The Air Pumps (Smog Pumps) had an additional effect on exhaust gas emissions. By adding air to the exhaust, tailpipe emissions were diluted giving cleaner readings.
I mentioned this in post # 3. The smog/air pump wasn't meant to dilute the exhaust, it was to mix fresh air to the exhaust to create a chemical reaction, raise the exhaust temperature to burn more completely.
Catalytic converter science improved dramatically since 1975 too. High flow converters, multiple converters, heated oxygen sensors, etc.
