A few months elapsed since we last tested our 2008 Subaru STI, producing over 75 whp with six power-adding enhancements. These gains were achieved by improving airflow to the compressor, optimizing exhaust flow and reflashing the ECU while using the factory-installed top-mount intercooler. For our third round of parts testing, addressing the remaining areas that could rob horsepower became vital. By improving the efficiency of the ignition and intercooler systems, not only will reliability be increased but significant horsepower gains should be obtained.

The Parts

Increasing boost pressure results in higher cylinder pressures that also increase the possibility of misfires. The higher pressures within the cylinder raise the amount of voltage required for a proper spark to occur. By replacing the factory ignition coils with Okada Projects' Plasma Direct coils, greater power is provided to ensure the spark plugs never miss a beat. Okada Projects states that these coils increase the secondary current (or spark amperage) by 100 percent while generating a multi-spark discharge to ensure all fuel combusts completely. Okada Projects designs the Plasma Direct coils as application-specific replacements for a wide range of platforms, requiring no cutting or splicing of any wires. We tested a set in issue 55 on our GDB STI. So it was time to see if similar results could be achieved on the newer-generation GRB STI.

The top-mount intercooler design found on every turbocharged Subaru simply does not promote increased horsepower production. The sole benefit of the factory design is the little piping required to complete the tract from compressor outlet to throttle body. However, having the intercooler sitting atop the engine completely soaks the core with engine heat. Turbo XS offers a front-mount intercooler kit that places a larger core in the mouth of the front bumper. This location greatly improves air charge cooling efficiency, promoting engine reliability while also increasing charge-air density for greater power production. A Turbo XS cold-air intake is required in order to accommodate the piping layout and an optional adapter plate is needed for any Subaru-style bypass valve.

Vehicle performance is always limited to the capabilities of the fuel being used. Until recently, the only option for more aggressive tuning was the use of expensive race fuels. However, technology has blessed tuners with modern water-methanol (a.k.a. alcohol injection) systems. By spraying a mixture of water and methanol into the air stream before the throttle body, the charge air is dramatically cooled and the effective octane of the fuel is raised. This further deters detonation, allowing tuners to advance ignition timing and increase boost pressure without harming the engine. DevilsOwn offers universal alcohol-injection kits with a number of add-on options. For the STI, a DevilsOwn DVC-30 Stage 2 kit will be used along with the optional 2.5-gallon tank with float sensor. The DVC-30 Stage 2 kit utilizes a digital progressive controller that activates the 250-psi pump at a user-defined boost pressure reading.

The Dyno

The shop providing the previous tuning and dyno testing, HB Speed, has since closed its doors while transitioning into the new-coming COBB Tuning SoCal. Although the new facility wasn't available for this round of testing, tuner Jon Drenas was able to meet us at XS Engineering's DTS dynamometer to continue providing his services. Because of the different dyno being used, a new baseline power figure needed to be established. Horsepower checked in at 313.7 whp, dropping by 5.1 horsepower compared to the previously-used DYNO-mite dynamometer.