I have read from many sources that it opens at 3500. Mine opens at a certain acceleration rate and appears to be vacuum controlled as would make sense. For example, cruising uphill at about 70 mph in the automatic's top gear, it opens and I do get a drone, especially if there is a dividing wall next to the car. I am just curious if someone knows the facts or maybe the 2011 is different?

 

From forum member Talon90
The exhaust is modelled after the exhaust on the Z06. The exhaust uses a butterfly valve over one of the exhaust exit pipes (two total, one per muffler) to vary the exhaust output. It works similar to exhaust cut-outs popular on the street rods.

The exhaust control is operated by engine vacuum pressure. The system is tied to the pcm and monitors rpm and throttle position to measure "driver intent".

Under normal circumstances the exhaust valve is closed and it routes the exhaust gas out of the other pipe after it passes through the baffle chamber of the exhaust. In this case it is quieter. If the driver accelerates hard the valves will open at 2,800 rpm as long as the driver is above 80% throttle. Between 30% and 80% throttle the exhaust will open during acceleration at 3,500 rpm.

Here is a diagram of the routing of the exhaust gas and operation of the muffler system.

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Here is a graph of the opening and operating characteristics of the exhaust.

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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.

 

That isn't how it's supposed to work. There's a solenoid/1 way valve somewhere in the system that should prevent that from happening unless the ECM commands the flaps open.. which it shouldn't in that situation.

For people who retrofit NPP exhaust and connect the vacuum actuators directly to a vacuum source, that is exactly the behavior they get... so it sounds like something isn't quite right on yours to me (unless NPP was retrofitted later)

 

Thank you for the excellent answer and good diagnosis. My NPP is an original system so I will look further. Not that I mind but it does explain my experience.My 2011 has few miles but the owner ran with the fuse out all the time, probably stuck in place.

The solenoid part appears to be 25794610.

 

On a long upgrade in high gear, I get a drone also on my retrofit system.
Easiest thing to do is to down shift to 5th to get the rpm's up.