My 2024 Stingray with NPP exhaust note is changing when I am on the throttle. It is sounding better for no apparent reason. Maybe as the engine is breaking in? Not quite sure how to describe the change other than more aggressive, but I like it. The note when it is not under load is the same but when it is making power there is a very pronounced change. Has anyone else noticed this happening?

 

I put 8000 miles on my previous 23 SR w/NPP and never noticed a change. I drove almost 100% in Track mode on the exhaust.

 

I believe there's a slight change that may not be noticeable to others.....especially during the breaking In period.

 

Just a thought, could the change in sound be the difference between V8 and V4 mode?
When in V8, accelerating and in certain modes, the AFM and NPP valves are open.
When in V4 mode, the AFM valves are closed and the NPP valves open and close depending on the settings for the mode you are in.

 

Bingo

 

There may be a slight change in the intensity of the pop at shift points, as the car warms up. Also, I think maybe that pop at shift points may change with fuel blend. As for NPP operation, it works like this.
It has AFM valves that close in V4 mode, but are open all other times. And it has tailpipe flow control valves that open or close depending on various factors.

1. The tailpipe flow control valves will close when the car is in V4 mode (as noted in above posts), regardless of the engine sound setting.
2. It the engine sound setting is stealth - the tailpipe flow control valves will be closed all the time.
3. If the engine sound setting is tour, the tailpipe flow control valves will be closed at idle, and during normal cruising, but will open when hitting the gas.
4. If the engine sound setting is sport, the tailpipe flow control valves will be open at idle and most other times, but will close under some conditions to meet federal bypass standards.
5. If the engine sound setting is track, the tailpipe flow control valves will be open at idle and mots other times, but will close under some conditions to meet federal bypass standards. The conditions may be a little different than sport, but that detail is not provided in the service manual.

During the 500 mile break in period, there is an engine torque limit in first and second gear that is lifted after 500 miles. I think that limit is accomplished with timing. But I think it could result in a different exhaust sound (although I must say I did not notice it).

 

I suspect the NPP valves close in Sport to prevent drone or objectionable noise. In Track they are open other than to meet Federal drive-by requirements.

 

That makes sense. If the Track setting of engine sound closes to meet the Federal bypass standards, and it closes in additional circumstances in the Sport setting of engine sound, then those instances must be for some other reason than the Federal bypass standards. The detail is not explained in the Service Manual. But from experimentation and forum posts, there are certain combinations of speed, rpm, and throttle angle where they may close in sport, but not in track. The reason you suggest for that makes sense to me.

In my HTC, I often cruise on back roads at a leisurely pace with the top down of course - and even in Tour I sometimes feel the the exhaust note is annoying, and more so in Sport or Track settings of engine sound. Not saying I don't like it during spirited driving. In fact, as many have said, I wish it was louder (and deeper) in the Sport of Track settings. I think I would probably like the one of the Borla systems - which are not only louder in those settings, but also have more lower frequency tones.

 

I don't remember on my Stingray as I ran it in Track for the NPP almost 100%, but on the Z06 there is a definite difference between Sport and Track. On the Z Sport is closed a little more at lower RPM (somewhere around 2500?), which cuts out a little more drone while cruising.

 

The Stingray valves are binary - open or closed. I believe that the Z06 valves are variable - they can have positions in between both open and closed. The actuators are not different - but they can be controlled differently.

A look at the part numbers for Z06 and Stingray on gmpartsnow shows that they are the same

Exhaust Control Valve Actuator - GM (85520525)

2021-2025 Chevrolet Corvette - 85520525

But....the schematics in the 2020 Service Manual show that the control signal is pulse width modulated, which in simple terms means the control system is in place to allow variable positions for the actuator.

 

What exhaust mode are you in (Track, Sport, Tour)?

In Tour or Sport, the valves are closed much of the time but open up when you accelerate. They're closed more of the time in Tour than Sport, and open sooner in response to the pedal in Sport than in Tour. In Track they're open most of the time, but even in that mode they close at steady throttle when speeds are about 35mph or lower (to pass federal noise regs).

Also, except when the powertrain is set to Track, the car will to into V4 mode at steady speeds (exactly when depends on the driving mode), which closes the AFM valves and quiets things down a lot, but as soon as you accelerate a bit the engine goes into C8 mode and the AFM valves open.

 

Here is the wording from the Service Manual. It tracks what I said in post 6 and is consistent with what others said: The settings refer to the setting of the Engine Sound which also defaults to the Driver Mode (Weather, Tour, Sport, or Track) , but can also be set as desired in MyMode or Z-mode. As noted above, if you actually use Track Driver Mode, or if you set the Engine/Shift parameter (not to be confused with the Engine Sound paramter) to Track in Z-mode, or if you use Manual Shift Mode, then AFM is prevented.

"Note:
You may notice an exhaust tone change due to the AFM system activation, or during certain driving conditions to meet Federal noise pass-by
regulations. Review the cylinder deactivation exhaust flow control system operational modes listed below for additional details.
1. Weather/Stealth Mode: Exhaust tailpipe valves will be closed at all times when the engine is running.
2. Tour Mode: Exhaust tailpipe valves are closed when the engine is idling and during normal/non-aggressive driving. Valves open during
aggressive driving.
3. Sport Mode: Exhaust tailpipe valves are open when the engine is idling and during most driving scenarios. See note above.
4. Track Mode: Exhaust tailpipe valves are open at all times. See note above." [my note added: they will still close if AFM kicks in, which could happen in Z-mode if engine/shift is not Track, or MyMode which defaults engine/shift to Tour. And they will still close under some conditions to meet Federal Bypass standards).

 

One small correction. I don't think the NPP valves close in track mode even if AFM kicks in. The completely separate AFM valves close in that case, which also drastically quiets the exhaust.

 

Thanks. Sometimes the car has not read the Owner's Manual or the Service Manual and so it does not behave as described by those documents. But my statement that the tailpipe flow control valves always close in AFM (which would include even if engine sound is in Track mode), is based on the following description from the Service Manual. Other than that, I have never attempted to determine its accuracy.

"Cylinder Deactivation Exhaust Flow Control Valve System
As a means to improve fuel economy, the engine management systems in some vehicle applications include provisions to deactivate half of the
engine cylinders under certain operating conditions. For example, under a light load. The cylinder deactivation exhaust flow control valve
subsystem is used to mitigate the impact that powertrain engine cylinder deactivation (active fuel management) has on the quality of the exhaust
sound. When the active fuel management system is in operation, the cylinder deactivation exhaust flow control valve subsystem takes over control
of the exhaust tailpipe flow control system. When active fuel management is operating, the exhaust tailpipe flow control valves are commanded shut
resulting in a quieter exhaust note until the active fuel management system deactivates with increased throttle demand at which time the exhaust
tailpipe flow control returns to the operational status determined by the vehicle specific mode that is being used."

 

Interesting. The NPP valves definitely didn't work that way in the C7, (I had a homemade mild to wild that let me keep the NPP valves open, or let the system control them, and there was no difference in sound in track mode either way when AFM kicked in. So it certainly sounded like the NPP valves stayed open in V4 mode.) but I don't know for certain whether the C8 works the same way, and it would be harder to test than in the C7.

And I wish GM would just call them AFM and NPP valves, instead of long strings of words that sound almost the same. .