Bought another pack of the D-16s with the shorter delay to continue the evaluation.  One thing I noticed is that the delay charge well on the Q-Jet case will interfere with the forward tab on the standard Estes engine hook.  Usually it just takes some gentle twisting to get the flat side of the charge well to slide past the hook, but its definitely a "scrape" fit, and I needed pliers to get the spent motors out after flight.  Another decent boost, quick and straight, arcing into the wind after motor burnout.  The 4 second delay is definitely too short, as the rocket was still climbing at ejection.  Good chute, recovered on the field with no damage.