With the NPP simulators and the valves open 100% of the time, the NPP exhaust is as loud as it can be.
Any mode, any RPM.

 

Do you have to take off the rear bumper to install the simulators ? I watched a couple videos on doing the exhaust and/or simulators and it looks very tight in there.

 

no need to pull the bumper.
0) NPP valve can be seen if you look where the two pipes exit on each side
1) disconnect the battery (optional)
2) remove rear wheel
3) remove stone guard
4) remove rear fender liner screws
4a) pull fender liner away from fender
5) unplug NPP wire
6) plug in simulator
7) zip tie simulator up away from tailpipe
8) place a piece of 3m foil tape over the NPP valve female plug to prevent water/dirt/etc from entering the female plug.
8a) put back together
8b) repeat on the other side
9) connect battery (must index windows, see owners manual)
10) NPP valve will now be open 100% of the time
11) no CEL

 

Thanks for the detailed procedure. Once this is done, is the exhaust sound perceived to be louder? As I understand it, running in z-mode or my mode with the exhaust set to track, the valves may be open all the time anyway. I attached the relevant paragraph from the 2020 Service Manual that says that. It says "open all the time" in the track setting. But it also says see "note above". The "note above" says "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."

 

Wrong, that was said about that way for the C7. It was easilly proved wrong by folks who installed Mild to Wild where they could be held open and toggled off and on. That was done a number of times validating what I found.

My tests of the C7 made it clear.
In Track Mode (where some Service Manual words said they stayed open- it DID NOT. I have the 4 volume set of printed C7 Manuals)
In Track they were closed until about 1700 rpm.
In Touring that was until about 2500 rpm,
There was some influence by throttle position.

To reinforce, in my C6 I had installed a toggle switch which kept them open when wanted. I had it "open most of the time." BUT at 60 to 70 mph had to shut them as there was an objectional Drone.

My perception from tests in Manual Mode the C8 rpm's are similar to the C7. The valves are NOT open at lower rpm's except on starting where they are purposely left open!

Because of the GM wording this was a debate with the C7 until a number of folks who installed Mild to Wild Proved it Was Wrong! Looks to me like they repeated those same words re Track!

 

Thanks Jerry. Most of what you say is about the C7 though. Except for that sentence I quoted above where you talk about tests of the C8. Can you say more about that? I will note that long after start up, and at any time, with the car idling, I can hear the exhaust get louder in track mode than in tour mode, so they clearly are open at idle in track mode, long after start up. I have the HTC, and when driving with the exhaust in track mode, top down, I notice the louder exhaust note at almost all speeds, all the time (like just cruising at steady speed at 40 mph). So, those are two "lower rpm" examples where they are open. And I can switch to tour and it gets quieter right away. I saw a video somewhere, where someone did bypass the valves, and the result "claimed" was little or no increase in sound (relative to just using the track setting). By the way, if does turn out to be that there are still times in the track setting where the valves are closed, the service manual is not necessarily "wrong", because it includes that note, that I mentioned, that seems to specifically caveat the track setting statement about being open all the time. It remains unclear to me at this time.

 

Since no one has reported being able to use Mild to Wild or like I did with my C6 installed a toggle switch to open the NPP valves we don't have a definitive answer.

But yep, you're hearing what I would suspect as in Track, expect the valves open at a lower rpm that in Touring. Since I have a coupe and never remove the roof it's hard to define the rpm as it's so quite compared to my C7 Grand Sport Coupe. Really need to have some folks define a way to use a switch or whatever to manually assure they are open than toggle at some fixed speeds in a specific gear!

One observation I make is at a somewhat step turn in the twisty road I live on. Have a freind who lives near that turn. She accused me of "Gunning" my C7 when I passed their home. Depending on what lower gear I was in, I could be close to that 1700 rpm and It did make more noise! I stated to use a higher gear! When driving my C8 in manual mode if I was close to the ~1700 rpm (+/- can't say it's 1700) it did make a difference at that location! I'm generally in My Mode when I leave my house until I reach the 4 lane divided highway at the end and press Z Mode and it's always in a higher gear at that turn and it's quiet!

As I said, this discussion is similar to what was done with the C7 (my first was an early September 2013 built C7 Z51) until enough folks added a Mild to Wild and validated they could toggle back and forth on the highway at a fixed rpm and the sound difference was obvious.

 

On my C7 there was 2 spots to control exhaust volume. If you ran in track mode, fully open, and you coukd set it up to be fully open all the time in all driving modes in prefernces on computer. I will disclaim though NPP woukd close during car warm up and if rolling at low rpm at times. Slight accelaration or highway driving it was fully open all the time. I am hopeful C* allows you to set exhaust to Track via computer once again.

 

Stock:
The "NOTE" is the final rule. The exhaust valves are NOT open 100% unless you have your foot in it.
With the simulators:
In the cabin, I don't notice much difference from stock.
I am told by other C8 owners that it is louder when I drive by.
With the valves open 100%, it is as loud as it can be.

 

The reason pulling the fuse for opening the valves all the time is no good is because there are TWO circuits running through that fuse (in the C8). I don't know what the second use is for but we probably don't want to eliminate it. I believe this is the same reason the "Mild to Wild" doesn't work correctly in C8's

 

The 3 legged fuse can be replaced with a standard 2 leg fuse in the circuit that needs to remain enabled. I believe someone tried this and still had problems.

 

^^^
Yep, have followed since folks were trying.

Posted this pic of what I had done with my C6 and since the Fuse panel is on the back of the console thought it would be easy to add a simple switch


Yelocarbuff posted this. He and several others tried- no luck.

 

So in my experience in a C7. Leisurely drive on a country road around the lake, some turns and slight hills. 40mph or below. Exhaust in track mode, noticeable exhaust note but low RPM. As I get to a slight hill the car slows a bit as I don't give it more gas, just let it slow a bit and go up the grade. As the car slows the rpm drops a bit and the exhaust suddenly gets quiet. Bugged the **** out of me, fake. Pulled the fuse. Sounds like you cant just do this in the C8. Simulators I guess?

 

Regarding the attempt to bypass the part of the fuse that goes to the NPP and exhaust flow control valves, what does "it did not work" mean? Did it get louder, but throw a code? Or did it not seem to result in opening of the valves?

 

Have to ask those who tried but my recollection is it did not open a close vallve.

 

I wonder what would happen if you both pulled the fuse and disconnected the valves. Disconnecting the valves should keep them open. Pulling the fuse may prevent the CEL. But I suspect if this were the case then the simulators wouldn't exist.

 

Agree with both comments and it is louder outside than you realize sometimes.

I pulled into a gas station the other day in Z Mode and gave it a light rev before shutting down and I scared the heck out of this woman. I apologized to her and said I did not realize it was that loud outside the car because inside it is really quiet.
She was cool, laughed and then said "Nice car BTW"

 

On the C7 if you unplugged the V4 valves or blocked them from activating, it would trip a CEL BUT not the NPP valves. Don't know about the C8 as no one has been successful. However looking at the C8 V4 valve can understand why if it did not return in V8 mode it would be a problem (see pic.)


These are the two exhaust valve butterflies. Note in V4 mode the valve closes and all exhaust must go thorough the two small holes. Logical as with the ~25/35 hp it's probably all it's producing in V4 to overcome the aerodynamic drag and tire rolling resistance drag. As I recall above about 75 mph where those forces start to increase significantly it will be in V8 mode. Also if on a modest incline it switches to V8 mode. So if the valve was closed and it went into V8 mode the exhaust back pressure would cause issues! For NPP if the valve did not open it's like a car without NPP


I'm afraid this discussion will be just like it was with the C7 until a number of folks with Mild to Wild who toggled between open and closed and reported the sound difference we're reading the GM manuals and trying to draw a conclusion. Just checked my 2014 C7 4 volume paper manual and this is what it says. That is what created the discussions early until as I said several with Mild to Wild proved in Track the vales do not open until about 1700 rpm!

Yep as noted it says in Track the valves are opened at all times!

 

The C8 NPP valves are not open 100% of the time.
The only way to have them open 100% of the time with no CEL is to install the NPP simulators as defined in POST#23 above.

C8 service manual: (Be sure to read the Note: after reading 3-SPORT and 4-TRACK)

 

I found, in the 2020 Service Manual, what appears be two sets of exhaust flow control valves that will affect the sound. One pair are between the CATs and the muffler, the other pair are in the tail pipes. The pair between the CATs and muffler are associated with the AFM. The pair in the tail pipe are associated with NPP. According to the 2020 Service Manual, when AFM is active, it will take control of the tailpipe exhaust flow control valves and close them. It also apparently activates the other pair of valves that are between the CATs and the muffler. I have attached the exhaust flow control control valve schematic. I think the actuators associated with AFM are the two components on the lower left. The two on the lower right are the tail pipe actuators (I think). I highlighted the paths providing power to the actuators in red. Note that the fuse being discussed is in the power path only for the tail pipe actuators, meaning pulling it, or replacing it with a switch will not disable the AFM exhaust flow control valves, and so they will continue to open and close if/when AFM is activated (as a minimum). I also highlighted what I believe are the control paths, in blue. Control appear to come from the fuel pump power control module to all of the actuators. And in-turn, control appears to come from the Engine Control Module (ECM) to the fuel pump power control module. The control paths from the fuel pump power control module to the two actuators on the lower right show the wires going though a box labeled NPP. The power path to those, from the fuse, also shows the wire going through a box labeled NPP. Notice too that there appears to be a control path coming back from the AFM associated actuators to the fuel pump power control module.

I also attached photos showing the valve locations and the connectors, and also some other photos that might be of interest.


Exhaust Flow Control Valve Schematic

Green Circle - Tailpipe Exhaust Flow Control Valves. Red Circle - Exhaust Flow Control Valves associated with AFM.

Here is the tailpipe exhaust flow control valve connector.

Show actuator removed from the valve.

Shows connectors to the AFM associated exhaust flow control valves.

Linkage to the AFM associated exhaust flow control valves can be disconnected.