  It is well known that helical magnetohydrodynamic (MHD) turbulence exhibits
an inverse transfer of magnetic energy from small to large scales, which is
related to the approximate conservation of magnetic helicity. Recently, several
numerical investigations noticed the existence of an inverse energy transfer
also in nonhelical MHD flows. We run a set of fully resolved direct numerical
simulations and perform a wide parameter study of the inverse energy transfer
and the decaying laws of helical and nonhelical MHD. Our numerical results show
only a small inverse transfer of energy that grows as with increasing Prandtl
number (Pm). This latter feature may have interesting consequences for cosmic
magnetic field evolution. Additionally, we find that the decaying laws $E \sim
t^{-p}$ are independent of the scale separation and depend solely on Pm and Re.
In the helical case we measure a dependence of the form $p_b \approx 0.6 +
14/Re$. We also make a comparison between our results and previous literature
and discuss the possible reason for the observed disagreements
