I wouldn't think that gave you any more than 2-3hp, but it does give the car a throatier growl....and hey, it was free.
Your car is an S13.
May I suggest you replace the exhaust piping from the cat back with a pre-made aftermarket unit, or a custom one from a muffler shop...mandrel bent, of course. Just a muffler isn't gonna do a whole lot.
BTW, welcome. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/cool.gif" border="0" valign="absmiddle" alt='
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