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A computer program (PISTON) has been developed for evaluating air and exhaust flow parameters within the cylinder and the exhaust manifold of single or multi-cylinder reciprocating engines. The PISTON Code has been experimentally verified to accurately determine such 1) air and exhaust flows (including volumetric efficiency, trapped mass, and blowthrough), 2) piston pumping horsepower, and 3) cylinder and exhaust stack pressures, temperatures, and horsepowers (both instantaneous and average), A turbocompounded system, including a blowdown exhaust stack designed using the PISTON Code, increased the engine BHP output of the Army’s 525 CID VHO diesel engine by over 25%. This program serves as a useful tool for designing reciprocating engines for optimum performance, i.e. increased airflow, reduced pumping horsepower, and/or increased exhaust energy utilization.