What differences can I expect in results from running the ISA S75.02 valve capacity tests (compressible - air) under vacuum (e.g. downstream “blowing engines”) versus pressurized (e.g. upstream air compressor)?
It depends how much vacuum you have downstream.  If the absolute pressure ratio is about 2 or more, it exceeds critical drop.  Velocity becomes sonic, there is a standing shock wave in the vena contracta, and downstream pressure becomes independent of flowrate.  Starting at atmospheric pressure, for a fixed ball valve position if you drop the downstream pressure, flow will increase. Progressively lower downstream pressure increases the flow, until you hit the critical pressure drop.  This is also known as choked flow.  I don't like the term "choked" because it is misunderstood.  If you open the valve more, you still get more flow, and if you increase upstream pressure you also will get more flow.  "Choked" is merely a thermodynamic expression that to us means it's time to use a different Cv equation. 

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