This paper describes why thermal engine data of the main components around the combustion chamber are main issues when calculating ring pack dynamics. Examples are given to illustrate and physically explain how important it is to accurately evaluate thermal condition for a blow-by simulation. A simple fast innovative way to calculate these data based on a thermal mechanical model is presented. Then a complete speed/torque range engine blow-by map is calculated and compare to engine bench test. It shows that simulation give results in the range of the engine test measurement tolerance.