF intake comes in two pieces: the main aluminum tube and the rubber MAF coupler. Slide the MAF coupler onto the intake tube. The coupler has a ridge that aligns with a groove in the tube. Loosely tighten the clamp on the coupler for now. Install the included rubber grommet and plastic vacuum fitting into the designated hole on the intake tube. This fitting will connect to the crankcase vent hose (the one that was attached to the factory intake duct).

Step 2: Install the Intake Duct to the Throttle Body

The Cobb kit includes a silicone intake duct that connects the intake tube to the throttle body. Slide the duct onto the throttle body opening and tighten the clamp. Then attach the other end to the intake tube. Orient the tube so that the MAF sensor mounting location faces up and slightly toward the driver’s side. Tighten all clamps to 7-9 ft-lbs (do not overtighten; the silicone can deform).

Step 3: Mount the Air Filter

Attach the large cone filter to the end of the intake tube. Use the provided clamp and position the filter so it sits in the lower fender area where the stock airbox was. Ensure the filter has clearance from the inner fender liner and the wheel when turning full lock (test later).

Step 4: Install the MAF Sensor

Carefully insert the MAF sensor into the new MAF coupler. Use the supplied screws (or re-use the factory ones). The sensor must be oriented with the airflow arrow pointing toward the turbo. Connect the wiring harness. Use zip ties to secure the harness away from hot engine components like the turbo heat shield.

Step 5: Connect Crankcase Vent and Boost Reference

Connect the crankcase vent hose from the valve cover to the vacuum fitting on the intake tube. Some Cobb kits include a reducer. If your car has a second port on the factory intake (used for the boost control solenoid), you may need a T-fitting to connect it. Refer to the Cobb instructions for your specific vehicle—models vary between the SF and SG Forester.

Installing the HKS Blow-Off Valve

Installing the HKS BOV requires removing the factory bypass valve, which is located on the intercooler or the charge pipe, depending on your Forester gen. For SG5 and SG9 models, the bypass valve is on the top-mount intercooler near the throttle body. For SF models, it is often on the driver side of the turbo outlet.

Step 1: Locate the Factory Bypass Valve

It is a black plastic valve with a vacuum line attached. On the intercooler, it is held in by two 10mm bolts. Some models require removing the intercooler to access it—this is a good time to inspect the intercooler seals.

Step 2: Remove the Factory Valve

Disconnect the vacuum line. Loosen the two bolts and pull the valve off. The valve may have a gasket; if it is damaged, replace it. Do not reuse a leaking gasket.

Step 3: Prepare the Mounting Flange

Most HKS kits come with a universal mounting flange. You may need to weld or bolt it to the existing intercooler or charge pipe. For a bolt-on fit, purchase a Forester-specific HKS adapter. Alternatively, you can mount the BOV directly to the intercooler if the flange pattern matches. Use high-temperature threadlocker on the bolts.

Step 4: Install the HKS Blow-Off Valve

Mount the BOV to the flange using the supplied O-ring and bolts. Tighten evenly. Connect the included vacuum hose to the small nipple on the valve. Route the hose to a boost reference source, such as the intake manifold or a dedicated port on the intake tube (the Cobb intake has an extra port). Use a vacuum line that is long enough to avoid kinks. Secure with zip ties.

If you plan to keep the car without a tune, install the HKS recirculation adapter. This adapter connects a hose from the BOV back to the intake tube before the turbo. This prevents unmetered air from escaping and keeps the MAF sensor readings stable. Without recirculation and without a tune, the car may run rich between shifts, causing possible catalytic converter damage.

Tuning Considerations

Both modifications significantly change airflow and fueling. The Cobb SF Intake is designed to work with an Accessport tune, and Cobb provides off-the-shelf (OTS) maps for many Forester models. If you do not have an Accessport, you can still run the intake safely on the stock ECU, but the car will likely run slightly lean at high loads because the factory MAF calibration is calibrated for the restrictive stock intake. It is strongly recommended to get a professional tune or use an Accessport with a map designed for the Cobb intake. Similarly, if you run a fully vent-to-atmosphere BOV without a tune, the momentary richness can cause misfires and long-term fueling issues.

For best results, install both parts together, then flash an appropriate tune. Many Forester owners pair the Cobb intake with a Cobb Downpipe and an Accessport for a complete stage 2 setup.

Final Checks and Reassembly

  • Double-check all clamps, especially the intake duct and the intercooler connections. Loose clamps can cause boost leaks.
  • Inspect all vacuum lines for cracks or loose connections. Use a smoke test if available.
  • Reinstall the intercooler if it was removed. Torque the intercooler bolts to spec.
  • Check that the air filter has clearance from the wheel liner and is not touching any moving parts.
  • Reconnect the battery. Before starting the engine, turn the key to the “On” position without cranking, wait 5 seconds, then turn it off. Repeat once to allow the fuel pump to prime and the MAF sensor to initialize.

Testing Your Installation

Start the engine. The idle may be rough for the first 30 seconds while the ECU re-learns idle air control. Let it run until the coolant temperature starts to rise. Listen for any sucking or hissing noises that indicate a vacuum leak. If you have an Accessport, monitor the fuel trims (AF Learning) and MAF voltage. Then, take the car for a gentle drive around the block, avoiding heavy boost until the engine is fully warm. Gradually increase throttle under light load and check for any check engine lights. If all is well, perform a few full-throttle pulls in third gear to test the BOV operation. The HKS valve should produce a crisp “psshh” sound when you lift off the throttle. If you hear fluttering or compressor surge, the valve may be too stiff—adjust the spring preload using the included shims (refer to the HKS manual).

Additional Tips and Common Issues

  • Check engine light (CEL) – P0101 (MAF circuit range/performance): This often indicates a leak after the MAF sensor or an improperly seated sensor. Recheck all clamps and the MAF coupler alignment.
  • Rough idle after BOV installation: Likely a vacuum leak or incorrect recirculation setup. Verify that the vacuum hose to the BOV is connected to a manifold pressure source, not a boost-reference source before the throttle plate.
  • Oil in the intake: Some oil mist from the PCV system is normal. Consider adding an oil catch can to prevent fouling the MAF sensor.
  • Heat soak: The Cobb SF intake sits in the engine bay and can suffer from heat soak at low speeds. Wrapping the intake tube with DEI heat wrap or installing a cold air box helps reduce intake air temperatures.

Conclusion

Installing a Cobb SF Intake and HKS Blow-Off Valve on your Forester Turbo is a rewarding project that delivers noticeable gains in power, sound, and throttle response. By following this detailed guide and paying attention to the tuning requirements, you can avoid common pitfalls and enjoy a reliable, high-performance setup. Remember that proper engine management is the key to safe and consistent performance. For further reading, check out the official Cobb Tuning support page (Cobb Subaru Support) and the HKS installation guide (HKS Subaru Products). For community advice, the Forester forums at SubaruForester.org are an excellent resource. Get your tools ready, take your time, and enjoy the transformation of your Forester Turbo.