Are straight pipes illegal in South Carolina?

Actually they are. It is a Federal law that makes it illegal to straight pipe and be driven on roadways. It can be done for offroad racing. South Carolina does not have emissions tests, but Federal law trumps State law. If you do it, just understand if caught it is a $10,000.00 fine. Just like speeding, its all good till you get caught! Are you talking about straight piping the mufflers? That what most are doing.

But I have heard muffler shops do straight pipes.

No