Early A, 273, 340 manfolds dual exhaust.

OK, that takes care of that. The numbers are simply mathematical calculations. I don't disagree with the comment above about exhaust gas cooling off as it speeds toward the tail end. The factory exhaust systems use tail pipes that are smaller than the exhaust pipes for just that reason, although most aftermarket systems use the same diameter all the way through. Still, the chart gives you some idea about flow potential for different diameter pipes. Not everyone realizes, for example, that increasing pipe diameter from 2 inch to 2 1/4 inch increases flow potential by a third - including some of the self-styled "experts" working at muffler shops.