Bach 8½A vs Kelly 7C
The Kelly 7C has a wider cup diameter, the Bach 8½A features a deeper cup.
Cup Diameter
The Kelly 7C has a cup diameter of 16.51 mm / 0.6500 in compared to 16.10 mm / 0.6339 in on the Bach 8½A — a difference of 0.41 mm / 0.0161 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Bach 8½A has a very deep cup while the Kelly 7C has a medium cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
Throat diameters are essentially identical at 3.66 mm / 0.1441 in, so resistance feel will be comparable.
Material
The Bach 8½A is described as unspecified, and the Kelly 7C as Lexan polycarbonate. Material affects mass, warmth, and how quickly the piece responds.
Which is better?
Neither mouthpiece clearly dominates the other — the right choice depends on your embouchure, playing style, and the sound you're after. Try both if possible.