Like the Mitsubishi Evolution and Toyota Supra before it, turbocharged Imprezas can boast of something that few other cars can: 50 extra horsepower and just as much torque for $4 worth of hardware-store parts. This is one of the first tuning tricks ever applied to domestic turbo imports and remains a popular upgrade for modern enthusiasts regardless of their budget.
Most modern turbocharged vehicles including the Impreza use an exhaust waste-gate to control turbocharger speed at high rpm. This waste-gate is essentially a valve that allows exhaust gases to bypass the turbo, and it's controlled by an electronic or vacuum-operated solenoid (a mechanical actuator used to open the waste-gate). Imprezas use an electronic solenoid that is triggered by boost pressure.
The basic idea is to install an spring-loaded restrictor in the line between the waste-gate solenoid and the intake. When properly tuned, this restrictor cuts off the pressure signal between turbo and solenoid, tricking the solenoid into believing the boost pressure is lower than it is.
All you need to make your own adjustable boost controller is some basic brass fittings from Home Depot or Lowe's. Purchase two 1 x 1/8" NPT 1/4" Barbed Nipples (one each male and female fitting, $2), one 3/16" ball bearing ($0.15), one 1/8" utility spring ($0.30) and two 1/2" hose clamps ($0.75).
Place the ball bearing inside the female-end brass fitting, and seat the utility spring inside of the male-end fitting. Hold the two fittings together so the spring is touching the back of the ball bearing and carefully screw the fittings together. Putting it together is the easy part, the trickier part is tuning the assembly.
You're going to need a compressor, bench-blower tip and an inline pressure regulator to tune the assembly. Install the adjustable pressure regulator between the blower tip and line, and push the blower tip tightly against the ball bearing side of your controller. Set the pressure regulator to 18 to19 psi, which is a fairly safe boost pressure for unmodified Subaru engines (stock boost is 14.5 psi). Apply the air.
If the air blows clean through your restrictor, take it apart, and stretch the spring a little with your hands. Put it back together, making sure the brass nipples are as far engaged as they can be and re-test. Repeat this procedure until the valve stops opening when you apply air pressure. Once the valve stops opening, unscrew the nipples from each other a little bit. This will reduce the load on the spring and allow you to fine-tune the boost setting. Keep re-testing and unscrewing the nipples until the valve opens at exactly your desired boost setting and no higher.
Now, install your tuned restrictor into the rubber line which connects the turbo outlet to the waste-gate. This tube will be located on the upper/front/passenger side of the engine. Make sure to install the restrictor with the ball bearing side facing the turbo so that boost pressure is pushing the valve open instead of shut.
