Tighter emmisions standards

Changing cam timing/specs and ignition timing was to lower the NO'X's....which later was the job of EGR Valves.
EGR in basic terms, it's like eating your own **** and still expected to perform.

We went to EGRV's here in about 1976, and then they disappeared (~1988) for a few years, then started to make a come back in about 2000.
We started using Unleaded in 1986.....no CATs before that.