  The influence of $\Delta$ isobar components on the ground state properties of
nuclear systems is investigated for nuclear matter as well as finite nuclei.
Many-body wave functions, including isobar configurations, and binding energies
are evaluated employing the framework of the coupled-cluster theory. It is
demonstrated that the effect of isobar configurations depends in a rather
sensitive way on the model used for the baryon-baryon interaction. As examples
for realistic baryon-baryon interactions with explicit inclusion of isobar
channels we use the local ($V28$) and non-local meson exchange potentials
(Bonn$_{2000}$) but also a model recently developed by the Salamanca group,
which is based on a quark picture. The differences obtained for the nuclear
observables are related to the treatment of the interaction, the $\pi$-exchange
contributions in particular, at high momentum transfers.
