In the framework of the qualifying process of the new small European launcher Vega several experimental tests and numerical studies were done. Among them the qualification of P80 is particularly interesting: P80 is the only motor of the Zefiro-like family that was tested with different pressurizing gas both with nitrogen and helium. The static firing tests of P80 have shown that the pressure oscillations during ignition transient are more intense using nitrogen rather than helium. Several previous numerical studies1-5,7,8 have pointed out that the pressurizing gas is fundamental for the behavior of the interaction between the hot igniter jets and combustion chamber geometry variations: it can favor or limit the generation of pressure oscillations depending on its compressibility. The present paper reports the numerical analysis of the P80 ignition transient both for nitrogen and helium and a comparison with the experimental date provided by the P80 static firing tests.