The ECU End of It
The turbo 2.4L is the first boosted engine to use DaimlerChrysler's Next Generation Controller. As the SAE paper describes, "The NGC algorithms are 'model-based,' continuously calculating the appropriate control parameters to keep the engine at its desired performance. This is a departure from past control strategies that relied on pre-programmed tables of operating conditions." The system is speed density rather than mass air.
Interestingly, the wastegate control is dictated by throttle demand rather than a simple blowoff at a set boost pressure. This way, the computer can estimate the power needs of the driver and either provide boost pressure or not. The good news is that you may get more boost in some conditions than a basic mechanical blowoff might provide; the bad news is that the computer "can also reduce the torque as required to ensure powertrain durability," per the SAE paper. The system uses an air charge temp (ACT) sensor after the intercooler as well as a throttle inlet pressure (TIP) sensor. The combined information can predict turbo compressor speed and pressure drop across the throttle blades and the IAC. It's this combination of information that has been giving fits to aftermarket companies trying to modify the Mopars with electronic throttle control (ETC). With the 2.25-bar (33-psi) sensors on the turbo 2.4, hopefully there will be more room for increased airflow and performance. Mopar Performance is already working on Stage I, II, and III power packages for, we hear, up to 300 hp.