Online combustion efficiency optimization in a variable-valve-timing (VVT) gasoline engine requires the real-time knowledge of in-cylinder pressure and its various derivatives. The in-cylinder pressure measurements, however, are still inapplicable to current light duty vehicles due to the high cost of fast pressure sensors. In this paper, an effective combustion model is developed to provide online prediction of crank-angle resolved (CAR) in-cylinder pressure evolution given five representative initial states at intake valve closing (IVC). The prediction of the combustion pressure is made by incorporating mean-value mass/energy flow models with the first law thermodynamics. To achieve real-time calculation for end-use engines, this paper improves the validity region of the existing mass/energy flow models while preserving their simplicity. In particular, the model performances of chemical heat release, crevice flow and heat transfer are closely examined for a wide operation range and then renovated accordingly. This paper also proposes a new calibration scheme which uses in-cylinder pressure measurements only to parameterize all model coefficients as functions of the initial states. The overall calibrated model is implemented online using high-power chip and validated for 3787 test runs with a wide range of engine speeds, loads, residual gas fraction (RGF), air-fuel ratios and ignition timings.