In the US: ZC and D16A1/D16A3 are most similar.
Elsewhere: ZC and D16A8/D16A9 are *almost* the same motor.
In the US: The 86-89 Integra engine is basically just a detuned version of the ZC. It features flat-top pistons (ZC has domed), less aggressive cams, different rods, and a smaller intake manifold.
Elsewhere: The A8/A9 only lacks the cams of the ZC to be the same engine.
There is no D16A6ZC, there are never more than 5 characters in a Honda block code.