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

Hydrocarbons (HC): 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.

Reducing emissions

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.