What a mechanic once told me when I had him install true duals is that there was no need to go larger than 3". and an xpipe or something similar will help balance out the back pressure(at least that is how I understand it), so they are a good thing. I had a magnaflow muffler with an internal xpipe and it worked and sounded really good. not too loud at idle nor low speeds, but it sounded good on the highway too
