  Creeping flow of polymeric fluid without inertia exhibits elastic
instabilities and elastic turbulence accompanied by drag enhancement due to
elastic stress produced by flow-stretched polymers. However, in
inertia-dominated flow at high $\mbox{Re}$ and low fluid elasticity $El$, a
reduction in turbulent frictional drag is caused by an intricate competition
between inertial and elastic stresses. Here, we explore the effect of inertia
on the stability of viscoelastic flow in a broad range of control parameters
$El$ and $(\mbox{Re}, \mbox{Wi})$. We present the stability diagram of observed
flow regimes in $\mbox{Wi}-\mbox{Re}$ coordinates and find that instabilities'
onsets show unexpectedly non-monotonic dependence on $El$. Further, three
distinct regions in the diagram are identified based on $El$. Strikingly, for
high elasticity fluids we discover a complete relaminarization of flow at
Reynolds number of the order of unity, different from a well-known turbulent
drag reduction. These counterintuitive effects may be explained by a finite
polymer extensibility and a suppression of vorticity at high $\mbox{Wi}$. Our
results call for further theoretical and numerical development to uncover the
role of inertial effect on elastic turbulence in a viscoelastic flow.
