Lean mixture combustion might be an important feature in the next generation of SI engines, while diluents (internal and external EGR) have already played a key role in the reductions of emissions and fuel consumption. Lean burn modeling is even more important for engine modeling tools which are sometimes used for new engine development. The effect of flame strain on flame speed is believed to be significant, especially under lean mixture conditions. Current quasi-dimensional engine models usually do not include flame strain effects and tend to predict burn rate which is too high under lean burn conditions. An attempt was made to model flame strain effects in quasi-dimensional SI engine models. The Ford model GESIM (stands for General Engine SIMulation) was used as the platform. A new strain rate model was developed with the Lewis number effect included. A 2.5L V6 4-valve engine and a 4.6L V8 2-valve modular engine were used to validate the modified turbulent entrainment combustion model in GESIM. Results showed that current GESIM can differ by as much as 10 crank angle degrees compared with test data. The modified GESIM can predict burn duration to within 1-2 CA of experimental data, which is considered very good for engine models. This improvement will make quasi-dimensional engine models such as GESIM more predictive and enhance its, applications in new engine development.