The GT spec, if I'm not mistaken is made for race use and is made of titanium to preserve weight. Also, the smaller diameter of the GT spec is supposed to leave you with a bit more torque. The N1 muffler end is made of stainless steel but I'm not sure about the pipe. From what I hear, the N1 Dual is supposed to be good because the smaller twin mufflers will keep your back pressure descent and with full open throttle it is able to let the engine breathe good. I personally like the look of the GT spec better though <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/wink.gif" border="0" valign="absmiddle" alt='
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