3D calculations of homogeneous charge spark ignition engines were carried out using the KIVA III code. A modified wall function was introduced by an approximate solution of the one -dimensional simplified equations of energy and mass balance. The model takes into account the pressure unsteadiness and the mean rate of combustion in boundary layer. Moreover a modified turbulent conductivity law was proposed following the classical Prandtl approach.The predictions of heat transfer model were compared with the mean heat flows calculated by thermodynamic processing of pressure cycles in motored engines. Two engines with different geometry were used. Namely: a CFR engine running 900 rpm and an AVL engine, running at 2200 rpm. The results regarding heat transfer seem very encouraging.The combustion phase was simulated using a Fractal Flame Model (FFM) elsewhere describe. Simulations in firing conditions were compared with measurements carried out on a CFR engine and on an AVL engine. Test parameters were spark timing and EGR amount.The numerical experiments carried out suggest that, on the same engine, it is possible to simulate the effects of spark timing and EGR variation. However different constants of the combustion model are required for different engines.