Emission Impossible

The Second '73 System

In response to federal demand, a revised emissions control system was incorporated into engine production on March 15, 1973. The new system contained some drastic changes, many of which are detailed in Pontiac Technical Service Bulletin 73-T-21. The bulletin reveals that the changeover occurred at Motor Unit Number 532727. To distinguish engines with the new system from old, they were painted a darker shade of blue and received a new series of engine codes. It goes on to say that approximately 700 engines before said breakpoint were also converted to the new system and repainted the darker hue. Not only would most transmissions reflect the changes, but rear axles on certain applications would also.

Though performing the same basic function, the operational characteristics of the second-type emissions system were different than the first. It no longer allowed for TCS and EGR operation at opposite times--they now operated independently. A thermostatic vacuum switch in the intake manifold replaced the electric switch and allowed EGR any time coolant temperature was above 95 degrees. TCS, however, allowed for vacuum advance only when coolant temperature was above 125 degrees and the transmission was in high gear. Vacuum advance was limited to just 20 seconds at cold startup to assist idle.

Skip McCully's View

Over the years, different publications have printed conflicting information about why the emissions system change occurred. And though some reports may have been fairly accurate, trying to determine just which story was closest to the truth has plagued researchers. So we approached former Pontiac engineer Skip McCully and asked for his comments on the matter. Though he was typically involved with Special Projects, as with all other Pontiac engineers, McCully was kept abreast of the current situations though periodic engineering team meetings.

McCully conveyed that it was common practice for the engineering team to focus on maximum performance, street manners, and economy before addressing emissions. It seemed, however, that increasing federal emissions standards changed the order of priority to meet emissions first, with performance and economy coming from what was left. In an intensely competitive new-car market, Pontiac was not unlike any other auto manufacturer--they were all searching for some type of compliance loophole without compromising performance.

In Pontiac's instance, the first-type emissions control system was engineered to barely meet the requirements needed for emission certification. But other manufacturers found their own ways of partially disabling the emissions system. Once the EPA caught on, it quickly revised the laws to remove any chance of misinterpretation and required that each engine be fully compliant at all times. It was these types of situations that may have negatively affected the manufacturers' future credibility with the EPA.

While Pontiac's second-type system was entirely compliant, it negatively affected overall engine performance. According to McCully, many sources emphasized the elimination of the EGR solenoid, but vacuum advance suffered just as much, if not slightly more. Though EGR reduced engine efficiency, the engineers really wanted to provide vacuum advance whenever possible to maximize throttle response in all conditions. But that was not an option with the second-type system. McCully went on to add that neither the first- nor second-type emissions control systems affected full throttle performance since engines typically produce little to no vacuum under these conditions.

Discussing the emissions system of the Super Duty 455, McCully stated that preproduction SD engines with the 041-spec camshaft such as those found in the engineering test mules and press cars should have had the first-type system. But EPA certification for a cam change delayed the SD's release until late April 1973. Since the changeover to the second-type system occurred prior to that, it is unlikely that any production SD engines received the first-type system.

CONCLUSION

Many manufacturers tried their own ways of dodging the emissions bullet, but the EPA ultimately had the upper hand, and that could not have been proven any better than at Pontiac in 1973. After a closer look at the first- and second-type emissions control systems for 1973, we see that both were designed to provide a specific function--one allowed for technical compliance with maximum part-throttle performance, while the other was a compromise between the two. By 1975, the catalytic converter was not only filtering pollutants, but it seemed as if it was also filtering horsepower.

During that time, it appeared as if low emissions and performance could not coexist. But as engineers began understanding how to hurdle these emissions obstacles that robbed precious horsepower just a few years prior, we began seeing engines like the W72 400ci--the first post- catalytic-converter performance engine designed around low emissions and maximum performance. Since then, we have escalated to fully compliant, 400hp mills that provide 25 mpg and 12-second timeslips. So the next time you see a '73 Pontiac model, take a look at the emissions system and think back about how far compliance has come since then!

Special thanks to both Skip McCully and Tom Vaught for their assistance in preparation of this article.