| Opening the throat with a drill, without adjusting for any taper in the throat, will turn a tapered throat into a cylindrical one.
As a general rule, the longer the cylindrical section in the throat, the more focus or center to the sound, but the more it tends to condense or shrink the octaves, meaning the lower register goes sharp and the upper register becomes flat. As the cylinder is shortened (either from the backbore or the cup side) the octaves then augment or spread apart--all relative to the other aspects that makeup the mouthpiece.
Considering you can have a professional do this correctly for less than five dollars, you are crazy if you take a black and decker to it in the garage.
-marc |