  This article presents an investigation of isospin equilibration in
cross-bombarding $^{40,48}$Ca$+^{40,48}$Ca reactions at 35 MeV/nucleon, by
comparing experimental data with filtered transport model calculations. Isospin
diffusion is studied using the evolution of the isospin transport ratio with
centrality. The asymmetry parameter $\delta=(N-Z)/A$ of the quasiprojectile
(QP) residue is used as isospin-sensitive observable, while a recent method for
impact parameter reconstruction is used for centrality sorting. A benchmark of
global observables is proposed to assess the relevance of the antisymmetrized
molecular dynamics (AMD) model, coupled to GEMINI++, in the study of
dissipative collisions. Our results demonstrate the importance of considering
cluster formation to reproduce observables used for isospin transport and
centrality studies. Within the AMD model, we prove the applicability of the
impact parameter reconstruction method, enabling a direct comparison to the
experimental data for the investigation of isospin diffusion. For both, we
evidence a tendency to isospin equilibration with an impact parameter
decreasing from 9 to 3 fm, while the full equilibration is not reached. A weak
sensitivity to the stiffness of the equation of state employed in the model is
also observed, with a better reproduction of the experimental trend for the
neutron-rich reactions.
