Building a boost controller is a “slippery slope” sort of mod. Once you complete this you’re just going to want/need to modify something else in the system. A turbo-charger improves the performance of an engine by cramming as much air and fuel into the engine as possible. The engine exhaust drives an impeller in the turbo which drives the compress wheel in the intake path. Engines can’t handle extreme pressures so once the intake pressure reaches a factory set point a “waste-gate” opens to let the exhaust gas bypass the drive impeller so the turbo won’t produce any more boost. Factory boost settings a usually very conservative (otherwise, they’d be doing a lot of repairs) so there is a lot of potential for improvement.
A manual boost controller is placed in the path of the waste-gate’s sensor. The controller bleeds off some of the pressure in the line so that the pressure measured at the waste gate is lower than actual. This tricks the waste gate into staying closed longer than usual so a higher level of boost is reached. They’re really cheap to build, but the most important expense in this project is to getting accurate boost gauge. A little bit of bleed goes a long way and there is no way to tell where you are at without a proper gauge. DSMtuners has quite a few articles on how to build these devices and should work on almost all turbo cars.