This is an exchange on a Corvette newsgroup:
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w/Corvette.

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Actually it's not the same engine but it does share a similar block,
may even be the same block now. To answer your question, yes, there
are times during the casting process that porosity can occur. Many
reasons, cold mould, incorrect melt temperature, poor core material
adhesion, impure metal, impure core sand or mix, bad wax, missing glue
in core buildup, slow pour, vent obstruction, sprue restriction,
improper riser heights, cold shut, and core shift plus a few I forgot.
Bound to happen in mass production from time to time and I've seen 2
in my life time of which one was in cast iron block the other in an
'04 LS1. The LS1 is owned by a retired employee of GM from the foundry
that cast the block. How sad is that?

By the way the LS2 started out life as a truck block.