The measurement of instantaneous angular speed in flywheels has been used in Internal Combustion Engines for the diagnosis of faults in the fuel injection systems, combustion quality at idle, ignition system, and especially in the performance assessment of starter motor and battery accumulator, among others. It is the aim of this paper to forecast the use of an experimental flywheel-based test bench, driven by a single-cylinder Internal Combustion Engine and a starter motor, developed to perform research studies in the laboratory of internal combustion engines. It is illustrated the measurement and processing of the instantaneous angular speed of a flywheel to characterize the dynamic response of the inertial system during the run-up and run-out regimes. The mathematical model of the transfer function is presented, relating the angular velocity with the torque transmitted by the starter motor as a first-order system. For the acquisition of the signal, a NI 6009 card is used, while a Matlab computer program is employed to plot the instantaneous angular velocity curves, and also to identify the time response of the system. The time constant is 1.54 seconds, which corresponds to 63.2% of the value of steady state signal in run-up regime.