Transient operation of automobile engines is known to contribute significantly to regulated exhaust emissions, and is also an area of drivability concerns. Furthermore, many on-board diagnostic algorithms do not perform well during transient operation and are often temporarily disabled to avoid problems. The inability to quickly and repeatedly test engines during transient conditions in a laboratory setting limits researchers and development engineers ability to produce more effective and robust algorithms to lower vehicle emissions.To meet this need, members of the Powertrain Control Research Laboratory (PCRL) at the University of Wisconsin-Madison have developed a high-bandwidth, hydrostatic dynamometer system that will enable researchers to explore transient characteristics of engines and powertrains in the laboratory. Unlike most conventional dynamometers that typically operate under relatively steady state operation, this device is capable of providing a repeatable virtual transient engine load on engines under test during motoring as well as absorbing conditions.In this paper the authors review the hardware design considerations for hydrostatic dynamometers and describe in detail the components used in the current system. The dynamometer performance is then illustrated with test results.