2008 Subaru WRX Project

Intercooler
Although we're getting rally car-like response from our dinky turbo, there's a major difference. Rally cars use larger, twin-scroll, ball bearing units that are far more efficient and can take the abuse of anti-lag. These motorsport turbos require constant service-a luxury we don't have. So instead of a high-tech turbo tuned to boost fast while still having enough high-end flow for decent power, we'll resort to using a small turbo. Small responsive turbos have a major drawback though. Because the compressor wheel and housing is so small, these turbos were not meant to deliver a large amount of air. To compensate, we'll have to push the boost levels up, which also potentially shifts the turbo's operating line out of its optimal efficiency island on the compressor map. Compressor efficiency ultimately boils down to how much energy you put into the turbine side of the turbo and how much useful energy you get out of the compressor. On a small turbo, oftentimes when the boost is turned up, the result is a drop in efficiency, which usually means an increase in compressed air temperatures.

To help with the increased boost we plan on having, we need a much bigger intercooler to help compensate for the additional heat. Most would go with a front-mount intercooler, which offers one of the best options for exchanging heat from the charge air to ambient air. The problem is that the additional plumbing will slow down response and create an overall drop in charge pressure due to pumping losses through the pipes. Since Subaru already updated the hood ducting to their top-mount intercoolers, we stayed with a top-mount option for our small turbo to maintain response, minimize pressure drops, and save ourselves a lot of work and money.

The stock ducting on the WRX is a small box section that takes incoming flow from the hood scoop, turns it 90 degrees, and directs it down through the stock intercooler. The entire assembly is ducted and well sealed with aerodynamic aids like fences and flow separators (somewhat like in a twin-scroll turbo housing) to maximize flow and reduce turbulence and stagnation zones. Hacking the stock ducting to fit a large top-mount intercooler with more frontal surface area would probably hurt us more than help us, so the only solution is to increase the thickness of the core and thus the total cooling fin surface area.

Perrin Performance's intercooler for the Legacy 2.5 GT offers just the solution with a top-mount bar and plate-type intercooler that has a core almost three-times thicker than stock. Overall dimensions, excluding the end tanks, are 11.5 inches wide by 10.75 inches tall with a thickness of 4 inches, compared to the stock tub and fin intercooler's 10.5-inch width, 9.5-inch height, and 2.5-inch thickness. Although the Perrin unit is about an inch wider than stock, which means it extends roughly half an inch past each side of the intercooler ducting, the added surface area will still help since the pressurized intake charge still has to flow across those portions. The main advantage comes with the added thickness, which not only increases the intercooler effiency, but also the overall thermal mass of the unit.

Depending on how you look at it, more thermal mass can be good or bad. For drag racing, more thermal mass means that it takes longer to heat up the intercooler from dead cold. On the other hand, it also takes longer to cool down by force convecting air through it, especially since the intercooler sits on top of the engine, if you're just idling in traffic. We might have preferred a tube and fin-type core with less thermal and physical mass instead, especially with the 15.25-pound weight of the Perrin intercooler compared to the stock unit's 5.9 pounds.