The AIV Fitting on the Nissan 300ZX'S is a fitting that is welded into the Down Pipe that attaches to a Metal Tube that screws into the fitting.

From the manual: “The air injection valve sends secondary air to the exhaust manifold, utilizing a vacuum caused by exhaust pulsation in the exhaust manifold. When the exhaust pressure is below atmospheric pressure (negative pressure), secondary air is sent to the exhaust manifold. When the exhaust pressure is above atmospheric pressure, the reed valves prevent secondary air from being sent back to the air cleaner.” The AIV system allows air from just before the air flow meter (after the air filter) to be sucked into the exhaust if and when the exhaust is at a negative pressure. The AIV can be turned off by the electronic control module (computer) using a vacuum signal to open/close the valve. Reed valves act as a check valve to prevent any flow from the exhaust to the air intake. The AIV purpose is to add air to the exhaust when the engine is cold to help bring the catalytic converter up to operating conditions faster. It also comes on during deceleration just to purge any moisture that may have collected in the valve. The AIV system is inactive at normal operating temperature (except when decelerating).