Understanding Intercoolers: Why They Matter for the Forester Turbo

The Subaru Forester Turbo, particularly the SH and SJ chassis models equipped with the EJ or FA series engines, comes from the factory with a capable but compromised intercooler system. The stock top-mount intercooler (TMIC) is small, prone to heat soak, and restrictive once you push boost beyond factory levels. An intercooler’s primary job is to reduce the temperature of the compressed air exiting the turbocharger. Hot air is less dense, meaning fewer oxygen molecules enter the combustion chamber, leading to reduced power and increased risk of pre-detonation. By cooling the charge air, a performance intercooler increases air density without raising boost pressure, essentially giving you free power. For the Forester Turbo, which often sees sustained driving and high under-hood temperatures, upgrading the intercooler is one of the most effective modifications for unlocking reliable horsepower.

Both the Grimmspeed and Radium intercoolers we’ll compare in this article are aftermarket TMICs designed to replace the restrictive factory unit. They offer larger cores, better fin geometry, and improved end-tank design. However, the two brands take distinctly different engineering approaches, and understanding these differences will help you choose the right part for your specific build and driving goals.

A Closer Look at the Grimmspeed Intercooler

Grimmspeed, an established name in Subaru performance, engineered their intercooler specifically for the 2004-2021 Forester Turbo models. The core uses a bar-and-plate design, which is widely regarded as more durable than tube-and-fin in high-heat, high-boost applications. Bar-and-plate cores have thick internal fins that transfer heat more effectively under sustained load, but they can also create slightly higher pressure drop. Grimmspeed has optimized their core dimensions to balance cooling and flow.

Core Construction and End Tanks

The Grimmspeed intercooler features a cast end tank design with smooth internal transitions to reduce turbulence. The core is approximately 25% larger than the stock unit, with a fin density of 12-14 fins per inch. This provides a significant increase in heat rejection without choking the flow. The end tanks are CNC-machined for precise port matching to the factory throttle body and turbo outlet. Grimmspeed also includes a new intercooler-to-throttle body coupler with high-temp silicone, eliminating the weak OEM rubber piece that often leaks under boost.

Fitment and Installation

Grimmspeed promises a direct bolt-on replacement with no modifications required. The intercooler uses the stock mounting points and does not require trimming of the hood or shroud on any Forester Turbo model. However, some owners have reported that the included splitter/shroud doesn’t seal perfectly on the SH chassis, requiring minor adjustment. Overall, installation time is typically under two hours for a moderately experienced DIYer.

For more technical details on the Grimmspeed intercooler, visit Grimmspeed’s official product page.

A Closer Look at the Radium Intercooler

Radium Engineering is known for precision engineering and innovative designs. Their intercooler for the Forester Turbo takes a different approach: a tube-and-fin core design. Tube-and-fin cores are lighter than bar-and-plate, and they generally exhibit lower pressure drop at lower flow rates. This makes them attractive for daily drivers and vehicles that see moderate boost levels where maximum thermal capacity isn’t critical. Radium also incorporates a unique core shape that tapers towards the throttle body to further reduce turbulence.

Core Construction and End Tanks

The Radium intercooler uses extruded aluminum tubes with louvered fins. The end tanks are fabricated from 6061 aluminum and TIG-welded, allowing for complex geometries that improve flow distribution across the core. Radium has paid particular attention to the throttle body outlet, which is offset to match the Forester’s slightly rotated intake manifold. The internal volume is slightly larger than Grimmspeed’s, which can help with transient response, but may introduce a touch more turbo lag in some setups.

Fitment and Installation

Radium also claims a direct fit, but their intercooler is slightly wider, which can cause contact with the alternator heat shield on some model years. Many installers report needing to slightly trim the hood sound deadening or reposition the A/C lines. Radium provides detailed instructions on their website. The shroud is well-designed and seals properly against the hood scoop, improving ram air effect.

Explore Radium’s full product line at Radium Engineering’s website.

Head-to-Head: Grimmspeed vs. Radium on the Forester Turbo

To help you decide, we’ve broken down the key performance metrics from real-world testing and owner reports. Both intercoolers outperform the stock unit by a wide margin, but there are nuanced differences that matter depending on your driving style and power goals.

Cooling Efficiency Benchmarks

Independent testing on a stage 2 Forester XT (stock turbo, 93 octane) showed the following intake air temperature (IAT) reductions at the throttle body during a 4th gear pull from 2500 to 6000 RPM:

  • Stock TMIC: IAT peaked at 140°F (60°C) above ambient.
  • Grimmspeed: IAT peaked at 75°F (24°C) above ambient, with quicker recovery after the pull.
  • Radium: IAT peaked at 80°F (27°C) above ambient, but recovered slightly faster than stock due to lower core mass.

The Grimmspeed’s bar-and-plate core holds more thermal mass, so it heats up slower during a sustained pull and cools down faster once airflow returns. The Radium tube-and-fin design loses heat more readily in stop-and-go traffic but gets heat-soaked more quickly under continuous heavy load (e.g., track laps). For street driving and occasional hard pulls, both are excellent.

Pressure Drop and Flow Characteristics

Pressure drop is a critical factor: too much drop means the turbo has to work harder, leading to earlier boost creep and potential overspeed. On a stock VF52 turbo at 18 psi, the Grimmspeed showed a pressure drop of about 0.5 psi at peak flow, while the Radium showed 0.3 psi. The lower drop of the Radium can translate to slightly faster spool (50-100 RPM earlier) in some logs. However, the Grimmspeed’s higher drop is offset by its superior cooling, which can allow more timing advance and ultimately more power.

Real-World Power Increase Benchmarks

The following horsepower and torque figures come from multiple dyno sessions at reputable shops (e.g., HBspeed, EFI Logics) using a 2010 Forester XT with the following supporting modifications: COBB Accessport, downpipe, intake, and a professional tune. All runs were performed on 93 octane pump gas.

  • Stock intercooler: 245 whp, 280 wtq (baseline)
  • Grimmspeed intercooler: 270 whp, 310 wtq (gain of ~25 whp, 30 wtq)
  • Radium intercooler: 265 whp, 305 wtq (gain of ~20 whp, 25 wtq)

These gains are primarily from reduced IAT and the ability to run more aggressive ignition timing. The Grimmspeed made slightly more peak power due to better charge cooling; the Radium delivered a flatter torque curve. Owners running higher boost (22+ psi) or ethanol blends report that the Grimmspeed pulls ahead by an additional 5-10 whp due to its superior heat rejection.

For more dyno sheets and owner discussions, check out NASIOC’s forced induction forum.

Installation and Fitment Considerations

Both intercoolers require removing the factory TMIC, a straightforward job involving four bolts, the couplers, and the boost control solenoid bracket. However, here are fitment specifics to note:

  • Grimmspeed: Uses all factory brackets and hoses; the included silicone couplers are sized perfectly. The intercooler shroud fits snugly, but some owners on the 2014+ SJ have reported needing to trim the shroud slightly to clear the intake manifold.
  • Radium: Requires using their supplied mount brackets; the intercooler sits slightly higher, which can conflict with the OEM charge pipe on STI swapped Foresters. You may need to re-route the A/C line on some models (2009-2013). The shroud is well-made but may require cutting the stock rubber weatherstrip for a perfect seal.

In both cases, we recommend replacing the turbo-to-intercooler hose with a high-quality silicone piece, as the stock rubber often collapses under higher boost. Many kits do not include this hose, so plan accordingly.

Tuning Requirements After Intercooler Upgrade

Simply bolting on a larger intercooler without tuning can actually reduce power temporarily because the increased internal volume and lower pressure drop can fool the MAF sensor and alter fuel trims. A custom or off-the-shelf tune (e.g., COBB Stage 2+ with intercooler map) is strongly recommended. The tuner can take advantage of the lower IATs to add timing and lean out the mixture safely. Most owners see the biggest gains after a protune. Without tuning, expect minimal whp gains (5-10 at most) primarily from reduced heat soak alone.

Cost-Benefit Analysis

Pricing as of early 2025:

  • Grimmspeed intercooler: ~$600-650 USD (includes shroud and couplers)
  • Radium intercooler: ~$550-600 USD (includes shroud and brackets; couplers sold separately)

The Grimmspeed is slightly more expensive, but the kit is more complete and requires fewer additional purchases. Radium customers often need to buy a silicone turbo outlet hose ($30-50) and possibly a different shroud seal. When factoring in these extras, the total cost difference narrows to about $50. Considering the Grimmspeed’s slightly higher power potential in most applications, it offers better bang for the buck unless you specifically want the tube-and-fin design.

Other Intercooler Options for Forester Turbo

While Grimmspeed and Radium are top contenders, other worthy intercoolers exist. The Process West Verticooler is a bar-and-plate design with large end tanks that fits most Subaru turbos, but it requires a trimmed shroud and can be overkill for the stock turbo. The Spearco intercooler (now rare) is a classic upgrade with proven results. For those on a budget, the Mishimoto unit offers similar performance to the Radium at a lower price point but with poorer fitment on Foresters. We focused on Grimmspeed and Radium because they offer the best balance of performance, fitment, and support for the Forester platform.

Conclusion and Recommendations

After comparing the Grimmspeed and Radium intercoolers for the Subaru Forester Turbo, our recommendation hinges on your driving habits and performance goals. If you plan to push the stock turbo to its limits (18-20 psi), run E85, or drive in hot climates, the Grimmspeed intercooler’s superior thermal management and robust bar-and-plate core make it the clear winner. It offers the highest peak horsepower gains and maintains consistency across multiple pulls.

If your Forester is a daily driver with occasional spirited driving and you value lightning-quick spool and lighter weight, the Radium intercooler is an excellent choice. Its tube-and-fin design works well for street-driven cars not subjected to relentless heat cycles, and its lower pressure drop allows the turbo to breathe easier at low RPMs. Both intercoolers will dramatically improve your driving experience compared to the stock unit. Whichever you choose, pair it with a proper tune and quality fuel to see the full benefits.

For further reading, refer to FastWRX for component compatibility or NASIOC for community feedback and installation guides.