To amplify on the posts of others here (especially AndyBump and JerryU) Here is the entire service document from which Andy posted excerpts. I am also going to create a new thread from this post to disseminate this information to a wider audience. I can provide this as a MS Word document if you PM me. The document is fairly long, so here are the cliff notes:

Valves are controlled by pulse width modulated signals, but only have two commanded positions - full open and full closed. The difference I and others have noticed in each drive mode are because the valves move between these two positions at different times depending on the drive mode.

I assume the valves move fairly quickly because there is a lot more motion commanded than I had realized.

The position of the NPP valves are also affected by the commanded position of the AFM valves.

And here is the full document:

2022 Chevrolet Corvette | Corvette Service Manual 15356649 | Engine / Propulsion | Exhaust | Description and Operation | Document ID: 5830642

Exhaust Tail Pipe Flow Control System Description and Operation

Engine Exhaust Flow Control Valve System

The engine exhaust flow control valve system Has two subsystems:

1. The exhaust tail pipe flow control valve system, also known as the exhaust sound quality valve control system.

2. The cylinder deactivation exhaust flow control valve system, also known as the active fuel management (AFM) exhaust flow valve control system.

Exhaust Tailpipe Flow Control Valve System

The exhaust tailpipe flow control system is used to tune the exhaust note for high performance vehicles. The vehicle is equipped with two tailpipe exhaust valves, one in the left tailpipe and one in the right tailpipe. Each exhaust tailpipe valve is installed in the low restriction exhaust path of a dual outlet muffler, near the exhaust tip.

When a tailpipe exhaust valve is open, the low restriction exhaust path is opened to the atmosphere, and the exhaust note becomes more aggressive.

An output circuit from the fuel pump control module is used to control the actuator that opens the left and right exhaust tailpipe valves. The exhaust flow control valve open and closes when the fuel pump control module commands the actuator by pulse width modulation of the control signal.

To provide a more aggressive exhaust note when the vehicle is started, the exhaust tailpipe valves are opened during an engine crank event during specific mode of operation if that option is available. Once the engine is running, accelerator pedal position, transmission gear, and engine speed are used to determine the commanded state (open or closed) of the exhaust tailpipe valves.

There are four specific exhaust performance modes that result in different behavior of the exhaust tailpipe valves. These modes follow:

Note: You may notice the exhaust tone change due to the AFM system activation, or due to 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. Winter/Eco 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.

It is possible to personalize the exhaust sound. When engine sound enhancement is turned off via the personalization menu, the exhaust tailpipe valves will be put in track mode no matter which vehicle mode (Winter/Eco, Tour, Sport, or Track) is displayed on the driver’s instrument panel.

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 of that powertrain engine cylinder deactivation (active fuel management) has on the quality of the exhaust sound. When the active fuel management system 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.

The cylinder deactivation exhaust flow control system controls the undesirable exhaust noise generated during the cylinder deactivation process. This is accomplished by the use of two electronically actuated cylinder deactivation flow control valves in the vehicle exhaust system. The cylinder deactivation exhaust flow control valves are commanded to the closed position by a pulse width modulated signal from the fuel pump control module.in response to active or pending cylinder deactivation. When the cylinder deactivation exhaust flow control valves are in the closed position, the engine exhaust flows through a hole machined in the plates integral to the valves. When the engine is operating an all cylinders, the cylinder deactivation exhaust flow control valves are commanded to the open position in order to minimize exhaust restrictions. To provide some control hysteresis, once the cylinder exhaust flow control valves have been commanded to the closed position, they are not reopened until the engine torque reaches a calibrated minimum value that is determined as a function of the driver selectable performance mode.

 

A few observations. 1. "Valves are controlled by pulse width modulated signals, but only have two commanded positions - full open and full closed. The difference I and others have noticed in each drive mode are because the valves move between these two positions at different times depending on the drive mode." I always try to include the statement "according to the Service Manual......". Although I believe there are only two positions of the exhaust flow control valves , not everyone does. Also, I think another reason (and its speculation) it can sound different in the different drive modes is that the engine timing, fuel injection, and throttle response may be different in each drive mode. This will make a difference in exhaust sound, as for example, when we get the loud bark at start up when the valve are open, but also the timing and fuel injection are adjusted for a quick warm up of the CATs. 2. "I assume the valves move fairly quickly because there is a lot more motion commanded than I had realized." They do move very quickly. I recently "discovered" that in the service mode, entered by holding the start button down a few seconds without putting your foot on the brake, I can hear the valves open and close while cycling thought the Drive Modes, or putting it in Z-mode and cycling through different exhaust sound settings. They make a distinct clicking sound both opening and closing. I hear the noise moving from tour to sport, but not from sport to track, because the valves are already open at that point. 3. I notice you are referencing a 2022 Manual. How did you acquire it?

 

In my C7 I could remove the fuse for the valves. Can that be done with the C8?

 

^^^
Nope!

 

I was given a copy of this document in the 2022 manual by my dealer because I specifically asked for information on how the exhaust valves worked. I was at the dealer to have an exhaust valve re-learn performed on my Borla S-type exhaust, which was throwing CELs. I asked specifically because of this thread (see post 29).

 

Do you have the whole 7000 page manual, or just that section? And also, did your mechanic agree that there were only open and closed positions?

 

I was only given that section, and yes, the master Corvette mechanic agreed that there were only open and closed positions. He also showed me using their computer equipment to command those positions and the car reporting that the positions were reached with no error. I didn't see any way on his screen to command a partial position.

 

Well, no one else was dumb enough to try so I unhooked mine again as described previously. Verified all spacer nuts were in place and actuators were free to move without engaging valve. CEL was tripped in less than 20 miles. Must have some type of servo monitoring sytem in there. Computer could tell valves weren't operating correctly. It was not much louder after cold startup but did sound more aggresive when driving away at slower rpms.

 

It may work if an "exhaust control valve learn" procedure is performed. That procedure is called for any time the exhaust flow control valves are serviced. I have an idea that when the valves are installed, the control module for the valves, which is the "fuel pump power control modules" learns something about when the actuators hit the stopping points at the end of the valve travel. When removed from the valve, that actuator is free to travel a further without "hitting the stops." Perhaps if the learn procedure were performed it would "learn" the new endpoints. All just a guess. But I do know that there is a learn procedure. You need a scan tool to do it. Did you feel like it made enough of a difference to pursue further?

 

That was what folks with C7s said who installed Mild to Wild, which is a remote relay that essentially pulled the fuse when activated. BUT the C7 did not monitor the NPP valves!

In the C7, the V4/V8 valves (prior to the muffler) sent signals back to the computer and would trip a CEL if the valves did not close when commanded by the computer. The C7 NPP valves did not! That is why pulling the NPP motor fuse (using a switch or remote relay like Mild to Wild) to bypass the fuse- worked! Now the C8 NPP valves send signals back to the computer!

Solution is to install bypass devices like PHASTEK sells (see pic) that trick the computer OR as I said worth trying: SEE PIC, Disconnect the motor to valve connection and keep the motor plugged in. Just like the skip shift delete on manual C7's the computer will think the motor "did its job!"

The difference in having a "switch" as I installed for my C6 NPP, is when flipped you can hear the sound difference. As you note all that will happen at Moderate Throttle is it will sound like what you found! " but did sound more aggressive when driving away at slower rpms."

At partial throttle that is all you can get with reduced exhaust flow. IMO your comments show some proof that, as expected and as happened with the C7, Although the Service and Owner's Manual may say the NPP valves are "always" open in Track Mode THEY ARE NOT ALL THE TIME! At low rpm they shut! The rpm that occurs is dependent on Drive Mode.

What you are finding is what folks who tried removing just that side of the 3-prong fuse that activates NPP (easy to do just short that section) with the FUSE REMOVED- TRIPPED CEL.

PS: The louder Cold Start Sound is also due to the richer air/fule mixture and higher idle that MOST cars use. Many cars sound louder at starts and don't have NPP! Including what C8 owners have posted of their non NPP cars!




These work like the C6 and C7 manual trans to defeat "Skip Shift." The computer thinks the NPP motor closed the valves and "did its job" but didn't! ~$119.

 

Recently posted this I found on the Net re Cold Start Sound totally unrelated to Corvette, NPP etc!

This Is Why Cold Starts Are So Damn Loud

A cold start can turn a humble hatch into a roaring monster truck, and something more powerful like a V8 Mustang can send shivers down your spine. Revel in the power (and the glares from the neighbours) for those few seconds, until the car starts idling normally again and you can set off.

To start with, a car’s ECU decides that the engine has to idle higher when the engine is cold to stop it from stalling. This is programmed into the ECU via a start-up map, which allows a richer air/fuel mixture to enter the cylinders. Ideally, efficient combustion occurs at a higher temperature than that of the ambient temperature of a cold engine, where the fuel can vaporize naturally and be burned.

When it is cold, more fuel is added to the air/fuel mixture to make up for the lack of natural vaporization. So the AFR (air-fuel ratio) is seen by the ECU to be leaner than usual (below the Stoichiometric ratio of 14.7:1) so it compensates for this with extra fuel to make up for the unburned fuel due to the cold.

Although it might wake up your neighbours in the morning – especially when coupled with a lack of exhaust-silencing – a cold start-up function within the ECU is a necessity in this day and age, seeing as we have abandoned the choke as a mechanical device. The cold starts can surely put a smile on the face of any true petrolhead, whatever the weather.

 

The loudness on cold start is due to the mixture and timing during the crank event regardless of whether one has the NPP or standard exhaust. However, with NPP, the exhaust flow control valves are fully open during the crank event - "to create a more aggressive exhaust note" according to the service manual. If closed, they would muffle to some extent the otherwise loud note during the crank event. But, on my car, with NPP, I know that the valves will close almost immediately after the crank event if the car starts in the default tour mode. I know this, because immediately after the crank even I can toggle the exhaust setting between tour and track, and I can hear the volume increase in the track setting. And I do mean immediately after (I just verified that right now). I also know that at idle, with exhaust setting to tour the valves are closed, and with it set to track they are open. And, I have also confirmed this at other speeds up to even 40 mph, steady cruising. Valve closed if the exhaust setting to tour, and valves open if exhaust setting is track (powertrain setting in track to avoid V4 mode). I do this in my HTC with the top down so I can hear it very well. Its why I remain skeptical that forcing the valve open is going to be perceived as a louder exhaust than just setting the exhaust sound to the track mode, but I would still like to hear from someone who has done it. I am aware that there are supposed to be times, even with track setting, that those valves will close. I know one is this case: If in Z mode, with the exhaust setting to track, but with engine/shift (powertrain) setting to tour, AFM will kick in at steady speeds. When it does that, it closes both sets of valves - (according to the service manual) - the AFM valves, and the tailpipe exhaust flow control valves, and I have heard that when it happens.

 

Folks were also skeptical with the C7 saying what you are the "book says!" That was until a few folks installed Mild to Wild and sure enough even in Track where the book said they were "always open" they were not!

In the C6 no drive modes to change sound BUT I installed a simple switch to bypass the NPP fuse. I kept it open most of the time but at highway speeds there was an objectional drown and I had to flip the switch and allow NPP valves to close.

We just had a poster say "it was louder at lower rpm's with NPP's disconnected. BUT like Thomas, some may need to insert their hand in the wound! As I said, we really need a way to turn on or off OR someone with a fiberoptic camera to look!

 

I know that. But I'm not sure how that could be. As I said, in the Z-mode, with power train set to track, I can drive at different steady speeds up to at least 40 mph, and I can switch the exhaust setting between tour and track - and I can hear the louder exhaust in the track setting, at steady speeds. It gets harder at higher speeds due to wind noise. One possiblity - when I tested that - I noted that in Z-mode it likes to shift in a way that keeps the RPMs up just a bit - in tour mode the RPMS can be lower. As recall, I attempted to repeat the test in manual mode, still with power train set to track to say out of AFM, but trying to keep the RPM lower and then switch between tour and track. I'll have to repeat that test, but I think it was inconclusive for me. The may very well close under that condition.

 

^^^^
Don't have data on the C8 but what you describe would be easy to show in the C7. In Touring the NPP valves opened at about 2500 rpm. In Track (or as I and many did when NPP set to always Track) they opened at about 1700 rpm. So if driving at 2000 rpm and changing drive mode from Touring to Track it would get louder!

As I said until it can be switched to hear the change there will always be folks who won't agree. Lasted for several years with the C7 and some even with posters saying it changed with Mild to Wide some would still quote the "book!"

Frankly it's so much quieter in my C8 coupe than even 2014 C7 or 2017 Grant Sport, perhaps I would not here much!

 

Exactly. When I tried it at lower RPM (using manual mode to keep it around 1700) my recollection is that I was not sure - its naturally quieter anyway - but valves could have closed (or not). I will try it again. But to be clear, at idle, the valves are definitely open in track mode, and closed in tour mode. So, if they are closing at low RPM in track mode, its above idle and below 2000. And again, when I drive out of my neighborhood in track mode, slowly, at 25 or so, the valves are definitely open most if not all of the time because it clearly louder in that mode than in tour mode. Meaning to me that while they may close in track mode, but it sure is hard to find that spot where it makes a perceptible difference (with the exception that with the powertrain in tour, but the exhaust setting to track, both sets of valves close if AFM kicks in and thats easy to hear when it happens). Agree that if we had a switch where were were sure of the position - it would be easier to test. The poster above that said it seemed louder could not do the switch test.

 

It was not an obviously perceptible difference and I don't think the juice is worth the squeeze to pursue further for me. I'll have to wait until my warrenty is up and ditch the 🐈.

 

Someone said you can't remove the fuse, but I believe you can. I believe the stock fuse is a double fuse, where the center post on the fuse supplies power to two circuits. You could do one of two things, pull the fuse and clip the blade that goes to the exhaust valve circuit, or pull the fuse and just replace it with a two blade fuse for the other circuit (don't recall what the other circuit is). Pulling the fuse just on the exhaust valves shouldn't throw a CEL.

 

But will it be perceived as louder than just using track mode? The problem with comparisons is to find that spot (speed, RPM, throttle opening) where the valves were closed in the track mode setting, but now open, and noticeably louder. By switching back and forth between track and tour, I can find many, many combinations of speed, RPM, throttle opening where its clearly louder in track setting, meaning the valve is open in the track position. I have been finding it difficult to conclusively find an instance where I could hear no difference between those settings. I think if a switch were inserted, where the fuse was, so that you could switch between using the drive mode setting vs open circuit, you might be able to compare. Drive in track mode, then at various speeds, RPM, throttle setting, move the switch back and forth. When closed, the valve is controlled by the car. When open, the valve defaults to the no power position which everyone claims is the open position.

 

On both my 2013 Camaro SS 1LE I had the mild to wild switch, so could switch back and forth any time I wanted to. When cruising along at around 1800 rpm the valves are closed, I hit the switch and opened them, they were louder, but didn't drone and weren't that loud that you couldn't carry on a conversation. When I got my 2017 Camaro ZL1 I just pulled the fuse as installing the mild to wild was harder as it had to be done under the hood. Same thing, not loud enough to bother me on the highway. I plan on doing the same thing on the C8, just pull the fuse and leave the valves open all the time. Since the engine is between you and the exhaust, pretty confident the exhaust won't be that loud when you're cruising on the highway.