Note: Special thanks to Def on bimmerforums for his post on there. I didn't remember too much of this and his guide definitely filled gaps for me. He's actually a 240sx owner, I wouldn't be suprised if he popped up on here and took a shit on me for mooching off his guide.
Compressor map time!
Yet again, a little background information.
A turbine is the "hot side" of a turbocharger. Exhaust gasses go through it, then expand and spin the turbine wheel that drives the compressor shaft.
Compressor is the "cold side" which sucks in intake air and compresses so the engine can combust it.
Surge is the absolute lowest amount of air a compressor will supply at a certain ratio of pressure, and anything above this will make your compressor "gulp" air...which isn't good for your turbo/power, but from what I understand you would have to have a HUGE compressor and tiny turbine to be concerned with gulping.
x-axis is mass air of the compressor
y-axis is the pressure ratio(x in the equation below);
Atmosphere Pressure+Boost Pressure= X
Atmospheric Pressure
Ratio of 1.0 is atmospheric pressure, normal dookie that gravity does on it's own.
2.0 is equivalent to ~14.7psi in your IM.
Usually by default I say the pressure is ambient at the compressor's inlet. Pressure drops result in less power than mass flowrates would suggest.
Those cool looking rings are effiency islands. This is where the compressor has about the same efficiency at compressing air. You guessed it, the higher the efficiency, the better cause there will be less unnecessary heat provided to the charge air by the compressor.
4 main things;
The motor's displacement
The volumetric efficiency (engine "breathing")
Pressure at your inlet valves (the boost, ya big dummy)
Teh RPMz reached
Changing any of the four above things will usually yield more combustion and generally more power.
If this is brief, please tell me.