Engine exhaust is the sole source of a vehicle's carbon monoxide (CO) and nitrous oxides (NOx) emissions, and accounts for about 62% of a vehicle's hydrocarbon (HC) emissions. In a perfect situation, the only emissions from a gasoline engine would be carbon dioxide and water. However, the combustion process within an engine is never complete. This incomplete combustion is the cause of the unwanted and dangerous exhaust emissions. An engine set up to run "clean", without any pollution control devices may be a theoretical possibility, but such an engine would be an intractable beast to use as a power source for a vehicle. Cold starting and acceleration, for example, require air/fuel mixtures much richer (i.e. more fuel) than can be properly burned. Deceleration produces an effectively rich mixture. Atomization of fuel is never optimum for complete combustion, and the means of ignition (sparks from the spark plugs) is rarely as good as it should be. In order to clean up the results of these inevitable pollution-producing conditions, exhaust emission control devices and modifications have become the most numerous and varied of all emission controls on a car.

The main exhaust emission controls are: exhaust port injection, intake manifold injection, exhaust gas recirculation (EGR), catalytic converters, and intake air temperature controls. Modifications to existing components and systems include vacuum advance cut-off and delay, leaner fuel mixtures, redesigned intake manifolding, reduced compression ratios, longer stroke, redesigned combustion chambers, camshafts with valve overlap changes, revised centrifugal advance curves, and other similar changes.

An air injection system consists of a belt-driven air pump, pressure relief valve, check valve(s), hoses, and an anti-backfire device (either a gulp valve or a diverter valve). The pump provides a continuous flow of low pressure air (typically at around 3 p.s.i.) into the exhaust ports. This air promotes oxidation of the unburned hydrocarbons and carbon monoxide in the exhaust gasses to produce water and carbon dioxide. Under conditions of high manifold depression (deceleration), the gulp valve or diverter valve will allow some of the air from the air pump into the intake manifold. This helps the carburetor's deceleration valve in controlling excessive hydrocarbon emissions by adding fresh air to lean the rich mixture caused by the deceleration (overrun) condition.

Catalytic converters are another control to minimize hydrocarbon and carbon monoxide emissions. These continue the job begun by exhaust air injection by using a catalyst (platinum or palladium) to further the oxidation of the unburned hydrocarbons and carbon monoxide, by essentially burning them, but at lower than burning temperatures. It must be noted that even small amounts of lead severely contaminate the catalyst material, rendering it useless, leading to blockage and mechanical failure within the converter. This condition quickly leads to loss of power, engine overheating, and expensive repairs.

Exhaust gas recirculation is generally the most effective means to control nitrous oxides (NOx) emissions. As its name states, this system recirculates a small percentage of exhaust gasses back into the intake system.

This reduces the combustion temperatures by diluting the intake fuel/air mixture. The main physical feature of this system is the vacuum controlled E.G.R. valve, which opens during periods of low manifold depression (eg. high speed).

Intake air temperature controls use a temperature sensitive flap valve to give the engine warm air from around the exhaust manifold when the engine is cold to better vaporize fuel during engine warm-up. After warm-up, the valve changes position to allow the engine to use cooler air for better combustion and more power.

The leaner air/fuel mixtures required to reduce unburned hydrocarbons naturally lead to higher combustion temperatures. Vacuum advance cut-off and delay systems essentially retard the ignition timing from what it would otherwise be to reduce the combustion temperature and reduce formation of nitrous oxides. At the same time, higher temperatures are produced at the end of the combustion process, reducing the amount of unburned hydrocarbons in the exhaust. As these functions may be controlled by throttle position, engine temperature, and manifold vacuum, or a combination of these, these systems can be relatively complicated, as on 1972-'74 TR6s. (Yes, all of those skinny black nylon tubes and rubber connectors do have important jobs to do.)