The stock exhaust manifold has a lot more material in it as long as you keep the opening centered and know what you're doing with a grinder. Which I do not. But here is what my friend Freddie did for me on one manifold and the one I have currently has been opened up to 2.5" and we feel sure there's room for 3" although that would be getting a bit thin. I'm just crappy with a hand grinder, but thankfully Freddie is a legend genius with one.
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