I just recently installed NPP-in-a-box and a set of NPP mufflers on my 2010 Grand Sport. Everything works correctly with the remote control. When the car first starts up, the butterflies are open, then quickly close. So, as far as I can tell, everything is installed and functioning correctly.

However, when I drive and the RPMs are above 3500, it doesn't seem like the butterflies are opening. I have not really FLOORED it when doing this, and I'm wondering if you have to have a certain amount of throttle in order for the NPP to open.

 

If your referring to the NPP retro kit for non NPP cars, the mufflers butterflies only open/close with the remote. You can open the box up and switch open at start or closed at start.

 

Only works open or closed.

 

Only the factory installed system opens at that preset RPM. The NPP-in a box is a control that is either open or closed, totally independent of RPM.

 

As a side note .... I had the factory installed NPP on my 2011 GS.

I don't think the NPP valves open at a preset RPM.

On my car, the NPP butterflies would only open when you would get on the throttle about half-way or more (needed a good load on the engine).

Even if you downshifted into lets say 3rd gear while cruising at 65 mph ... Yes, the RPM would then be high, but the valves still would not open because there was no load on the engine.

In fact, you could be at a dead stop in neutral and hold the throttle down to achieve a steady 5,000 RPM, and the valve still would not open ... (no load on the engine)

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I had 3 C6s over the years with NPP. Since they are a cool feature I usually show people how they operate by accelerating in manual mode at moderate power and the valves open around 3500 RPM every time. Once the engine is warm we do some max accel runs where it seems like they are on all the time because the first time to 3500 rpm takes very little time. You won't notice it as much coasting above 3500 rpm but they are open.

 

 

Interesting. Thank you everybody for your responses. I guess I was under the impression that the NPP-in-a-box mimicked the factory setup. Still love the NPP.

 

 

 

Not my info, just what I've compiled and passing it along....

NPP is the Regular Production Option (RPO) code for the Dual Mode Exhaust Option first offered to non-Z06 Corvettes in 2008 as an $1195 option. LS3 engines equipped with NPP get a bump in horsepower rating from 430 to 436. Modeled 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.

NPP Exhaust Valves

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.

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

 

I guess the software in which ever module is responsible for opening the solenoid in the factory NPP is absent in the NPP in a box system. Probably could be added if some one is **** enough.

OP - Don't sweat it, most of us with factory NPP over ride the software and leave the system open ALL THE TIME.

I found the opening and closing of the factory system rather distracting while playing with the throttle when sliding the car around on tight mountain roads, my preferred environment.

 

That's a very good point about it being distracting. I've just been opening it whenever I feel like and that's been fun enough. I guess I was figuring that the lack of vacuum under heavier throttle would cause the butterflies to open.

Now I just have to figure if the car actually feels faster when it's open. Seat of the pants meter seems to say 'yes'.

 

Yes, you can have the valves open by lack of vacuum, especially on long mountain grades where it's not steep enough to require a downshift.