the pipe is called the manifold vacuum control tube its connected to the air pump and pumps air into the exhaust system..its part of the air injection system read info. below.
The air injection system is designed to introduce outside air into the exhaust stream to assist in burning the gases produced by the engine. Air is injected into the exhaust system at one of several locations including the cylinder head, exhaust manifold, or directly into the catalytic converter. Outside air is drawn by an air pump (sometimes referred to as a smog pump), a pulse air valve, or a reed valve. The air pump is run by a fan/accessory belt on the front of the engine or, in some cases, a small electric motor. The pulse air and reed valves work by sensing the drop in manifold vacuum produced by an exhaust valve closing. Each time the drop in vacuum is noticed, a "puff" of fresh air is pulled into the exhaust stream.
Some related parts of an air injection system can include, but are not limited to: air pump, check valve(s), gulp valve, diverter valve, air supply tube, and/or an air manifold.