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what's the size of the stock pipes on the ex? i have a 2.5 inch exhaust on it, but the piping from my manifold to the cat is still stock.. reason i want to know is b/c i just got my megan header and im thinkin of just boltin it up and keepin the stock connecting pipe there.. help.

 
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I belive from manifold to inlet of cat is 2 inch, from outlet of cat to muffler is 1 5/8 You have 2.5 piping where down pipe, mid pipe or from axle back?