You choose the hotside based on the exhaust flow of the engine at the desired operating parameters. If it is too small, a restriction happens, which effects the gas flow through the engine. If it is designed efficiently and matched well, it turns the shaft, which turns a compressor. This also has to be matched to the amount of air the engine uses. If this is too big or too small, adverse things happen here as well.
I also understand that the intake air turns into the exhaust. More intake=more exhaust. More exhaust=more faster turblow=moreintake=more exhaust.......They are related. In the end, a well designed system has both sides sized for the engines desired operating range, and matched to each others abilities.
It is a hell of a confusing process, as this thread illustrates.
I am trying to break the concept down into chunks that are simpler to understand.
If the bigger turbo places less exhaust restriction, it will flow more at 18psi than one that creates more restriction. This still works with what I said previously. A cylinder with a given intake will flow 200cfm at 18psi, with a given exhaust restriction. To increase cfm through the engine you can either increase intake pressure or decrease backpressure or both to increase flow. Its a pressure differential in which either side can be manipulated. They are independent, but related.
Psi is a direct measure of the backpressure before the intake. Put on a custom head and manifold, drop the intake backpressure and the same turbo will flow the same cfm at a lower psi. At the same psi, the cfm will go up, within the limits of the exhaust backpressure and turbines ability to transfer power to coldside compression.
It is an interrelated system no doubt but you can look it as separate pieces without losing sight of the whole.
Wrong. That is what you are failing to see. They are not at all independent and can't be treated as such. You don't get higher boost pressures without restricting the exhaust. There is "work" going on. To say otherwise is to believe in perpetual motion, or some such analogy. The turbine is a restriction no matter what size or AR turbine you run.