Top Mount Intercooler - Subaru 03-05 GD STI

The Process West Top Mount Intercooler for the 2004–06 Subaru Forester XT was developed with a single goal — to be the very best aftermarket top mount intercooler for the platform, without compromise.

Every competitor product was studied. Every common mistake was addressed.

The ducting problem nobody else solves.

The most common error in aftermarket top mount intercooler design is supplying a larger core without properly sized ducting to match. Fitting a larger intercooler and leaving the factory splitter in place means up to 50% of the new core receives no ambient airflow — a complete waste of the upgrade.

The Process West intercooler is supplied with a purpose-built splitter sized specifically for the upgraded core, ensuring 100% of the core receives full ambient airflow. This is how it should be done.

Process West intercooler and splitter combination — 100% of the upgraded core with full ambient airflow across the entire surface.

The image above shows a 2005 WRX with the bonnet scoop removed and the factory splitter still in place. The white line marks the actual footprint of the upgraded intercooler sitting beneath it. The gap between the two tells the whole story — upgrading the intercooler without upgrading the ducting is a pointless exercise.

Core thickness matters.

More is not always better when it comes to top mount intercooler thickness. With the intercooler mounted at 90 degrees to ambient airflow, air is already working hard after making a 90-degree turn through the bonnet scoop. Forcing it through an oversized 114mm core only compounds the problem, resulting in poor heat exchange and high intake temperatures.

The Process West core is 90mm thick — the optimum balance between cooling surface area and airflow resistance. The result is lower outlet temperatures and more power.

Y-pipe charge distribution.

Most aftermarket Y-pipes distribute charge air unevenly, with one well-flowing duct and a poorly designed second outlet. The Process West Y-pipe is engineered for even charge air distribution across the full width of the intercooler core.

Together, the oversized core, purpose-built splitter, and balanced Y-pipe form a complete, properly engineered top mount intercooler system — not just a bigger core in a box.

The complete Process West system — Y-pipe, intercooler, and splitter working together as one integrated solution.

Dyno testing.

It's worth noting upfront that dynos and top mount intercoolers are not an ideal combination. Regardless of fan positioning, airflow into the bonnet scoop on a dyno is minimal. With that said, both the factory intercooler and the Process West upgrade were tested under identical conditions — a fair and accurate comparison.

The test vehicle was an otherwise stock 2005 WRX running a BPM GT exhaust. Stock WRXs of this specification consistently produce around 90kW at the wheels on this dyno. Intercooler outlet temperatures were measured via probe placed in the silicone joiner at the throttle body.

Running the Process West intercooler and splitter first, the baseline power result was 115.5kW, with a peak intercooler outlet temperature of 51°C.

The factory intercooler was then installed and tested under the same conditions. Maximum power dropped to 107.5kW — and the factory unit's thermal performance was far more concerning than the power figure alone suggests.

Power comparison — Process West intercooler vs. factory unit. Both runs conducted back-to-back under identical conditions.

This graph (above) shows the before and after intercooler outlet temps. Dark line is the PW unit, lighter line the stock unit. Note, we made sure that we started the test with same intercooler outlet temp to give an accurate comparision between the two. You can see both intercoolers started at about 37C. The PW unit saw a max of 51C and the stock unit 66C. This, of course is after one run on the dyno.

A final test measured the effect of back-to-back runs on the factory intercooler. After three consecutive runs, outlet temperatures climbed to 84°C — and still rising. The implications for circuit use are significant.

Street testing.

Real-world testing tells the most honest story. Intercooler outlet temperatures were measured using a Fluke 87 digital multimeter with a K-type thermocouple positioned just before the throttle body — the same location used during dyno testing. Readings from this setup were consistently within one degree of the dyno's AIT probe throughout.

With the Process West intercooler and splitter fitted, hard driving through the mountains could not push outlet temperatures above 43°C regardless of how hard the car was pushed.

The factory intercooler and splitter were then refitted and the same route driven under the same conditions. Outlet temperatures peaked at 72°C — and were still climbing when testing was concluded. The difference in the car's response was immediate and noticeable.

The numbers speak for themselves. The Process West Top Mount Intercooler is the definitive cooling upgrade for the Subaru Forester XT.