Y–Pipe exhaust systems are in use on cars with six, eight and more cylinders when only one tailpipe is desired. This kind of exhaust system isn’t as efficient as a dual exhaust system. It’s because the single exhaust pipe can’t provide the same amount of flow and low resistance to flow that two separate pipes can provide to the tailpipe. There are many different designs of Y-pipe exhaust systems. In some, the Y-pipe looks exactly like a capital letter Y. In some the pipe more closely resembles a T. Most are designed to connect together right after the exhaust manifolds and before the catalytic converter. An example of one can be seen here.
For more info, see How Exhaust Systems Add Power