Tech Service Bulletin for 1969 Potential Overheating in 396 and 427 Engines - Page 1 of 2.
In previous threads and discussions, others (in particular JohnZ) have pointed out the benefit of using intake manifold vacuum instead of ported carburetor vacuum as the source for the distributor vacuum advance. One issue with lack of proper vacuum advance, is the potential for the engine to run hot at idle or in city traffic. Refer to the following for JohnZ's disccussion: http://www.camaros.org/forum/index.php?topic=64.0
In this Tech Service Bulletin, you will note that the bulletin states that: "Full vacuum advance at idle must be provided". On the second page, the drawing shows one of many possible ways to do this. The service bulletin is telling us the same thing that JohnZ has pointed out. Full manifold vacuum as the source for the distributor vacuum advance is a desirable thing.