The volume of air going into the motor is X. The only way you can change X is to change the pressure at which it enters the motor. The size of the turbo itself isn't particularly relevant at this point. Your 2zz with 8psi of boost going into it is going to make Yhp. Regardless of whether that boost is coming from a GT28 or a GT40.
That couldn't be further from the truth.. A gt25 at 8 psi may flow 300cfm of air and a gt40 will flow 400cfm of air (hypothetical values, not going to look it up but you get the idea here). The amount of air flow is what makes the power. A motor is a air pump, more air and fuel = more power. That's why for example, my subaru with the stock turbo at 22psi makes 220-230awhp and then with the 20g at the same psi it made 343whp! Same amount of boost and 100 more whp.
Edit: The pressure and compressor map are there to show you the amount of airflow that will be output and also to show you the limits of the turbo you are chosing.
This isnt really a great example since the gt25 or gt28 cant even operate at this range as you can tell from the map. Either a GT25 or GT28 on a 2l engine will choke out before any amount of boost at 7k rpm, and the gt40 would shine if revving to 8k+ rpm, but just to show you the difference I did it quick anyway. But the example here is using 22psi on either turbo and the figures I have for a 2l motor at 3k and 7k rpm to show the usable area on the map.
Edited by Twincharged&Auto, 26 March 2012 - 08:53 AM.