This paper numerically investigates the performance implications of the use of an electric supercharger in a heavy-duty DD13 diesel engine. Two electric supercharger configurations are examined. The first is a high-pressure (HP) configuration where the supercharger is placed after the turbocharger compressor, while the second is a low-pressure (LP) one, where the supercharger is placed before the turbocharger compressor. At steady state, high engine speed operation, the airflows of the HP and LP implementations can vary by as much as 20%. For transient operation under the Federal Test Procedure (FTP) heavy duty diesel (HDD) engine transient drive cycle, supercharging is required only at very low engine speeds to improve airflow and torque. Under the low speed transient conditions, both the LP and HP configurations show similar increases in torque response so that there are 44 fewer engine cycles at the smoke-limit relative to the baseline turbocharged engine. When the requested engine torque rise rate is increased from the FTP ramps to steps, the benefit of supercharging is extended to also include mid-range engine speeds, with over ∼ 70% fewer cycles at the smoke-limit line. In addition, the results show an improvement in the overall fuel economy of the supercharged engine during low engine speed transients compared to the baseline turbocharged engine. The study highlights the importance of supercharger by-pass valve control, where the transient response of the valve should be twice as fast as the electric supercharger drive motor for accurate and minimal supercharger power consumption during transient maneuvers. Finally, an engine re-calibration with increased exhaust gas recirculation at low engine speeds/loads, resulted 4.6% fuel economy improvements at that low speed/load region while the supercharger enabled fast air-flow increase during aggressive tip-ins.