OK so what happens

[BoostedFC]

Basically you have a few options where EMS is involved. A piggy back system in which the aftermarket unit is hooked in with the factory ECU with the ability to adjust Fuel & Ignition maps. There is the full stand alone in which the new unit replaces the factory ECU for control. They have chipped factory ECU's which are generally set to predetermined adjustments according to mods. There are units where all they really do is trick the factory ECU to add more fuel to compensate. You would have to find out what type of system would benefit your needs. The more feature & complex the system the more you will spend.