The 4AGZE suffers from a lean condition when the supercharger boost is increased (either by smaller SC pulley, or larger crank pulley). This lean condition shows up under heavy load and under ~4300 RPM. The main symptom felt is a bogging (lack of power) when trying to accelerate quickly, then a huge jump in power after 4300 RPM. Measured AFR from various dyno runs shows values as high as 17-18 AFR which not only costs the engine torque, but is also puts the engine at risk for detonation.
The Gruntbox was designed specifically to solve this issue. It monitors RPM and throttle position. When the logic detects the lean condition, it activates the cold start injector to add extra fuel enrichment. It brings the AFR back down to a proper 12-13 AFR and give a big boost in torque (at least 10ft-lbs at the wheels at 3k rpm). The Gruntbox, at the moment, is only good for US electronics Big Pulley'd GZEs, not Japanese MAP sensor'd GZE's came from the hot-rod JDM Corola's.
The cause of the leanout issue is a function of the stock ECU which will "ignore" airflow values greater than a certain threshold. At low RPMs and increased boost, the airflow meter reads more air than the OEM computer thinks it should be reading, and thus the computer ignores all the airflow above a certain amount and injects less fuel than is necessary to maintain the proper air-fuel ratio.
The Gruntbox was designed by two long time MR2SC enthusiasts, Mike Gruber and Rob Files. They are available on a build to order basis.