  Systematic examination of fusion for $^{39,41,45,47}$K + $^{28}$Si and
$^{36,44}$Ar + $^{28}$Si provides insight into the impact of neutron and proton
exchange on fusion for nuclei at and near the N=20 and N=28 shells. Comparison
of the reduced excitation functions reveals a marked difference between the
behavior of open-shell and closed-shell systems. While coupled channels
calculations provide a good description for the closed-shell nuclei they
significantly under-predict the fusion cross-section for open-shell nuclei. The
observed trends are examined in the context of a potential energy surface,
including shell effects, and multi-nucleon exchange with consideration of
Pauli-blocking.
