Why Upgrade Your Subaru Forester with a Garrett GT15 Turbo Kit?

Adding forced induction to a naturally aspirated Subaru Forester is one of the most rewarding modifications you can make. The Garrett GT15 Budget Turbo Kit offers a cost-effective entry point into turbocharging, delivering noticeable gains in horsepower and torque without requiring a complete engine rebuild. This guide provides a detailed, step-by-step installation process tailored to the Forester platform, covering everything from component selection to final tuning considerations.

The GT15 turbocharger is well-suited to the Forester's 2.5L or 2.0L engine displacement. Its relatively small turbine housing spools quickly, providing boost as low as 2500 RPM, which translates to strong mid-range pull ideal for daily driving and light off-road use. With proper supporting modifications and tuning, you can expect power increases in the range of 60-100 wheel horsepower over stock, depending on boost levels and fuel quality. Beyond raw power, a well-installed kit improves throttle response and overall drivability when calibrated correctly.

This expanded guide assumes you have intermediate mechanical skills and access to a basic set of automotive tools. If you are new to turbo installations, consider working with an experienced friend or a professional shop for the more critical steps involving oiling and tuning.

What You Will Need

Before laying a hand on your Forester, gather all necessary components and tools. Running to the parts store mid-installation wastes time and increases the risk of errors.

Mandatory Components

  • Garrett GT15 Turbo Kit – includes the turbocharger, cast iron or tubular manifold, oil feed and return lines, coolant lines, all necessary gaskets, and hardware.
  • Intercooler – Front-mount (FMIC) or top-mount (TMIC) intercooler with piping kit. FMIC offers better cooling but requires more cutting. TMIC is simpler but can suffer from heat soak.
  • Wastegate – Internal or external wastegate to control boost pressure. Most budget kits include an internally gated GT15 unit.
  • Blow-Off Valve (BOV) – Recirculating or atmospheric, depending on your ECU tuning strategy. Recirculating is recommended for MAF-based Subarus.
  • Downpipe – Stainless steel downpipe with provisions for the oxygen sensor.
  • Exhaust Gaskets – High-quality multi-layer steel gaskets for manifold-to-block, turbo-to-downpipe, and downpipe-to-cat-back connections.
  • Oil & Coolant – 5W-30 or 10W-40 synthetic oil, coolant (Subaru Super Blue or equivalent), and possibly an oil filter sandwich plate for the feed line.
  • Boost Gauge & Wideband O2 Sensor – Essential for monitoring boost pressure and air-fuel ratio during tuning.
  • Vacuum Lines – Silicone vacuum lines for wastegate, BOV, and boost reference.

Tools

  • Metric socket set (10mm, 12mm, 14mm, 17mm, 19mm)
  • Combination wrenches
  • Torque wrench (ft-lb and in-lb)
  • Gasket scraper
  • Thread locker (medium strength)
  • Anti-seize compound for exhaust bolts
  • Pry bar or breaker bar for stubborn bolts
  • Jack and jack stands (or a lift)
  • Safety glasses, gloves, and a fire extinguisher
  • Upgraded fuel pump (e.g., Walbro 255 lph)
  • Larger fuel injectors (if running over 8-10 PSI)
  • ECU reflash or standalone tuning solution (e.g., Cobb AccessPort, RomRaider, or a professional dyno tune)
  • Boost controller (manual or electronic) for fine-tuning boost levels

For sourcing parts, consult reputable suppliers like Garrett Motion for genuine turbo components and SubaruForester.org for community-validated kit recommendations.

Preparation

Park your Forester on a level, solid surface, preferably in a well-ventilated garage. Disconnect the negative battery terminal and wait at least 10 minutes to allow the ECU capacitors to drain. Drain the engine oil and coolant to prevent spills when disconnecting lines. Label all connectors and vacuum lines as you remove them with painter’s tape and a marker.

Lift the front of the vehicle using jack stands rated for the Forester’s weight (around 3,500 lbs) and chock the rear wheels. Having the front end elevated provides easier access to the exhaust manifold, oil pan, and intercooler mounting points. Remove the plastic engine undercover to expose the stock exhaust manifold.

Step 1: Remove the Stock Components

Begin by removing the stock air intake system, including the airbox, intake tube, and MAF sensor. Set the MAF sensor aside carefully – it is delicate and expensive to replace. Remove the heat shield from the exhaust manifold if equipped. Unbolt the stock exhaust manifold from the cylinder heads (12mm and 14mm bolts) and lower it away from the engine. You may need to disconnect the oxygen sensor wiring before completely removing the manifold.

On 2.5L Subaru engines, the stock manifold often has integrated catalytic converters. Removing these can trigger a check engine light unless the ECU is tuned to ignore the rear O2 sensor. Plan for a tune that disables the secondary O2 code.

Next, remove the stock oil dipstick tube – it is held by a single 10mm bolt and may interfere with the turbo placement. Carefully pull it out after loosening the bolt. You will reinstall it with the turbo kit’s new orientation or reuse the original if clearance allows.

Inspect the cylinder head exhaust ports for any debris. Use a shop vacuum to clean out soot or gasket remnants. Do not drop anything into the ports.

Step 2: Install the Turbo Manifold

Fit the new turbo manifold onto the studs of the cylinder heads. Apply a thin layer of anti-seize to the studs if they are new. Use the provided gaskets – do not reuse old ones. Tighten the manifold nuts in a cross-pattern to the manufacturer’s torque specification, typically 35-40 ft-lb for M10 studs. Overtightening can warp the manifold or strip the threads.

With the manifold bolted on, loosely install the turbocharger mounting studs if they are not pre-installed. A dab of thread locker on the studs will prevent them from loosening during operation.

Step 3: Attach the Turbocharger

Carefully lift the Garrett GT15 turbocharger and align its mounting flange with the manifold studs. Use a new gasket between the turbo and the manifold. Tighten the nuts evenly to 30-35 ft-lb. Do not use anti-seize on these nuts – the clamping force is critical to prevent exhaust leaks.

Now route the oil feed line from the engine block (typically a port near the oil filter housing) to the turbo’s oil inlet. Most kits include a banjo bolt or -4AN fitting. Use copper washers to seal the banjo bolts – tighten to 15-20 ft-lb. The oil return line connects from the turbo’s drain to the oil pan. Remove the oil pan drain plug, install a return line fitting (may require drilling and tapping the pan or using a pre-welded bung), and secure the hose with clamps. Ensure the return line drains above the oil level and has a gentle slope to prevent oil backing up into the turbo seals.

Connect the coolant lines from the turbo to the engine’s coolant system. Many kits splice into the heater core hoses or use a dedicated coolant bypass pipe. Use spring clamps or worm-gear clamps. Bleed the cooling system after installation to prevent air pockets.

Step 4: Install the Intercooler and Piping

This step differs significantly between front-mount and top-mount setups. For a front-mount intercooler (FMIC), remove the front bumper cover and crash bar according to the kit’s instructions. Mount the intercooler core using brackets provided. Cut the bumper beam if necessary – use a grinder or sawzall and paint exposed metal to prevent rust. Route the piping from the turbo compressor outlet to the intercooler inlet, then from the intercooler outlet to the throttle body. Use silicone couplers and T-bolt clamps rated for 30+ PSI.

For a top-mount intercooler (TMIC), remove the stock TMIC if equipped. Install the new TMIC using the provided brackets. Route a y-pipe or direct pipe from the turbo to the intercooler. TMIC installs are much simpler but the intercooler sits directly above the turbo, soaking heat during prolonged boost.

Regardless of style, pressure test the intercooler system before driving. Use a boost leak tester from the turbo inlet to pressurize the system to 15-20 PSI and listen for hissing. Fix any leaks immediately.

Step 5: Hook Up the Wastegate and Boost Control

The wastegate regulates maximum boost by bypassing exhaust flow around the turbine. On the Garrett GT15, the wastegate is typically internal – integrated into the turbine housing. Connect the wastegate actuator diaphragm to a boost pressure source using silicone vacuum line. For a simple setup, run the line directly from a port on the compressor housing (boost reference) to the wastegate. This gives a fixed boost level determined by the actuator spring (usually 5-8 PSI).

For adjustable boost, install a manual boost controller (MBC) or an electronic boost controller (EBC) between the compressor reference and the wastegate. Set the controller to a moderate boost level initially – around 7-8 PSI – until the engine is tuned. Never run more than 10 PSI without proper fuel system upgrades and a professional tune to avoid detonation and engine damage.

Install the blow-off valve (BOV) on the intercooler piping between the intercooler and the throttle body. Use a recirculating BOV to keep the metered air in the system if you are running a stock MAF sensor. This prevents rich condition during shifts and rough idle.

Step 6: Connect the Exhaust System

Install the downpipe from the turbo’s exhaust outlet to the Forester’s mid-pipe. Use a new gasket at both ends. Tighten the bolts at the turbo flange to 25-30 ft-lb. If the downpipe does not have a flex section, consider adding one to reduce stress on the turbo and manifold from engine movement.

Reinstall the oxygen sensor(s) – you may need to extend the wiring on the front O2 sensor to reach the new location. The rear O2 sensor can be deleted via tuning or left unplugged with the CEL tuned out. Check the entire exhaust system for clearance against the frame, transmission, and steering shaft. Fix any contact points immediately to prevent melted wiring or rattling.

Step 7: Final Checks Before Starting

Before you turn the key, perform a thorough inspection:

  • Oil level – fill with fresh synthetic oil to the full mark.
  • Coolant level – fill and bleed the system. Run the engine with the radiator cap off and heater on to purge air.
  • All clamps and bolts – recheck intercooler piping, vacuum lines, and turbo mounting nuts.
  • Battery connection – reconnect the negative terminal.
  • Vacuum lines – verify no cracks or loose connections. Use zip ties to secure all lines.
  • Fuel pressure – if you upgraded the pump, prime it by cycling the ignition a few times and check for leaks at the injectors.

Perform a boost leak test using a simple PVC cap and air compressor fitting. Pressurize the intake system to 10-15 PSI and listen for leaks. Spray soapy water on connections to identify bubbles. This step alone can prevent hours of troubleshooting later.

Step 8: Start the Engine and Initial Break-In

Start the engine and let it idle. The turbo will require a few seconds to build oil pressure – you should see oil pressure rise on the gauge. Check for oil and coolant leaks immediately. Listen for any unusual noises like a whistle (boost leak), ticking (exhaust leak), or rattle (loose heat shield).

Let the engine warm to operating temperature. Do not rev the engine above 2000 RPM during the first 10 minutes of operation. This allows the turbo seals to bed in and the oil to fully lubricate the bearings. Watch the boost gauge – it should read vacuum at idle and build boost smoothly when you blip the throttle (without load).

Once the engine is warm, take the vehicle for a gentle test drive. Keep boost below 5 PSI for the first 20-30 miles. Vary engine RPM and load, but avoid sustained high boost. After this break-in period, perform a full fluid change (oil and coolant) to remove any debris from the installation.

Tuning and Calibration

No turbo kit performs well without proper calibration. The stock ECU is not programmed for boost. You have three options:

  1. Cobb AccessPort – Load a pre-made off-the-shelf (OTS) map for turbo Subaru Forester. This is the easiest route but may require fine-tuning for your specific setup.
  2. Open-source tuning (RomRaider) – More labor-intensive but offers complete control. Requires a Tactrix cable, a laptop, and a solid understanding of fuel and ignition maps.
  3. Professional dyno tune – The safest and most effective option. A tuner will dial in air-fuel ratios, boost targets, and ignition timing on a dyno. Expect to pay $500-$1000.

Regardless of method, target an air-fuel ratio of 11.5:1 – 12.0:1 under full boost for pump gas (93 octane recommended) and keep boost levels conservative until you confirm the fuel system can keep up. For a detailed tuning guide, consult resources like Cobb Tuning or community guides on SubaruForester.org.

Maintenance and Longevity

Turbocharged engines demand stricter maintenance intervals. Adopt the following practices to keep your GT15 kit running reliably:

  • Oil changes – every 3,000-4,000 miles with high-quality synthetic (Mobil 1, Red Line, Motul). Turbo bearings tolerate abuse from old oil poorly.
  • Cool-down procedure – after hard driving, let the engine idle for 1-2 minutes before shutdown. This prevents oil from coking in the turbo’s center housing.
  • Boost leak check – annually, or after any front-end repair. Loose clamps are the most common boost leak source.
  • Listen for changes – a sudden whistle or loss of power usually indicates a boost leak or wastegate issue. Address it promptly to avoid lean conditions.

For more on turbo maintenance best practices, Garrett’s official maintenance guide offers detailed advice on oil selection and bearing care.

Final Thoughts

Installing a Garrett GT15 Budget Turbo Kit on your Subaru Forester transforms it from a reliable daily driver into a genuinely fun, torquey machine. The key to a successful build lies in meticulous preparation, clean work, and – most importantly – proper tuning. Do not cut corners on the fuel system or the ECU calibration. A well-built turbo Forester can deliver hundreds of thousands of miles of boosted enjoyment if treated with respect.

If at any point you feel out of your depth, seek help from a Subaru specialist or an experienced tuner. The investment in professional support is far cheaper than rebuilding a detonated engine. Enjoy the build and the new power under your right foot.