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This paper presents results of mass flow prediction study in progressive bore throttle body (TB), using CFD. A major emphasis of the study has been on capturing the effect of tolerance on clearance area and hence flow predictions, especially at low angles. In addition, effect of viscosity on mass flow predictions has been investigated. Comparison of experimental mass flow obtained from a “manufactured TB” with CFD prediction leads to significant difference, especially at low angles. One reason for this difference is that CFD models based on mean-CAD geometry do not capture the effect of tolerances. To address this difference, an analytical equation for predicting clearance area has been developed. This allows capturing of variation in geometry due to manufacturing. A Design of Experiments (DoE) approach utilizing the analytical work and GE proprietary six sigma tools has been used to capture the effect of tolerances on the clearance area and quantify the variation.