As I have understood it..... In '73, the first serious emission stuff was installed, and the timing was retarded to reduce peak combusiton temps to lower NoX emissions. It also worked better to avoid pinging with leaner idle mixtures used to lower HC emissions.
But even at that, the factory timing settings for the /6 were never very advanced. I assume this was just to avoid pinging problems with limited maintenance, bad gas, and so forth in a standard economy car. My '62 FSM says 2.5* and I have it up at 6-7* and could go further.
