Early A, 273, 340 manfolds dual exhaust.
That chart doesn't seem to compensate for gas expansion/contraction?? When the exhaust gas leaves the exhaust ports it very, very hot and has greatly expanded at high temperature...as the gas works its' way to back of the system it cools and contracts. As it contracts it takes up less space and requires less pipe diameter. You can hold your hand at the tail pipe when the engine is running but there's no way you could hold your hand at the exhaust port with the manifolds removed.
