The Smog System
As you know in the 70's GM started the vehicle emissions control. The AIR system was added to pump fresh air into the exhaust system to help burn partially burned fuel before its ejection from the tailpipe. There was significant unburned and partially burned fuel in the exhaust of early 1970s vehicles, and so secondary air injection significantly reduced tailpipe emissions. However, the extra heat of recombustion, particularly with an excessively rich exhaust caused by misfiring or a maladjusted carburetor, tended to damage exhaust valves and could even be seen to cause the exhaust manifold to glow red. The reason for the unburned fuel in the first place was the smog heads GM produced that would not let the engine breath. The timing on the engines were also set at TDC or ATDC. So it makes sense to get rid of this junk, use better heads, headers or free flowing exhaust, better intakes, and adjust timing to burn fuel in the combustion chamber more completely.