What are General Motors Nuclear Propulsion Package (NPP) Valves? How do they work?
The Nuclear Propulsion Package (NPP) option is a vacuum controlled valve on the factory exhaust which is controlled by the ECU. It will open and close based on RPM and throttle position.
The valves will be open for the first two seconds that the car is started. It requires time for the vacuum pressure to build to close the valves. After that The operation of the NPP is normally closed, open on demand. Have someone stand behind the car and bring up the RPM to 3,500 and you should see the valves open. The butterfly operation is tied to the throttle position sensor and the RPM. It uses the throttle position to determine driver intent. Under normal operation the valves work as follows:
Below 30% throttle and 3,500 rpm the valves are closed.
Above 30% throttle and 3,500 rpm the valves will open.
Between 30% throttle and 80% throttle and above 3,500 rpm the valves are open.
Above 80% throttle and above 2,800 rpm the valves are open.
Below 30% throttle and 3,500 rpm the valves are closed.
Above 30% throttle and 3,500 rpm the valves will open.
Between 30% throttle and 80% throttle and above 3,500 rpm the valves are open.
Above 80% throttle and above 2,800 rpm the valves are open.
Dual Mode Exhaust is a feature that allows your GM vehicle to stay within 'driveby noise' compliance. At 3500 RPM the valves auto bypass the factory mufflers to form a temporary straight back system which allows increased airflow at higher speeds.
So what is the difference between NPP and AFM Valves?
AFM valves are pre-muffler and shut when the engine enters V4 mode (the flaps have a hole in them to allow reduced exhaust flow).
NPP valves are post-muffler on the two outboard pipes on only the performance exhaust. When these close the exhaust quiets down.
The following video demonstrates the NPP valve functions on a stock Chevrolet Corvette C8 Exhaust system
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