  We use fully nonlinear numerical relativity techniques to study high energy
head-on collision of nonspinning, equal-mass black holes to estimate the
maximum gravitational radiation emitted by these systems. Our simulations
include improvements in the construction of initial data, subsequent full
numerical evolutions, and the computation of waveforms at infinity. The new
initial data significantly reduces the spurious radiation content, allowing for
initial speeds much closer to the speed of light, i.e. $v\sim0.99c$. Using
these new techniques, We estimate the maximum radiated energy from head-on
collisions to be $E_{\text{max}}/M_{\text{ADM}}=0.13\pm0.01$. This value
differs from the second-order perturbative $(0.164)$ and zero-frequency-limit
$(0.17)$ analytic computations, but is close to those obtained by thermodynamic
arguments $(0.134)$ and by previous numerical estimates $(0.14\pm0.03)$.
