Numerical simulation of working process of diesel engine with the 21/21 dimension was performed in ACTUS software (Switzerland, Baden, ABB Turbo Systems Ltd). The object of modeling was the eight-cylinder diesel engine with turbocharger. This engine was made by Ural Diesel Motor Plant (Russia, Ekaterinburg). Nume- rical study showed that the geometric dimensions of the intake pipe have a significant impact on the flows dynamics. In particular, it is shown that the use of intake pipe with a larger diameter (on the 8DM-21 engine) leads to an increase of the filling ratio on average by 0,5 %. This leads to an increase in engine power by up to 0,7 % and reduce the effective specific fuel consumption by about 0,5-0,75 %.