Top Mount Intercooler - Subaru 03-07 GD WRX & 06-07 STI

The Process West Top Mount Intercooler was developed with a single goal — to be the very best aftermarket top mount intercooler for the WRX and STI, without compromise.

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

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 bonnet splitter in place means up to 50% of the new core receives no ambient airflow. A complete waste of the upgrade.

The images below demonstrate this using the Process West intercooler installed in a 2006 Forester XT, with the factory splitter placed over the core as if the bonnet were shut. The result is self-evident.

The solution is straightforward: a purpose-built splitter sized specifically for the upgraded core, ensuring 100% of the core receives full ambient airflow.

Process West intercooler and splitter/combination showing 100% of the new larger intercooler with full ambient airflow over the entire core.

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 beneath it. The mismatch is obvious. Upgrading the intercooler without upgrading the ducting achieves very little.

Core thickness matters.

The second mistake we commonly see is building an intercooler that is too thick. Bigger core dimensions make for compelling marketing, but physics tells a different story.

A top mount intercooler sits at 90 degrees to ambient airflow. On a boxer engine, that air has already made a 90-degree turn through the bonnet scoop before it reaches the core. Asking it to then push through 114mm of intercooler core is too much to ask. The result is restricted airflow, poor heat exchange, and high intake temperatures.

The Process West core is 90mm thick. That is the optimum balance between cooling surface area and airflow resistance, and it shows in the outlet temperatures and power figures.

Y-pipe charge distribution.

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

Together, the correctly sized core, purpose-built splitter, and balanced Y-pipe form a complete, properly engineered top mount intercooler system for the Subaru WRX and STI.

This picture above shows the complete combination of Y-pipe (underneath), intercooler and splitter.

Dyno testing.

It is 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, making for 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.

This graph (above) shows the before and after power. Note the test number. These 2 runs were back-to-back.

Intercooler outlet temperature comparison — Process West (dark line) vs. factory unit (light line). Both intercoolers started at 37°C under identical conditions. The Process West unit peaked at 51°C. The factory unit peaked at 66°C. Results shown after a single run.

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 were 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 WRX and STI.