yeah the increase on an SPI isn't a whole lot with the cat back. get a header & cat back, now you're talking about the SPI breathing. 1hotfocus, when your car is struggling with MPG b/c of parts that need to be replaced, you're gonna see difficulty in seeing HP gains with added performance parts.
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Mods - FS Werks Cool-Flo,CFM Crankshaft pulley/WP Kit, obx header/flex, FS Werks Stealth Flo Muffler with 2.75" straight pipe, & Tom's Tune SCT. FRPP kit & svt front sway coming soon!