1. A method to control torque converter slip in a powertrain comprising a torque generative device and a torque converter comprising an impeller, a turbine and a torque converter clutch, the method comprising: monitoring an actual torque converter slip; monitoring a desired torque converter slip; monitoring a turbine speed of the torque converter; determining a desired hydraulic torque based upon the desired slip and the turbine speed; determining an actual hydraulic torque based upon the actual slip and the turbine speed; determining a feedforward torque converter clutch command pressure based upon the actual hydraulic torque, the desired hydraulic torque, a torque generative device torque, and a torque converter clutch gain; determining a slip error as the difference between the desired torque converter slip and the actual torque converter slip; determining an overall torque converter clutch command pressure by combining the feedforward torque converter clutch command pressure and a feedback torque converter clutch command pressure; and controlling the torque converter clutch slip based upon the overall torque converter clutch command pressure.