They took a stab at controlling lean burn raised NOx levels

Another interesting fact to add to the NOx argument is the fact that an EGR (Exhaust Gas Recirculation) Valve System was added to the D16Y5. This is the "2nd Gen" VTEC-E engine found in the 1996-2000 Honda Civic HX, which is the 6th Generation Civic's "updated" version of your VX and D15Z1.

Google search "VTEC-E" lead me to bring this thread back from the dead.