more food for thought: if you have 4 header pipes at 1.75 inches internal diameter each, thats a total cross section area area of 9.6 square inches. A 2 1/2 inch ID muffler = 4.9 sq in, (no wonder there is a flow restriction) whereas a 3 inch ID muffler = 7.1 sq in, a 44% increase in cross section area, but still much less than the 4 header pipes. Although there is probably a bunch of fluid dynamics math thats way over my head to add to this...
