Schilke 45B vs Schilke 51D
The Schilke 51D has a wider cup diameter, the Schilke 51D has a larger throat opening.
Cup Diameter
The Schilke 51D has a cup diameter of 25.55 mm / 1.0059 in compared to 24.30 mm / 0.9567 in on the Schilke 45B — a difference of 1.25 mm / 0.0492 in. A wider cup generally produces a fuller, darker tone but requires more air support.
Cup Depth
Both mouthpieces share a medium cup depth, so the sound character from depth alone will be very similar.
Throat Diameter
The Schilke 51D has a wider throat (7.04 mm / 0.2772 in vs 5.95 mm / 0.2343 in). A wider throat allows more air through, increasing volume and projection but reducing resistance.
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.