  In this paper we present an extended set of nonlinear convective pulsation
models at varying the metallicity and DY/DZ ratio. The predicted instability
strip and bolometric light curves are discussed by comparing the new models
with our previous ones. In particular, the dependence on both metal and helium
abundances is investigated. By transforming the bolometric light curves into
the observational bands we are able to derive both Period-Color-Luminosity and
Wesenheit relations for each selected chemical composition. Synthetic
Period-Luminosity relations are obtained by populating the instability strip
according to specific assumptions on the number of pulsators and the mass
distribution. These theoretical results are compared with recent accurate data
by Sandage et al. and Kervella et al., in order to test the predictive
capabilities of the models. We confirm our previous results that the
theoretical metallicity correction to the Key Project Cepheid distance scale
depends both on the period range and DY/DZ ratio, becoming important for
periods longer than 20 days and DY/DZ > 1.5.
