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Exhaust pressure does always increase with increasing boost pressure, this is because the boost pressure, or more correct pressure ratio and massflow will dictate the amount of power needed from the turbine, and since increased flow also means increased exhaust flow we can se that the pressure ratio over the turbine and compressor are related, simplified, only affected by exhaust enthalpi and efficiencies.
However, the VNT technology can by changing the turbine nozzle area adjust the speed of the gases that flows into the turbine, and this makes it possible to reduce exhaust pressure at high rpm's as well as lower the boost threshold and reducing lag.
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