Nakazoto, only thing you are missing is that the turbo assists the supercharger even when by itself it has not reached boost threshold yet.
So at say 3500 rpm the turbo may be producing half a bar, that means that the supercharger only needs to assist with the rest, thus its workload is reduced.
Don't forget that the supercharger increases exhaust gasses which in turn make the turbo spool sooner.
You can see this working if you put two boost gauges on the car one after the turbo and another after the supercharger.In the most simplistic form your trying to get a smooth transition from SC to Turbo, this can only happen when the turbo is self sufficient.
The trouble is there are many variables to this. The electric solenoid on the bypass valve for example is there to allow air to bypass the supercharger at idle and all light throttle loads where the demand for the supercharger is not met, this increases engine efficiency reduces wear etc on the supercharger (since it wont be spinning). This is also assisted with the vacuum tank which under these conditions will hold open the actuator, and thus butterfly valve on the bypass control valve.
The whole point of Nissans system is to allow a progressive transition from one to the other. The other interesting thing is how the ecu defines load since its only inputs to control the solenoids are throttle position and airflow. Its interesting to note that the ecu and electronics actually have no idea of boost level at all.
Ed
