Abstract: Apparatus for testing a power transmission drive intended for use with a reciprocating engine by use of an electric drive motor in lieu of the engine. The motor is provided in an adjustable input shaft driving unit which provides a variable speed drive of the power transmission drive correponding to a variable speed drive effected by a reciprocating engine. In one form, the drive speed is varied by a controlled brake which, in the illustrated embodiment, is an eddy current brake. The brake may be utilized in conjunction with a variable speed input shaft driving unit for providing a second speed variation or may be used independently thereof. Alternatively, the electric motor may be operated by a control arranged to provide a speed variation of the electric motor corresponding accurately with the reciprocating engine characteristics. Control circuity for effecting the desired brake control and motor control are disclosed.