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      09-05-2011, 10:52 AM   #13
BimmerRob08
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Drives: 2008 M3, Space Gray
Join Date: Mar 2008
Location: KIKR (ABQ), NM

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Normal, I sometimes can detect it with the stock exhaust system. With my EI X-pipe (HFC), it's more pronounced. That popping sound is, I believe, a small void in constant exhaust gases moving through the pipes from when the engine rev's down, then back up. I hear it more as after a downshift and rpm's drop. It's not back fire though. Back fire is unburned gases re-lighting and being expelled.