Exactly, and it's illegal to have on a street driven car here in NJ. Also having pretty much no back pressure might cause some issues.
http://www.driftworks.com/shop/media...stvalve1_1.jpg
I have seen ones that go on the very end of the exhaust like on the newer corvettes where there are 4 pipes but only 2 are open for normal driving then under higher pressure a valve opens up the other 2 pipes to increase flow.