Subaru Forester Performance Upgrade: APS Cold Air Intake and Upgraded Intercooler

The Subaru Forester has long been a staple of reliability, all-weather capability, and family-friendly versatility. However, the naturally aspirated and turbocharged Forester models (particularly the XT and SJ/SK generations) have substantial untapped potential. For owners seeking more power without sacrificing daily drivability, the combination of an APS cold air intake and a high-performance upgraded intercooler offers one of the most cost-effective and proven enhancement pathways.

This article provides a deep dive into the engineering behind these modifications, real-world dyno and street-testing data, installation best practices, and long-term maintenance considerations. Our focus is on verifiable results, not marketing hype, so you can make an informed decision and achieve the performance gains the Forester platform is capable of delivering.

How a Cold Air Intake Unlocks Engine Potential

The factory air intake system on the Subaru Forester is engineered for noise suppression, production cost efficiency, and adequate performance under average conditions. In many cases, it incorporates restrictive airboxes, convoluted ducting, and small-diameter filter elements that impede airflow, especially at higher engine speeds. An aftermarket cold air intake, such as the APS system, addresses these limitations by reducing restriction and sourcing cooler air from outside the engine bay.

The Science of Cool, Dense Air

Internal combustion engines operate most efficiently when fed with cool, dense air. Cooler air contains more oxygen molecules per unit volume, which enables more complete fuel combustion and, with proper tuning, higher cylinder pressures. The APS cold air intake relocates the air filter—often to an isolated compartment behind the headlight or bumper—to avoid heat soak from the turbo, exhaust manifold, and radiator. This can result in intake air temperature (IAT) reductions of 10–20°F (5–11°C) under stop-and-go driving and even more significant drops during sustained boost use.

Beyond temperature, the smooth mandrel-bent aluminum or composite intake tube of the APS system eliminates the turbulent flow caused by the factory’s molded plastic and resonators. Smoother flow means less pressure drop between the filter and the turbo inlet, which allows the turbo to spool more quickly and maintain higher boost at a given RPM. The result is a measurable improvement in throttle response and torque delivery, particularly in the mid-range where daily driving occurs.

Measurable Gains: Dyno and Real-World Data

Independent dyno testing of the APS cold air intake on a 2014–2018 Subaru Forester XT (FA20DIT engine) has consistently shown peak gains of 10–15 horsepower and 12–18 lb-ft of torque at the wheels when combined with a standard ECU recalibration. Without tuning, gains are more modest—typically 5–8 hp—but the intake alone can still yield a 2–5% improvement in volumetric efficiency. On the street, owners report sharper tip-in response and a more aggressive induction soundtrack that does not cross into drone or excessive cabin noise.

In controlled acceleration tests (using VBOX or Dragy GPS loggers), a Forester XT with an APS intake and a stock tune recorded a 0–60 mph improvement of approximately 0.3 seconds. Paired with an upgraded intercooler and a conservative OTS (off-the-shelf) tune, that improvement grows to 0.5–0.7 seconds. These numbers are consistent across multiple owner forums and independent testers.

Why an Upgraded Intercooler is Essential for Turbocharged Foresters

The intercooler’s job is to dissipate the heat imparted by the turbocharger’s compression process. Subaru’s factory top-mount intercooler (TMIC) for the Forester is adequate for low-boost, moderate-load driving, but it quickly reaches thermal saturation during aggressive driving, towing, or hot weather. Once saturated, intake air temperatures spike, and the engine management system responds by pulling ignition timing and reducing boost pressure to prevent knock. This robs the car of power just when it’s needed most.

Heat Soak and Its Consequences

Heat soak occurs when the intercooler core cannot shed heat fast enough, causing the metal itself to become hot and consequently heat the air passing through it. For a top-mount intercooler located directly above the engine, this problem is compounded by radiant heat from the turbo and exhaust manifold as well as hot air recirculating from the radiator area. Upgraded intercoolers address this through several design changes:

  • Larger core volume: More internal surface area allows greater heat exchange before the air leaves the cooler.
  • Improved bar-and-plate construction: More durable and thermally efficient than the factory tube-and-fin design, with denser internal fins.
  • Better end-tank flow: Smoother transitions from inlet to core minimize pressure drop and distribute airflow evenly across the core.
  • Optional charge pipe upgrades: Many upgraded intercooler kits include larger-diameter, smoother-bore charge pipes that further reduce restriction between the turbo and throttle body.

A quality aftermarket intercooler for the Subaru Forester can reduce peak intake temperatures by 20–40°F (11–22°C) under repeated full-throttle runs. This directly translates to more consistent power output and a lower risk of detonation, which is particularly important if you are also running increased boost from an ECU tune.

Real-World Intercooler Testing: Track and Street

In a series of 0–60–0 repeated pulls (simulating highway merging and autocross conditions), Foresters fitted with a larger capacity upgraded intercooler maintained intake air temperatures within 15°F of ambient after five successive runs, while the stock intercooler showed a temperature rise of 40–50°F above ambient by the third run. On a rolling road, a Forester XT with an upgraded intercooler and a stage 1 tune made over 30 whp more than the stock configuration in 90°F summer conditions—a difference that disappeared in cooler weather, confirming that heat management is the critical factor.

Combined Performance: APS Intake + Upgraded Intercooler Results

Installing both the APS cold air intake and an upgraded intercooler creates a synergistic effect. The intake supplies cooler, denser air to the turbo, and the intercooler more effectively manages the post-compression temperature rise. The result is a compound improvement in both peak output and sustained performance.

Peak Power and Torque Gains

  • +20 to 28 wheel horsepower on a 93-octane tune (vs. stock)
  • +25 to 35 lb-ft wheel torque in the 3,000–4,500 RPM range
  • 0–60 mph improvement of 0.6–0.8 seconds (e.g., from 6.2 sec to 5.5 sec)
  • Quarter-mile ET reduction of 0.4–0.6 seconds with a trap speed increase of 4–6 mph

These figures are from actual owner logs posted on Subaru enthusiast forums like SubaruForester.org and NASIOC, where multiple XT owners have documented their modification journeys with spreadsheet data and timeslip photos. While individual results vary based on altitude, fuel quality, vehicle condition, and specific tune, the trend is consistent and repeatable.

Daily Driving Improvements

Beyond raw numbers, owners report subtler but equally valuable enhancements:

  • Less lag during highway passing (the turbo reaches target boost quicker)
  • Consistent performance on hot days—no "mushy" pedal feeling
  • Better fuel economy by 1–3 MPG under normal driving (the engine requires less throttle to maintain speed)
  • Improved engine and turbo sound without excessive noise

Installation: What to Expect and Common Pitfalls

Both the APS cold air intake and an upgraded intercooler are considered moderate difficulty modifications. With basic hand tools, a jack and jack stands, and a Saturday afternoon, most mechanically inclined owners can complete the work. However, paying attention to critical details separates a successful install from one that causes drivability issues or even component damage.

APS Cold Air Intake Installation Steps

  1. Disconnect the battery negative terminal to reset the ECU learning.
  2. Remove the factory airbox and intake snorkel—keep the bolts organized.
  3. Unplug the MAF sensor from the stock housing and carefully transfer it to the APS intake tube (use the supplied gasket and screws, torquing to specification).
  4. Position the new intake components: verify that the filter sits in a cool zone away from the turbo heat shield and that no wiring harnesses rub against the metal tube.
  5. Secure all clamps to the specified torque (do not overtighten on silicone couplers).
  6. Reconnect the battery and perform an idle relearn procedure as described in the kit instructions.

A common mistake is failing to properly align the MAF sensor orientation. The MAF must be installed with the arrow pointing in the correct airflow direction, and the harness should not be stretched. A misaligned MAF can cause erratic idle, lean conditions, and check engine lights. Always double-check the sensor placement against the manufacturer’s diagram.

Upgraded Intercooler Installation Steps

  1. Remove the engine cover and disconnect the factory intercooler hoses (label them if reusing).
  2. Unbolt the stock intercooler from its brackets—on many Foresters the intercooler sits between the turbo and the throttle body, secured by four bolts.
  3. Lift the stock intercooler out, taking care not to damage the fins or the cooling hoses underneath.
  4. Install the upgraded intercooler using the supplied mounting brackets and bushings. Ensure it sits parallel to the hood line for proper airflow.
  5. Connect the charge pipes and hoses. Use the provided silicone or hose clamps; consider replacing factory spring clamps with constant-tension T-bolt clamps for better sealing.
  6. Check for clearance with the hood latch, hood insulation, and throttle body. Many larger intercoolers require minor trimming of the plastic engine cover or a small hood spacer.
  7. Pressure-test the system before starting the engine to verify there are no boost leaks.

Tuning Considerations: Optimizing the Combination

While both upgrades can be run on a stock ECU tune, the full performance potential is only realized with a custom or off-the-shelf (OTS) tune from a reputable Subaru tuner. The stock ECU's fuel and ignition maps are calibrated for the restrictive intakes and small intercooler. Adding a high-flow intake and large intercooler will change the mass airflow reading and the air density at the cylinder. Without recalibration, the ECU may run lean, run rich, or fail to take advantage of the increased airflow potential.

We recommend using an Accessport or an Opensource tuning solution with a tune specifically designed for the APS intake and your intercooler brand. Many tuners (like Cobb Tuning and Surgeline) offer OTS maps that support these mods, but a dyno tune will yield the best balance of power, driveability, and safety.

Long-Term Maintenance and Reliability

Aftermarket intakes require periodic attention. The APS cold air intake uses a high-flow dry or oiled filter that must be cleaned every 15,000–20,000 miles depending on driving conditions. Oiled filters should be lightly re-oiled to avoid contaminating the MAF sensor. Inspect couplers and clamps annually for signs of cracking or loosening, especially in cold climates where silicone can become brittle.

Upgraded intercoolers generally require no special maintenance, but their larger frontal area can accumulate debris (road tar, bugs, leaves) that restrict airflow. A gentle rinse with a garden hose per year or when washing the car will keep the fins clean. Avoid using a pressure washer directly into the core as high force can bend the fins.

Overall reliability impact is minimal. These modifications do not significantly increase stress on the engine if tuning is conservative and proper fuel is used. Many owners have driven 60,000+ problem-free miles with this combination. As with any performance upgrade, monitoring knock correction via an Accessport or scan tool is prudent, especially when using lower octane fuel.

Potential Drawbacks and Compatibility Notes

It would be disingenuous to claim only positives. The upgraded intercooler will add weight—typically 5–10 lbs compared to the stock unit—and may require trimming of the plastic engine cover. Some owners in snowy or salted-road regions report ice formation on the larger intercooler surface during winter commutes, which can slightly reduce airflow. However, this is temporary and does not cause mechanical issues.

The APS intake may also increase turbo spool sound (a pleasant whistle) and, in some cases, a slight increase in air intake noise under light throttle. If absolute cabin quiet is a priority, these modifications are not ideal. Most owners find the added sound to be sporty and inspiring.

Additionally, these modifications may void the factory powertrain warranty on newer Foresters if the dealer determines the modification caused the failure. It is important to understand your local warranty laws and, if concerned, keep the original parts for reinstallation before dealership visits.

Conclusion

For Subaru Forester owners seeking a tangible improvement in both everyday driveability and peak performance, the combination of an APS cold air intake and an upgraded intercooler delivers real-world results that are consistent, well-documented, and cost-effective. With verified gains of 20+ wheel horsepower, quicker spool, and better heat management, these modifications transform the Forester into a genuinely quick crossover without sacrificing the versatility and reliability that define the model.

Whether you are merging onto the highway, climbing a mountain pass, or simply enjoying a spirited backroad drive, the enhanced responsiveness and consistency are immediately apparent. As with any performance upgrade, careful installation and proper tuning are essential to realize the full potential and ensure long-term reliability. The Subaru Forester community has spoken with dyno sheets and stopwatch data—these mods work.

For further reading and owner experiences, check out the dedicated build threads on SubaruForester.org's performance section and the technical documents on Cobb Tuning's support site.