Air Pump and It's Function
Starting in the 1960s, there was concern over the significant amount of unburned and partially burned fuel in automotive exhaust. In an effort to decrease this source of emissions, the air injection system was created.
A.I.R. (air injection reactor) pumps, commonly referred to as smog pumps, help reduce emissions in two ways. While your car's engine is cold, it pumps excessive hydrocarbons back into the engine for re-combustion, providing a more complete combustion. It also diverts air to either the atmosphere or downstream to the oxygen sensor.
Engine exhaust (which contains unburned hydrocarbons and carbon monoxide) is routed into the AIR pump, then compressed injected into the exhaust port of each cylinder. The gases then combine with unburned hydrocarbons and carbon monoxides at higher temperatures, causing a chemical reaction. Now the spent exhaust is less harmful to our environment.
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Along with recalibrated carburetor and distributor settings and higher engine-operating temperatures, combustion efficiency was improved and harmful emissions were reduced.
The AIR pump is generally a sealed bearing unit that mounts to the front of the engine. They are most always belt-driven with other accessories such as alternator, power steering pump, and air-conditioning compressor.
A common AIR system would include the pump, a fuel-mixture control-valve or air diverter valve. Also in the system are air injection tubes and check valves.
Classic cars are worth more in factory-stock condition, but many cars from the late sixties and seventies have had their smog equipment removed. If your old car is missing the air pump, it's probably also missing the manifold tubes, check valves, diverter valve, mounting bolts and brackets, as well as hoses and clamps. Theses can be hard to find, but often these can be taken from a similar year and model donor car.
Needed or Not
The Fiat Spider, produced from 1966 to 1985, featured an efficient twin-cam four-cylinder motor, designed by Italian automobile and aircraft engine designer Aurelio Lampredi. But starting in 1974, regardless that these engines already ran cleanly and efficiently, all cars bound for North America were required to have pollution devices - regardless of whether they were really needed.
In order to continue selling to its largest market, Fiat, like many other manufacturers, complied by adding on smog equipment, which burdened motors, choked performance, and caused driveability issues. Air pumps were added, along with restrictive manifolds, tiny carburetors, scores of vacuum lines, and other emission controls. Performance and sales both suffered.
The good that came out of all this, was it prompted car manufacturers to design and produce more efficient fuel-injected engines, which started appearing in the early eighties.