Engine Instrument Systems (1) Engine pressure ratio (EPR) is a ratio of the exhaust gas pressure to the engine inlet air pressure, and indicates the thrust produced. (2) Engine pressure ratio (EPR) is a ratio of the exhaust gas pressure to the engine inlet air pressure, and indicates volumetric efficiency. Regarding the above statements, a. only No. 1 is true. b. only No. 2 is true. c. both No. 1 and No. 2 are true.