The factory at Shipley tested all engines on a dynometer from as early as 1927, and possibly earlier, but the earliest photo of the dynometer that I have shows straight-back crankcase Flyer engines waiting for test. It is safe to assume that they experimented with different sizes/volumes of coffeepot, and tailpipe diameters, so the standard one is probably the correct and optimum size. It is also worth noting that at least one of the 1929 TT bikes had a coffeepot system, presumably because the rider preferred the torque characteristics over those afforded by the 2″ L/H siamesed pipe system, ie. more bottom end grunt.
My 1927 Flyer with coffeepot and two smallbore tailpipes, and no silencers, really was LOUD, and I would have to kill the engine and freewheel past horses. (There used to be a racing stables in the village, and racehorses are even more skittish than other horses! The stables was owned by a man with the unusual name of Chuck Spares! One of his stablelads was Willy Carson’s son, and it was his steed that put hoofprints into the door of one of my vans. If it had kicked out at me on the ’27 Flyer I might not still be here posting this message….).