Combustion phasing of Spark Ignition (SI) engines is traditionally regulated with map-based spark timing (SPKT) control. The calibration time and effort of this feed forward SPKT control strategy becomes less favorable as the number of engine control actuators increases. This paper proposes a model based combustion phasing control frame work. The feed forward control law is obtained by real time numerical optimization utilizing a high-fidelity combustion model that is based on flame entrainment theory. An optimization routine identifies the SPKT which phases the combustion close to the target without violating combustion constraints of knock and excessive cycle-by-cycle covariance of indicated mean effective pressure (COV of IMEP). Cylinder pressure sensors are utilized to enable feedback control of combustion phasing. An Extended Kalman Filter (EKF) is applied to reject sensor noise and combustion variation from the cylinder pressure signal. The proposed combustion phasing frame work is validated with real-time dynamometer tests using a rapid-prototype engine control system. Test results indicate the SPKT control system is able to track the combustion phasing target (CA50) with a RMSE of 3.8 crank angle degrees without violating combustion constraints under both steady state and transient operating conditions. Furthermore, the successful execution of this SPKT control strategy using a rapid-prototype engine controller demonstrates the computational efficiency of the proposed system.