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Blowby phenomenon of fuel-air mixture from cylinder-piston crevices, which occurs due to difference of in-cylinder and connected crevice pressures, influences engine performance. In the current work, experimental data of gasoline fuelled motoring condition at equivalence ratio of 0.9 were collected from a single cylinder research engine using skip spark technique. A relatively simple non-thermodynamic polytropic-base model was introduced and orifice-volume theory was coupled it; and gas flow through crevices was studied. From positive points of the model, it can be implied that the model predicts cyclic blowby without performing complex heat transfer and thermodynamic calculations. A verified thermodynamic simulation model including blowby sub-model was used to validate the polytropic-base model. Cylinder pressure evaluated by the thermodynamic model had good agreement with the measured pressure in the gasoline fuelled motoring condition at the equivalence ratio. First, in the polytropic-base model, output cylinder pressure of the thermodynamic simulation model was defined instead of experimental cylinder pressure and its blowby was evaluated. Then entering experimental cylinder pressure at equivalence ratio of 0.9 to the current model, cylinder mass and blowby to crevices were evaluated and compared with the predictions of the thermodynamic model. A very good agreement was observed between the obtained results and the results of the thermodynamic model. The new model showed maximum 6.88% cylinder mass lost around peak pressure position decreasing to 0.45% along the late expansion stage.