Exploring the Differences Between Pontiac's First- and Second-Type 1973 Emissions Control Systems
The next time you see a '73 Pontiac model like Steve Schappaugh's SD-455Trans Am, check under the hood and take a close look at the emissionscontrol system. Many fail to realize that two separate systems were usedthat year. Although each had the same basic components and functionedsimilarly, the early system fell victim to the EPA's watchful eye.Continue on as we discuss the first and second types and explore whychanges occurred.
Inrecent years, independent research has helped clear up many myths within the Pontiac hobby. One area, however, that still remains shrouded in mystery is the midyear emissions system change that occurred during the '73 model year. Generally referred to as an EGR system change required by the EPA, an in-depth look reveals that the changes went much deeper than eliminating a timed solenoid. Follow along as we explore the function of the entire '73 emissions control system, discuss the differences between the first and second types, and hear some inside information about the changes.
For every General Motors division, the '71 model year meant lower maximum compression ratios, operation on unleaded fuel, and stricter exhaust emission standards. As years progressed, manufacturers were faced with stricter standards requiring them to control the amount of certain pollutants emitted from the tailpipe. The two major control systems that helped reduce emissions for the '73 model year were Transmission Controlled Spark (TCS) and Exhaust Gas Recirculation (EGR).
THE POLLUTANTS
To better understand the function of the entire emissions control system, we must recognize those pollutants that were subject to federal standards for the '73 model year--carbon monoxide (CO), unburned hydrocarbons (HC), and oxides of nitrogen (NOx). These compound molecules are all formed from molecules found in fuel and air and are byproducts of combustion. Carbon monoxide is the incomplete burn of carbon molecules found in fuel. Hydrocarbons are unburned or partially burned fuel molecules. And oxides of nitrogen are formed when oxygen and nitrogen molecules combine during the pressure of combustion.
Transmission Controlled Spark (TCS) used transmission gear position andcoolant temperature to regulate vacuum advance and reduce hydrocarbons.Vacuum passed through an electric vacuum solenoid located on the driverside of the intake manifold, like those shown above. The first-typesystem used a timer housed in black plastic to disable the solenoid(top) after a specific amount of time for maximum throttle response. Thesecond-type system did not (bottom).
Though there are different methods of controlling each type of pollutant, carbon monoxide is not quite as manageable as others. Several factors contribute to the production of carbon monoxide, but engine size, engine efficiency, and carburetor calibration are the most prevalent. In any given combination, larger engines typically consume more fuel and produce more carbon monoxide. But overly rich mixtures can also generate greater amounts. As a result, carburetor mixtures were typically set lean from the factory.
Hydrocarbons are the unburned or partially burned fuel molecules that remain when the flame front stops. The highest amounts are typically generated when mixtures are leanest. This mostly occurs at steady, part-throttle cruise when manifold vacuum usually reaches its highest point. In this state, the carburetor throttle plates are nearly closed, limiting the amount of fuel and air that passes into the engine. Since the fuel and air molecules of a lean mixture are spread farther apart in the combustion chamber, the mixture is less combustible, requiring additional spark lead to maximize combustion efficiency.
Additional spark lead at part-throttle cruise can significantly enhance throttle response and overall engine efficiency, but as engine workload increases, the additional timing could result in damaging detonation. So a load-sensitive, vacuum-operated canister mounted on the distributor was used to add spark lead in conditions when an engine may benefit from it. Since a lean mixture is less combustible, as the flame front spreads across the chamber, it is more easily extinguished by cooler surfaces such as the cylinder wall. This leaves some fuel molecules unburned or partially burned so they are emitted as hydrocarbons. By regulating spark under certain throttle conditions, the amount of unburned hydrocarbons can be controlled.
Oxides of nitrogen are formed when superheated nitrogen and oxygen molecules combine during the extreme heat and pressure of combustion. Because of this, the amount of NOx is directly related to combustion temperature. But since the temperature at both idle and steady, low-speed operation is relatively low, the greatest amounts of NOx are produced under acceleration or heavy load when flame intensity is greatest. By lowering the combustion temperature during these conditions, the amount of NOx generated can be reduced.
THE BASICS OF THE '73 EMISSIONS CONTROL SYSTEM
Since 1968, the federal Clean Air Act has required that auto manufacturers control hydrocarbon emissions. Because the highest levels are typically found at part-throttle cruise, a Transmission Controlled Spark (TCS) system was developed to regulate vacuum advance in this situation. A series of switches and sensors on the engine and transmission were used to limit vacuum advance to specific transmission gear positions and engine coolant temperatures. However, a thermal override switch allowed for full vacuum advance to aid engine operation any time the coolant temperature was above or below a preset operating range.
Federal oxides of nitrogen standards for 1973 forced Pontiac engineers to explore ways of limiting NOx. Since the amount of NOx produced is directly proportional to combustion temperature, the most effective form of control at that time was to inhibit proper combustion. Inefficient combustion within the chamber degrades flame intensity, reducing the combustion temperature. Since exhaust gas is typically noncombustible, by recirculating metered amounts back into the intake manifold, combustion efficiency can be reduced and the production of NOx controlled. It was from this that the Exhaust Gas Recirculation (EGR) system was developed.
An EGR system was installed on every Pontiac V-8 from 1973 forward. A typical system consisted of a modified intake manifold that allowed exhaust gas passing through the crossover under the carburetor to circulate back into the intake plenum and mix with the incoming intake charge. A vacuum-actuated valve mounted on the manifold regulated the amount of exhaust gas that circulated. The vacuum source for the valve was a special timed port on the carburetor that denied vacuum whenever the throttle plates were closed. To regulate EGR function, vacuum from the carburetor passed through a switch that limited operation to specific conditions.
The First '73 System
The emission control system used from the start of production in the '73 model year was fairly complex, and Pontiac engineers knew that part-throttle performance would ultimately suffer from it. To prevent any major loss, the system was designed to meet the exact letter of the law. By loosely interpreting the compliance requirements, it appeared as if the engine must be compliant only for the duration of the testing period. So engineers used a time-delay solenoid that allowed the system to operate long enough to pass emissions testing but provide maximum throttle response and efficiency thereafter.
In an attempt to reduce oxides of nitrogen, Pontiac installed avacuum-actuated EGR valve that circulated metered amounts of exhaust gasinto the intake manifold. The first-type system (top) used an electricsolenoid to control vacuum to the EGR valve. EGR operated in conjunctionwith TCS to provide maximum part-throttle performance. This solenoid waseliminated in the second-type system and a thermostatic vacuum switchplumbed into the coolant crossover regulated vacuum (bottom).
Conditions in which the systems operated were limited to a specific range. When coolant temperature was below 71 degrees, TCS allowed full vacuum advance to assist cold engine operation, and EGR was inactive. Normal operation began once coolant temperature reached 125 degrees. TCS then deniedvacuum advance any time the transmission was in first gear or for up to 55 seconds after an upshift from First. Under the same condition, the EGR system operated exactly opposite where EGR was present any time the engine was in First gear or for up to 55 seconds after an upshift.
Together, TCS and EGR controlled the amount of hydrocarbons and NOx emitted during testing or for a short period under normal acceleration. Once the time delay solenoid timed out, however, TCS allowed for full vacuum advance for maximum throttle response, while a second switch connected to TCS denied vacuum to the EGR valve for maximum combustion efficiency. The entire system would reset whenever the transmission dropped back into First gear and the timed process began again.
In the technical sense, the emissions system was compliant. But it did not take long for the EPA to discover that the system was not fully so. Pontiac was given a limited amount of time to find a solution and recertifyevery engine within each respective group or engine production would come to a halt. Caught trying to provide the best running car yet comply with loosely interpreted testing requirements, Pontiac engineers worked diligently to produce a fully compliant alternative to the time-delay system.