A compound cycle engine (10, 110, 210) comprising: an output shaft; at least two rotary units each defining an internal combustion engine including an engine rotor sealingly and rotationally received within a respective housing, each housing defining an inlet port and an exhaust port, the engine rotor of each of the at least two rotary units being in driving engagement with the output shaft; a first stage turbine including a flowpath and a turbine rotor having a circumferential array of blades extending across the flowpath, the turbine rotor being in driving engagement with the output shaft; and a turbocharger including a compressor and a second stage turbine in driving engagement with one another; wherein: an outlet of the compressor is in fluid communication with the inlet port of each housing; the exhaust port of each housing is in fluid communication with a first portion of the flowpath of the first stage turbine, the first portion of the flowpath being located upstream of the circumferential array of blades of the first stage turbine; an inlet of the second stage turbine is in fluid communication with a second portion of the flowpath of the first stage turbine, the second portion of the flowpath being located downstream of the circumferential array of blades of the first stage turbine; the first stage turbine has the output shaft extending therethrough; characterized in that: the at least two rotary units include a first rotary unit (12a, 12b, 12d, 12e) located in front of the first stage turbine along the output shaft and a second rotary unit (12c, 12f, 12g) located behind the first stage turbine along the output shaft; in that the first stage turbine has a reaction ratio of at most 0.2; and in that the second stage turbine has a reaction ratio of at least 0.25. an output shaft; at least two rotary units each defining an internal combustion engine including an engine rotor sealingly and rotationally received within a respective housing, each housing defining an inlet port and an exhaust port, the engine rotor of each of the at least two rotary units being in driving engagement with the output shaft; a first stage turbine including a flowpath and a turbine rotor having a circumferential array of blades extending across the flowpath, the turbine rotor being in driving engagement with the output shaft; and a turbocharger including a compressor and a second stage turbine in driving engagement with one another; wherein: an outlet of the compressor is in fluid communication with the inlet port of each housing; the exhaust port of each housing is in fluid communication with a first portion of the flowpath of the first stage turbine, the first portion of the flowpath being located upstream of the circumferential array of blades of the first stage turbine; an inlet of the second stage turbine is in fluid communication with a second portion of the flowpath of the first stage turbine, the second portion of the flowpath being located downstream of the circumferential array of blades of the first stage turbine; the first stage turbine has the output shaft extending therethrough; an outlet of the compressor is in fluid communication with the inlet port of each housing; the exhaust port of each housing is in fluid communication with a first portion of the flowpath of the first stage turbine, the first portion of the flowpath being located upstream of the circumferential array of blades of the first stage turbine; an inlet of the second stage turbine is in fluid communication with a second portion of the flowpath of the first stage turbine, the second portion of the flowpath being located downstream of the circumferential array of blades of the first stage turbine; the first stage turbine has the output shaft extending therethrough; characterized in that: the at least two rotary units include a first rotary unit (12a, 12b, 12d, 12e) located in front of the first stage turbine along the output shaft and a second rotary unit (12c, 12f, 12g) located behind the first stage turbine along the output shaft; in that the first stage turbine has a reaction ratio of at most 0.2; and in that the second stage turbine has a reaction ratio of at least 0.25. the at least two rotary units include a first rotary unit (12a, 12b, 12d, 12e) located in front of the first stage turbine along the output shaft and a second rotary unit (12c, 12f, 12g) located behind the first stage turbine along the output shaft; in that the first stage turbine has a reaction ratio of at most 0.2; and in that the second stage turbine has a reaction ratio of at least 0.25.