The EGR valve was located on the intake manifold. A passage from the exhaust port of the cylinder head fed the inert gas to the valve and then distributed it to the intake-manifold plenum. A ported vacuum signal and a thermal switch would usually operate the valve. EGR was only introduced when the coolant was at operating temperature and the engine was at part throttle and light load. At idle and full throttle, the mixture is substantially richer and the fuel acts as a coolant, so no EGR is required.
The EGR passage in the intake manifold and cylinder head often resembles—and in some instances, is shared with—the exhaust feed for the heat riser.
Design elements in the engine, such as a lowered compression ratio helped to eliminate a good amount of NOx at its source. For the '71 model year, Pontiac dropped the compression ratio in almost all its engines with a value of 8.5:1 becoming nominal. Model-year 1974 saw some engines with a 7.6:1 compression ratio. The first EGR valve appeared on a '72 Buick engine and was standard equipment on most if not every Pontiac for the '73 model year.
Precise control of the air/fuel ratio is the most effective means for limiting CO production. A carburetor calibrated to create a mixture of 14.7:1 at idle and part throttle goes a long way in eliminating CO.
To limit the amount of alteration to the mixture, a plastic limiter cap was placed on the mixture screw ('70s). This device would only allow a ½- to 7⁄8-inch turn rich from the factory setting.
Pontiac paid attention to the choke circuit, also. During cold-engine operation, the mixture needs to be substantially richer than when at operating temperature. Pontiac revised the choke design into a dual vacuum break with spring tension and fast-idle specifications that would remove the choke sooner than was considered normal in years prior.
The accelerator pump shot and power enrichment circuit were also finely tuned. The edict [before the emissions era] was to overfuel these conditions to ensure acceptable engine performance. The new emission laws did not allow for this luxury. The accelerator pump shot and power enrichment were leaned to the point that there was just the proper amount of fuel available when every other aspect of the tune was on specification. If any part of the ignition or fuel system was degraded, then the end result was a flat spot or hesitation from the lean air/fuel ratio.
Hydrocarbons and carbon monoxide
By installing a small, crankshaft-driven pump to provide oxygen to the exhaust manifold, HC emissions were reduced. The proper name of this pump was Air Injection Reaction (A.I.R.), but became known as a smog pump. Interestingly, the First-Gen Camaro Z/28 with the 302ci V-8 was the first factory installation of an A.I.R. system in all 50 states.
The circuit included a filtered, fresh-air intake and a check valve that did not allow the exhaust gas to migrate into the pump and create combustion in-between the vanes and destroy it. Anti-backfire measures called diverter valves were also used for protection.
Though its main intent was to reduce HC emissions, the A.I.R. system had a positive impact on CO also. It allowed for a less sensitive carburetor calibration, and, contrary to belief, consumed almost no power.
Though many look at the federal emission regulations of the '70s as a hindrance, the laws are responsible for a much healthier environment and if used properly will allow you to tune your Pontiac better.
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3. The charcoal canister is used to keep gasoline vapors from being introduced into the environment. The fumes are then purged to the intake manifold and burned in the engine.
4. The EGR valve is responsible for a reduction in NOx emissions.
5. Pontiac introduced HEI in '75 to counter the increased HC emissions of breaker points.