The ecm can adjust to the smaller diameter when it notices that the o2 reads rich, but it shouldnt have to and the pipe should not of been smaller then stock. Bigger maybe, but definetly not smaller since it now limits how much air can pass through the smaller pipe compared to a 3" pipe.

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1993 talon tsi awd - stock on the outside, modded on the inside
2007 solstice gxp - Solo Performance 3" mach exhaust + hiflow cat - dyno sheet