Since we plan to use our car for track days, we knew cooling was an important issue. To help keep our thermal situation under control, since we removed the radiator to install the O2 sensor housing, we installed an ARC radiator, radiator air guide, and cooling system pressure regulator while we were at it. The ARC radiator is a very high-quality unit, using all TIG-welded construction in the aluminum end tanks to withstand high pressures coupled with a super efficient core. The radiator cools much better than stock but is no thicker-a good deal if you're installing a big turbo. The unit bolts directly in, including the stock fans, making installation a snap.

If a cooling system's pressure is kept higher, the boiling temperature increases, with less localized boiling in the head and less cavitations of the water pump. Therefore, the cooling system works much better. To increase pressure, we installed an ARC cooling system pressure regulator between the radiator and the coolant overflow tank. The regulator allows us to adjust the cooling system's pressure from the stock 1.1 bar to slightly over 2 bar.

The ARC air guide is a simple plate that helps prevent air entering the grille opening from pilling over the top of the radiator. Although it's simple in concept, being cut from titanium, it's probably one of the more esoteric parts on Project Evo IX.

Since we were upping the pressure of our cooling system, it was prudent to upgrade our coolant hoses as well. Nukabe sent us some of their molded four-ply silicone hoses to evaluate. Silicone has much better temperature-resisting properties than rubber and four-ply is twice as strong as stock units. These heavy-duty features mean that the Nukabe hoses will sport a much longer service life over the stock parts.

With our turbo-back exhaust in place, we returned to the dyno of XS Engineering to test what gain our new parts provided. Once our power levels stabilized, we determined that the new exhaust parts had given us a respectable 15 whp boost, bringing us to 304 whp-not bad for some very simple work. Our knock count, however, started to rise due to the increases in cylinder pressures our free-breathing engine was developing. On an Evo a knock count of more than three means that the ECU is starting to pull timing and the power will fluctuate. This is shown by a rough-looking power trace on the dyno chart as well as registering as knock events on data logging software that XS uses. We were skirting a knock count of three to five at a higher rpm and things were starting to look rough.

While the car was strapped down, we added another goodie, an ARC airbox. While studying what worked well on a local Evo forum, the ARC airbox was pretty highly regarded among some of Southern California's local tuners. We also liked it for its bling effect. The ARC airbox is constructed of welded aluminum and simply looks tricker than any other Evo airbox we've seen. Karmen vortex airflow meters like the one used on the Evo are notoriously sensitive to variations in airbox and air filter geometry. Often, simply installing an air intake will mean that an Evo's fuel map must be extensively retuned to maintain the same air/fuel ratio. The ARC box was known for its minimal affect on the MAF's reading, which is another good reason to use it besides looks.