You can't directly read what the compressor map says without doing some calculations. Basically you need to work out the values on the x and y axes (air flow and pressure ratio) and then look the point where the values intersect on the graph. The location of that point indicates how effective the turbo will be for your setup (I'll let someone more experienced elaborate on that).
The basics of it are, you want to aim to be within the smallest island or as close as possible to it as this is where the turbo is more efficient/effective. The further away you are the less effective the setup will be. The direction you are away from the smallest island indicates what would be the cause of it not running at its peak.
- Too far to the left and you'll get compressor surge, due to stalling compressor blades, generally means turbo is too large
- Too far to the right and you get choking, the turbo is too small and may be prone to overspeed depending on how far away you are from peak efficiency.
- The horizontal lines, which are speed lines, indicating in this case the rpm of the turbine assembly. Overspeed must be avoided to prevented the turbo failing.
That's the real basics of it, there's a lot more the graph can tell you but I'll leave that to someone more experienced than myself to cover.
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