Turbo, I have spent a great amount of time (years) researching the development of the SD-455. I have read many different stories from when these engines were new all the way to what’s published today. The most disappointing part of it all is how widely varied the stories are and the general inaccuracy amongst many of them. Next time I speak with Pete, I’ll make it a point to ask him where he found that information. Other than his book, I’ve never seen the 485640 listed as anything other than a ’72 HO application.
I have had the opportunity to speak with two of the key engineers who were responsible for the development and production of the SD-455. I specifically asked Skip McCully about the SD-455 aluminum manifold and was told that preliminary SD-455 dyno testing included a standard production RAIV aluminum manifold that was modified to accept both a Holley and QJ carburetors and pair of ported #614 RAIV heads. Once they started casting pre-production parts that were very close to the final production pieces, he said the aluminum manifold was canceled and a cast iron unit went in its place because of poor cold-start emissions and drivability and the fact that EGR had to be plumbed back into it. Another engineer validated those points on a different occasion.
That being said, we know that no production SD parts were cast earlier than mid-calendar year 1972. The most conclusive point in my eyes is that not a handful of these manifolds exist, there are several out there and they all have July and August ’71 cast dates (so far). I’ve got a 488945 right here with a Dec 1971 cast date. Even if these pieces were intended to be used on the SD-455, they cast several of them early in the ’72 model year—well before the heads or blocks, which just doesn’t make sense.
You said that people recall knowing this was available from the Pontiac parts counter. I’ve never heard this story from anyone nor do any of the parts manuals list the 485640 as anything other than the ’72 455 HO. I am convinced that it is nothing more than a early-72 model year 455 HO intake. I’ve never had one side-by-side to a 488945 but looking at several pictures I’ve archived, there is nothing much different between the two. Something had to change for the PN to change, but what?! Research continues…
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