another reason
Pistons too far below zero deck height. making it more likely to detonate. For more on this read..
Jim Hands compression article
I found this article by Jim Hand especially informative and reading it may give you insight.
http://www.boyleworks.com/ta400/psp/compression2.html
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1967 Firebird 400 conv T400 3.08, 1989 Formula 350 700r4 3.27