Title says it all.

 

They don't. I suppose you probably would like an explanation then... All they do is add additional air at more than off-idle throttle settings. Unless you emmissions test at WOT, where they should be running slightly rich, it won't affect the idle or roller tests.

 

Ahh, I'm so happy to hear that... emissions are evil.

 

This is assuming your setup is properly tuned:

SCs shouldn't affect emissions, or at least very minimally.

Turbos however, take some of the heat from the exahust and use it to spool up. So what? you say. Well, that heat is what makes your cat converters work. Basically your startup emissions will be higher until the cats do reach operating temp. This is easily gotten around for an emissions test though, by driving around for a while beforehand.