Thermostatic Air Cleaner: Also called T.A.C. or Thermostatically-Controlled Air Cleaner. Started to be be common in 1972.
This system existed to heat the incoming air on a cold engine. It basically lessened the need for the carburetor’s choke to be engaged fully, effectively leaning out the air/fuel ratio. This process helped to control the levels of hydrocarbons exiting the exhaust. It also helped cars warm up faster and run better in cold temperatures.
The entire system consisted of a ‘heat stove’ on one exhaust manifold, a riser tube coming off the stove, a few vacuum lines, a valve in the air cleaner snorkel, and a thermostat valve tapped into the car’s cooling system and/or inside the air cleaner.
Basically, heated air came from the stove and went into the bottom of the air cleaner snorkel. When the system’s thermostatic valve got to a predetermined temperature, it would let engine vacuum open up the regular valve in the air cleaner snorkel. This let in cooler outside air once the engine.
The advent of electronic fuel injection negated the need for this system since fuel control became more precise. Replacing the system with a standard open filter type air cleaner, to this day, is still frowned on by emission inspectors. Just don’t scrap the original system if this is what you plan on doing.