For the first time, we have deduced theoretical predictions concerning the pulsational proprieties of classical Cepheids at sovra-solar metal content and for different assumptions on the Y/Z ratio. The results of this investigation combined with previous nonlinear pulsating models with lower metal abundances, show that the location in the HR diagram of Cepheids instability strip is dependent on both metal and helium abundance. In particular, an increase of the metal content (at fixed Y) moves the instability strip towards lower effective temperatures, while an increase of helium abundance (at fixed Z) moves it bluewards. On this basis, we discuss helium and metal combined effect on the theoretical period-luminosity relations and their implication on the Cepheids distance scale.