It does not affect emissions, only sound. The change in volume is not extreme in any way, just tasteful and appropriate.
As thoroughly explained in the thread posted by silver-S above, by making a hole in the outlet pipe into the middle equalization chamber of the muffler, you are allowing some of the exhaust and sound waves to bypass the other equalization chambers. It is not drastic and does essentially the same the thing as the Porsche Sport Exhaust, but to a lesser degree.
Take a look at the pictures of the PSE in
this thread and you'll see what I mean. The crios mod will not be as loud as the PSE because the bypass pipes on it compared to the bypass holes we are making into the outlet pipes are much larger.
It's a $50 muffler...and a free mod with very nice results.
EssexPorsche doesn't like us doing the mod for some reason
