I'm not 100% sure, but I recall something about the 28" H20 pressure standard for flowing heads coming from Smokey Yunich, and 28" being chosen because it was fairly close to the 'average' pressure differential actually seen by the port (28" H20 is about 14" Hg, I think). Perhaps someone else remembers better.
The tradeoff is, of course, that it takes a lot of power to generate 28" H20 pressure across a port that flows well at high valve lifts, and not all flowbenches have that kind of power. Conversions from one test pressure to another are available, but not as accurate as measuring at the desired pressure to begin with.
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