what size engine?
if its a 2.4L i would go with the 2.5 inch.
if its 2.2L i would go 2.25 inch.
and exhaust isnt a huge deal with exhaust, but just make sure you do the header and cat (if you already havent) before you nitrous anything.
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I don't seem to think so. I run 2.25" and I didn't even have a header when I ran 13.3 @105 mph and put down 231 WHP.
<img src=http://ourworld.cs.com/jwithspray/Nitrosig13303.JPG>
bigger piping dont affect it as much as it would boost
1989 Turbo Trans Am #82, 2007 Cobalt SS G85