To many Pontiac enthusiasts, the U.S. government-mandated emission-control systems of the '70s are an unwelcome addition under the hood. Regardless, they forced an understanding of the chemical to mechanical energy exchange process, and you can apply that knowledge to any Pontiac to achieve a better-running and more-powerful engine. A proficiently tuned carburetor and ignition system produces the most horsepower. Accordingly, we need to look at tailpipe discharge as a bell weather of efficient combustion. This installment of Quick Tech will provide an overview of that.
Identifying the offenders
This pollutant is unburned fuel that did not become consumed during the combustion process. HC can be considered a misfire meter. It is measured in parts per million (PPM).
Carbon monoxide (CO): This chemical is identified as partially burned fuel or fuel that did not have enough oxygen during mixture to support the level of combustion required to consume the fuel in the cylinder bore. CO is a direct function of the air/fuel ratio. If the mixture is excessively rich, the CO will be high. It is measured in percent.
Oxides of nitrogen (NOx): This byproduct of combustion occurs when the leading-edge flame temperature exceeds 2,500 degrees F. It is the result of pressure, heat, and exposure time. NOx was identified as a major factor in the creation of smog and its impact on ground-level ozone production. If an engine experiences detonation, NOx production is exponentially increased. It is measured in PPM.
Evaporative emissions: This is an HC emission but is different from what comes from the tail pipe. It is the result of the gasoline evaporating into the atmosphere as would happen if you left the cap off a can of fuel. Evaporative emissions originated from the then-common vented-gas-tank cap and the bowl-vent of the carburetor.
Oxides of nitrogen
Pontiac achieved a reduction in NOx during the '70s through a system approach that included a lower compression ratio, camshaft profile redesign, and the most effective tool, an exhaust gas recirculation (EGR) valve.
The concept of EGR is at first a hard one to grasp. One may wonder how partially filling a cylinder with hot exhaust gas can cool down the combustion chamber enough to lower the production of NOx.
At idle and light load, the volumetric efficiency (VE) of the engine is very low. VE describes the amount of cylinder fill the bore experiences. At peak torque, the VE is the highest (approximately 80- to 85-percent on a production engine) and the lowest at idle. This is due to the piston velocity being low and the throttle plate being fully closed. By introducing inert exhaust gas into the cylinder, it consumes area that would normally be filled with combustible mixture, thus lowering the flame's temperature because there is less fuel and oxygen in the bore.
When hot exhaust gas is introduced into the bore, it is identified as EGR dilution. Pontiac's goal was to introduce a sufficient amount of EGR to cool the cylinder to below the critical temperature while not impacting power and/or creating a rough and noisy burn.
The amount of dilution varies based on the engine design. Most early Pontiac systems ('70s) would introduce no more than six percent (of VE) inert gas into the cylinder.
1. This '77 Trans Am with a factory-fitted 400 had most of the emission controls found in that era except an A.I.R. pump.
2. Originally this mixture screw was fitted with a plastic limiter cap, but it was removed. The air/fuel ratio predominately impacts the carbon monoxide emissions.
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.