Discussion and questions related to the course Boost Control
It's my understanding that Boost Pressure and CFM can be different. For instance, say you're running 8 psi manifold pressure on one turbo, and then you switch to another still running 8 psi, will one turbo produce more flow than the other while still at 8 psi? Perhaps one turbo can have an increased air velocity than the other while still maintaining the same boost level, thus increasing airflow into the engine?
The boost pressure and airflow definitely aren't one and the same. We only need to look at a compressor map to see that for a given pressure ratio the airflow out of the engine can vary dramatically. This really comes down to how much restriction to airflow the engine places on the turbo. In other words if we place the same turbo on a 2 litre 4 cylinder engine and then a 3 litre V6, the turbo is going to move more air at 15 psi on the 3 litre V6 because the engine is less restrictive.
The way I like to formulate it is this, "boost is the side-effect of the engine's flow restriction".
The turbo fills the intake system faster than the engine can empty it. More air going in than is going out = increase in pressure.