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The most used technology able to inhibit NOx formation in combustion chamber is the Exhaust Gas Recirculation (EGR). Smoke limits, however, can be easily reached when EGR grow up to fulfill the emission limits. Further percentage increase, necessary to observe the oncoming more severe regulations, can be performed only if a control strategy is implemented on A/F ratio lower limit, based on turbo-charging level. In this paper the authors present an analysis performed on a medium size light duty turbocharged direct injection Diesel engine aimed to the definition of the current engine layout limit in terms of smoke emission and turbocharger functionality, in case of increased EGR rates, both in short route and in long route configuration. The theoretical activity based on a mathematical model of the engine has been supported by a focused experimental campaign on test bench, with engine in original configuration and equipped with a long route EGR pipe.