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i have heard that its good to make the diamete of the exhaust piping larger to enhance airflow. i was wondering if it is worth it to change my piping from the 2.5" stock, i believe, to 3" piping. thanks
 

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first of all, you DON'T have 2.5" stock.
2nd of all, 2.5" is too big for your car let alone 3"

I think your system is either 1.75" or 2.0" stock and the optimum size for the QR25 is 2.25"

If you have a non-turbo car, you want to make it big as possible to reduce the backpressure, but at the same time, going too large will make the flow "choke" kinda like sipping a glass of water through a 3" PVC pipe rather than a normal straw.

I recommend you read www.nissanperformancemag.com's last 3 issues titled "revenge of the nerds" it talks about how echaust systems work.
 
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