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Reference.org
Engine pressure ratio
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<p>The engine pressure ratio (EPR) is the total pressure ratio across a <a href="/facts/Jet_engine/CJjFpeqt">jet engine</a>, measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor. </p><p>Jet engines use either EPR or compressor/fan RPM as an indicator of thrust. When EPR is used, the pressures are measured in front of the compressor and behind the turbine. </p>