A method for improving the performance of a bypass turbojet engine mounted on an aircraft that is to accomplish a determined flight mission comprising a phase of cruising flight, said turbojet engine comprising, around its longitudinal axis (L-L): - a nacelle provided with a nacelle outer cowl and containing a fan generating the cold flow and a central generator generating the hot flow; - an annular cold flow duct formed around said central hot flow generator; - a fan outer cowl delimiting said annular cold flow duct on the nacelle outer cowl side; - an annular cold flow outlet orifice of which the edge, which forms the trailing edge of said nacelle, is determined by said nacelle outer cowl and by said fan outer cowl converging toward one another until they meet; - a fan inner cowl delimiting said annular cold flow duct on said central hot flow generator side, passing through said cold flow orifice and forming a projection out of said cold flow outlet orifice toward the rear of said turbojet engine; and - a cold flow nozzle throat which is formed, forward of said cold flow outlet orifice, between said fan inner cowl and said fan outer cowl and of which the annular cross section has a nominal area (Ac) fixed by the thermodynamic cycle of said turbojet engine and smaller than the area of said cold flow outlet orifice, so that a convergent/divergent nozzle is formed at the rear part of said cold flow duct, characterized in that: - the extreme values for the expansion ratio of said cold flow corresponding respectively to the start and to the end of said cruising phase are determined; - from said extreme values, a reference value for said expansion ratio is chosen; - for this reference value of the expansion ratio, the theoretical value (Ath) of the area of said cold flow outlet orifice is determined from an auxiliary theoretical value representative of the ratio between said theoretical area of said cold flow outlet orifice and said nominal cross-sectional area of said nozzle throat; and - said cold flow outlet orifice is positioned along said longitudinal axis (L-L) in such a way that its area corresponds to said theoretical value (Ath). - a nacelle provided with a nacelle outer cowl and containing a fan generating the cold flow and a central generator generating the hot flow; - an annular cold flow duct formed around said central hot flow generator; - a fan outer cowl delimiting said annular cold flow duct on the nacelle outer cowl side; - an annular cold flow outlet orifice of which the edge, which forms the trailing edge of said nacelle, is determined by said nacelle outer cowl and by said fan outer cowl converging toward one another until they meet; - a fan inner cowl delimiting said annular cold flow duct on said central hot flow generator side, passing through said cold flow orifice and forming a projection out of said cold flow outlet orifice toward the rear of said turbojet engine; and - a cold flow nozzle throat which is formed, forward of said cold flow outlet orifice, between said fan inner cowl and said fan outer cowl and of which the annular cross section has a nominal area (Ac) fixed by the thermodynamic cycle of said turbojet engine and smaller than the area of said cold flow outlet orifice, so that a convergent/divergent nozzle is formed at the rear part of said cold flow duct, - the extreme values for the expansion ratio of said cold flow corresponding respectively to the start and to the end of said cruising phase are determined; - from said extreme values, a reference value for said expansion ratio is chosen; - for this reference value of the expansion ratio, the theoretical value (Ath) of the area of said cold flow outlet orifice is determined from an auxiliary theoretical value representative of the ratio between said theoretical area of said cold flow outlet orifice and said nominal cross-sectional area of said nozzle throat; and - said cold flow outlet orifice is positioned along said longitudinal axis (L-L) in such a way that its area corresponds to said theoretical value (Ath).