Most muffler tubing in 2.25-inch od is 0.065-inch wall thickness. You need to subtract the 0.130 inches of wall thickness, which brings you down to an id of 2.120 inches. This gives you a true area of the inside of the tube at 3.53 in2. You mentioned using 1.750-inch tubing for your quad tailpipes. Most 1.750-inch tubing you will find will be 18-gauge wall thickness. This has a nominal wall thickness of 0.049 inch. If we do the area calculation on the 18-gauge 1.750-inch tubing, you come out with a cross-sectional area of 2.14 in2. When you double the area (because of the dual tubes), you have an area of 4.28 in2. As you can see, this is well over the single pipe feeding your dual tubes. If you go with 1.625 × 0.049-inch tubing for your duals, you have a combined area of 3.66 in2. This would support your 2.25-inch feeder tailpipe with no problem.
Building unique cars is what this hobby is all about. Making your car different from the other guy’s or gal’s is where the fun is. Enjoy your peashooters! CHP
Technical questions for Kevin McClelland can be sent to him at firstname.lastname@example.org.