What happened to gasoline in the mid 1970s?

Bruce "Shrinker" Robertson on how different gasolines affect combustion in a real engine:
IF you give the engine fine droplets on street fuel and you keep going finer and finer you eventually go backwards. That the point where you have started to gas the heavy HC’s too early in the process and the engine will start to detonate. With street fuels you need the heavy HC’s to come to the party after TDC but not too late.
Deeper explanation here Motorsports Village • View topic - Dyno time