Kelly 7C vs Marcinkiewicz 2S
The Marcinkiewicz 2S has a wider cup diameter, the Kelly 7C features a deeper cup, the Marcinkiewicz 2S has a larger throat opening.
Cup Diameter
The Marcinkiewicz 2S has a cup diameter of 17.27 mm / 0.6799 in compared to 16.51 mm / 0.6500 in on the Kelly 7C — a difference of 0.76 mm / 0.0299 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
The Kelly 7C has a medium cup while the Marcinkiewicz 2S has a shallow cup. Deeper cups favor a darker, richer sound; shallower cups provide more brightness and easier upper register.
Throat Diameter
The Marcinkiewicz 2S has a wider throat (3.73 mm / 0.1469 in vs 3.66 mm / 0.1441 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
Material
The Kelly 7C is described as Lexan polycarbonate, and the Marcinkiewicz 2S as unspecified. 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.