In a hybrid electric vehicle powered by an internal combustion engine that produces torque and an electric machine that produces torque, a method for operating the electric machine to produce electric assist to the engine includes determining a magnitude of power requested by the vehicle operator, determining a minimum power limit and a maximum power limit of the engine that will produce a response to a torque request within a desired period following the torque request, using the magnitude of power requested, the minimum and maximum power limits of the engine to determine the upper and lower limits of the electric power assist such that the requested torque will be provided within the desired period, and operating the electric machine to provide electric power assist within said upper and lower limits.