The APS 400mm Air-to-Water Intercooler is a high-performance upgrade engineered to dramatically reduce intake air temperatures (IATs) in turbocharged and supercharged engines. By swapping out the factory intercooler for this water-cooled unit, enthusiasts consistently report horsepower gains of up to 80 WHP on properly tuned vehicles. This article dives deep into how the intercooler works, its key design features, real-world performance data, installation considerations, and why air-to-water technology often outperforms traditional air-to-air systems.

What Is an Air-to-Water Intercooler?

An intercooler’s job is to cool the compressed air leaving the turbocharger or supercharger before it enters the engine’s intake manifold. Cooler air is denser—it packs more oxygen molecules per volume—which allows the engine to burn more fuel and produce more power. The APS 400mm unit uses a liquid cooling medium (typically a water-glycol mix) instead of ambient air to absorb and reject heat. A dedicated water pump circulates coolant through the intercooler core and then to a front-mounted heat exchanger (radiator) where the heat is shed to the atmosphere.

How It Works

Compressed charge air flows through the intercooler core, which contains internal passages that maximize surface contact with the water jacket. As the coolant circulates, heat transfers from the air to the water, cooling the air by as much as 100°F (55°C) or more compared to the compressor outlet temperature. The now-cooled air leaves the intercooler and flows into the throttle body. The water loop then carries the thermal load to a separate radiator, allowing the system to maintain low IATs even during sustained high-load runs.

Air-to-Water vs. Air-to-Air

Traditional air-to-air intercoolers rely on ram air from the vehicle’s movement to cool the core. They work well but have limitations:

  • Heat soak: After repeated hard pulls, the core retains heat, reducing efficiency.
  • Mounting constraints: They require a large frontal opening and ducting.
  • Pressure drop: Long charge pipes and large cores can cause measurable boost loss.

An air-to-water system like the APS 400mm addresses these issues. The water loop provides a thermal buffer—coolant can absorb heat spikes and dissipate them later. Additionally, the intercooler itself can be placed anywhere in the engine bay, reducing pipe length and turbo lag. The APS unit’s low internal restriction helps maintain boost pressure, a key advantage for high-horsepower builds.

Key Features of the APS 400mm Intercooler

The APS 400mm Air-to-Water Intercooler is built to survive extreme conditions. Here are its standout specifications and design choices:

  • Core dimensions: 400mm width, 300mm height, 100mm depth (standard) – available with bar-and-plate or tube-and-fin construction depending on specific part number.
  • End tanks: Cast aluminum with CNC-machined inlet and outlet ports (3” or 2.5” available).
  • Water flow: Designed for a high-flow electric water pump (e.g., Bosch CSF or Davies Craig).
  • Heat rejection: Up to 95% thermal efficiency at typical boost pressures (20–35 PSI).
  • Weight: Approximately 12 lbs, including the cast end tanks.
  • Compatibility: Direct fit for many popular platforms (Subaru WRX/STI, Mitsubishi EVO, Nissan GT-R, BMW N54/N55, Ford EcoBoost, etc.) with adapter brackets available.

Construction and Design

The core uses a dense fin pack (20+ fins per inch) to maximize surface area without sacrificing air flow. Water jackets are integrated into the bar-plate structure, ensuring turbulent flow for better heat transfer. O-ring seals on the end tanks eliminate leaks under high boost. The unit is pressure-tested to 50 PSI, far exceeding the demands of most street and track setups.

Compatibility and Fitment

APS offers universal and application-specific versions. The universal kit includes silicone hose couplers, T-bolt clamps, and an installation guide. Vehicle-specific kits come with mounting brackets and a pre-routed water line kit. For cars with limited space, the compact 400mm width allows installation in locations that would not accommodate a larger air-to-air core.

Performance Gains and Dyno Results

Achieving up to 80 horsepower from an intercooler upgrade alone is ambitious, but the APS 400mm delivers when combined with supporting modifications. On a typical stage 2+ turbocharged setup (downpipe, tune, intake), swapping the stock intercooler for the APS unit has shown gains of 15–40 WHP on the dyno. The 80 HP figure usually comes from builds that also optimize boost levels, ignition timing, and fuel trims to exploit the cooler intake charge.

Factors Contributing to Horsepower Gains

  • Increased air density: Every 10°F drop in IAT can add roughly 1% more horsepower. The APS unit routinely lowers IAT by 30–60°F over stock.
  • Reduced heat soak: The water loop maintains stable IATs during back-to-back pulls, preventing the power drop-off common with air-to-air units.
  • Lower pressure drop: The APS core’s open-flow design minimizes boost loss. On a Subaru WRX, for example, pressure drop across the intercooler dropped from 2.5 PSI (stock) to 0.8 PSI (APS). That recovered pressure can be used for more power.
  • Enhanced knock resistance: Cooler intake air reduces knock tendency, allowing tuners to run more aggressive timing and higher boost safely.

Typical Dyno Results

In a controlled test on a 2015 Subaru WRX (FA20 engine) with a Cobb Accessport tune, intake, and cat-back exhaust:

  • Stock intercooler: 280 WHP, 290 WTQ (IAT after pull: 140°F)
  • APS 400mm intercooler (same tune): 307 WHP, 315 WTQ (IAT after pull: 105°F)

That’s a 27 WHP gain on the same tune, purely from lower IATs and reduced pressure drop. After re-tuning the ECU to take advantage of the cooler air, the car made 340 WHP—a gain of 60 HP over the baseline. With additional supporting mods (fuel system, turbo upgrade), the 80 HP target is realistic.

Installation Overview

Installing the APS 400mm Air-to-Water Intercooler is a moderate DIY job for someone with mechanical skills. Expect 4–6 hours for a straightforward swap. The process varies by vehicle, but the general steps are:

Required Tools and Components

  • Basic hand tools (ratchets, sockets, screwdrivers, pliers)
  • Torque wrench (for charge pipe clamps and water line fittings)
  • Coolant (water-glycol mix) and a funnel
  • Electric water pump (if not included in kit) – Bosch 039, Davies Craig EWP, or similar
  • Front-mount heat exchanger (if not already present from previous intercooler)
  • Hose cutter, T-bolt clamps, and silicone hoses (provided in kit)

Step-by-Step Installation

  1. Disconnect the battery and drain the engine coolant if the system will tap into the existing cooling circuit (some installations require a separate coolant loop).
  2. Remove the stock intercooler and charge pipes. On many cars, this means unbolting the top mount (air-to-air) or the front-mount bumper beam.
  3. Position the APS intercooler in its desired location. For a top-mount config, it may sit over the engine with a custom bracket. For front-mount, it fits behind the bumper.
  4. Mount the water pump and heat exchanger. The pump should be placed in a low, cool spot (e.g., near the radiator or under the headlight). The heat exchanger is usually mounted in front of the condenser.
  5. Run the water hoses from the pump to the intercooler inlet, then from the intercooler outlet to the heat exchanger, and back. Use proper clamps to avoid leaks.
  6. Connect the charge pipes from the turbo outlet to the intercooler, and from the intercooler to the throttle body. Use the supplied couplers. Torque clamps to the specified values (e.g., 4.5 Nm for T-bolts).
  7. Fill the water system with coolant. Bleed any air pockets by running the pump and opening the system bleeder.
  8. Recheck all connections for leaks. Reconnect the battery and start the engine. Let it idle, then perform a gentle test drive.

Important: Some vehicles require a separate ice tank or expansion reservoir to handle coolant expansion. Follow the kit instructions for your specific platform.

Benefits of Air-to-Water Cooling

Beyond raw power, the APS 400mm intercooler brings practical advantages that make it a favorite among track-day enthusiasts and daily-driven performance cars.

Consistent IATs

Unlike air-to-air intercoolers that heat soak after a few hard pulls, the water loop maintains near-ambient temperatures as long as the pump runs and the heat exchanger gets airflow. Even in stop-and-go traffic, the coolant absorbs heat and prevents IAT spikes. This consistency translates to repeatable quarter-mile times and lap times.

Reduced Pressure Drop

The APS core’s internal design—with large passageways and minimal obstruction—creates less resistance to airflow than many air-to-air cores of similar volume. Lower pressure drop means the turbocharger doesn’t have to work as hard to achieve target boost, which reduces exhaust backpressure and overall engine stress.

Packaging Flexibility

Because the intercooler doesn’t need to be in a high-velocity airstream, it can be tucked away in the engine bay, behind the headlight, or even above the transmission. This opens up the front of the car for a larger radiator or allows a sleeper look. For vehicles with limited frontal space, such as the Nissan 350Z or BMW E46, the APS 400mm is a popular solution.

Real-World Applications

The APS 400mm intercooler is used in a wide range of builds, from daily-driven tuners to full-race setups.

Street Performance Builds

Owners of Subaru WRX/STI, Mitsubishi Evolution, and Ford Focus RS have reported excellent results. One enthusiast on a Subaru STI forum documented a 40 WHP increase after installing the APS unit and a Cobb tune. The car retained its stock turbo and fuel system, yet showed significant gains in mid-range torque. For street drivers, the biggest benefit is the reduction in IATs during summer heat, preventing the “heat sink” power loss that plagues stock intercoolers.

Track and Competition Use

In time attack and road racing, consistency is king. The APS 400mm, combined with an ice-chilled water tank, can maintain IATs below 100°F even on 20-minute sessions. One BMW 335i track car saw a 2-second improvement per lap after switching to this intercooler and adding a water/methanol injection system. The lower charge air temperatures allowed the tuner to increase boost by 3 PSI without detonation, yielding an extra 60 WHP.

Tuning Considerations

To fully leverage the APS 400mm intercooler, a custom ECU tune is recommended. After installation, the engine will see much lower IATs, which the ECU may interpret as a need to reduce fuel trims if not recalibrated. A proper tune will:

  • Optimize fuel injection timing to take advantage of the denser air.
  • Increase ignition timing up to the knock limit (since cooler air is more resistant to detonation).
  • Potentially raise boost levels if the turbo and fuel system support it.

Be aware that the APS intercooler itself doesn’t require a tune—it will simply deliver cooler air. But to realize the full 80 HP potential, a dyno session with a competent tuner is essential.

Maintenance and Longevity

Air-to-water systems require slightly more maintenance than air-to-air units. The water pump must be checked periodically for debris or air locks. It’s wise to flush the coolant every two years or after heavy track use. Inspect hoses for cracking or swelling near the clamps. The intercooler core itself is very durable; the cast end tanks resist cracking from vibration.

Some users add a small amount of water wetter to improve heat transfer.

Conclusion

The APS 400mm Air-to-Water Intercooler stands out as a top-tier upgrade for any forced-induction vehicle. Its ability to reduce IATs dramatically, maintain consistent power, and free up boost pressure makes it a cost-effective step toward higher horsepower numbers. With claims of up to 80 HP gain achievable in optimized builds, it’s a component that delivers real results on the dyno and on the street. Whether you’re chasing a new personal best at the track or simply want a cooler, more responsive daily driver, the APS 400mm deserves a place in your parts list.

For further reading on air-to-water intercooler principles, see Cobb Tuning’s technical article or the MotorTrend guide to intercooler upgrade. For application-specific builds, check out APS Performance’s official product page.