  We numerically simulate the time evolution of the Ising field theory after
quenches starting from the $E_8$ integrable model using the Truncated Conformal
Space Approach. The results are compared with two different analytic
predictions based on form factor expansions in the pre-quench and post-quench
basis, respectively. Our results clarify the domain of validity of these
expansions and suggest directions for further improvement. We show for quenches
in the $E_8$ model that the initial state is not of the integrable pair state
form. We also construct quench overlap functions and show that their
high-energy asymptotics are markedly different from those constructed before in
the sinh/sine-Gordon theory, and argue that this is related to properties of
the ultraviolet fixed point.
