[frank.g]

All SD's built for production (the ones with an "X" in their VIN) were after the emission change over date, so all have the later emission system. Some intake manifolds were originally manufactured for the earlier system, and they were re-tapped and stamped with the later part number. Occasionally, you can see the double stamp on some of these manifolds.

[Alvin]

Originally Posted by frank.g

All SD's built for production (the ones with an "X" in their VIN) were after the emission change over date, so all have the later emission system. Some intake manifolds were originally manufactured for the earlier system, and they were re-tapped and stamped with the later part number. Occasionally, you can see the double stamp on some of these manifolds.

Cool thx. Reason for my Q here:

The next time you see a '73 Pontiac model like Steve Schappaugh's SD-455 Trans Am, check under the hood and take a close look at the emissions control system. Many fail to realize that two separate systems were used that year. Although each had the same basic components and functioned similarly, the early system fell victim to the EPA's watchful eye. Continue on as we discuss the first and second types and explore why changes occurred. Read more: http://www.highperformancepontiac.co...ntrol_systems/

[frank.g]

Interesting observation. Steve Schappaugh has had many SD's over the years, and it has been a long while since I have seen any of his, so I don't know to which car the observation is referring to. It's an open ended statement, so I can't tell from it whether Steve has a pre-production car, whether he retro fitted an early system to his car (unlikely as that would be) or whether he got the one that slipped out the door. I have learned thru the years with GM to never say "never". But, I will say, for all my years that I edited the SD Registry, none of the cars registered were built prior to the cutoff date that the emission system switch occurred.

[johnta1]

That blurb from the text under the picture was referring to the fact that the SD was different than the early engines I think?

Here is what was in the article later on:

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.

The thread on Micheal's 'test' TA probably had the 1st version egr.