I just watched a video by Squirrel Cage Garage which shows how the exhaust of the C8 works. I had no idea! Kinda disappointed how the exhaust works with a small back flow process.
My concern is that the 4 valves, 2 AFM and 2 NPP, are closed when in Tour mode, which I drive in all the time. This is a 2023 Z51 SR.
Probably most folks drive the C8 in a spirited way. I do a little but not enough for the Track mode. I like to see how I can improve the gas mileage with a 490 hp car. I did the same with my C6.
The C6 improved by 4mpg cruising at 74mph. (I'm in Florida and it is flat where I am). I drove that car for long distances so the 4mpg added up.

With the C8, I installed a K&N air filter, Soler Throttle Body and 2MFab High Flow Cats.
I have gone from 27.4mpg to 33.6. Again 74mph on flat road.
I am pleased with the additional mpg but concerned it could be much better! After startup, when the engine is loud, it gradually quiets down and the rpm drop. In Touring mode, these 4 valves STAY CLOSED after startup!!! The AFM valves have 2 approx 3/16 holes in them to allow some exhaust gas to pass through. The NPP do not open at all, instead there are 4 half inch holes in the exhaust pipe that does not have the NPP valves. With the NPP closed, all of the exhaust has to pass through the 4 holes and out the pipe.
The cars that have the aftermarket cat back delete systems must have a lot more power and better mpg due to much better exhaust flow out of the car. I can see how the cat back delete systems are so much faster.
I recommend watching that video to learn a ton about the exhaust system on the C8.

 

The AFM valves only close when AFM is active. Give the car a little throttle and you will exit V4 mode.
In Tour mode the NPP valves stay closed during normal driving and at idle but open during acceleration. In Sport mode the valves are open most of the time (including idle) except in conditions where drone would be a problem or to meet drive by noise standards. In Track mode they are open except when required to meet drive by noise standards.
You can set the exhaust to Stealth, Tour, Sport, or Track mode when using My Mode or Z Mode.

 

AFM only kicks in during steady cruising (not at idle), maintaining speed, or very very slight up hill or acceleration. Very slight. Tadge once described it as the case where the engine is only producing about 40 hp, and throttle plate is barely cracked. Because of this, the amount of air entering, and thus leaving the engine through the exhaust is quite small compared with WOT operation. Meaning exhaust back pressure during AFM is not particularly restrictive. The valves (all 4) are closing during AFM to reduce the "undesirable" sound that the exhaust would otherwise have. The AFM (aka Cylinder Deactivation Exhaust Flow Control Valves) are not closed at idle nor when AFM is not active (as described in the Service Manual anyway).

Regarding the tailpipe exhaust flow control valves (not to be confused with the Cylinder Deactivation Exahust Flow Control Valves), while they do close when AFM is active, they are also controlled by the engine/sound setting. If that setting is Tour, they are closed at idle and most benign driving situations, but open during aggressive acceleration. In Sport and Track settings, the Tailpipe Exhaust Flow Control Valves are open at idle, and most driving situations ( more often in Track then Sport) but will close to comply with Federal Bypass Standards. But yes, regardless of the engine sound setting, when AFM is active it overrides the engine sound setting and all valves close (according to the Service Manuall).

Below is the explanation from the Service Manual that I have. It does not say the Cylinder Deactivation Exhaust Flow Control Valves are closed at idle or all the time in Tour.

"Exhaust Tail Pipe Flow Control System Description and Operation
Engine Exhaust Flow 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 chassis control module is used to control the actuator that opens the left and right exhaust tailpipe valves. The exhaust flow control valve opens and closes when the chassis 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 modes 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 are:
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: 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.

Cylinder Deactivation Exhaust Flow Control Valve System
As a means to improve fuel economy, the engine management systems includes 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. The cylinder deactivation exhaust flow control system controls the undesirable exhaust noise generated during the cylinder deactivation process. This is achieved through the use of up to two electronically actuated cylinder deactivation exhaust 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 chassis control module in response to active or pending engine 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 on all cylinders, the cylinder deactivation exhaust flow control valves are commanded to the open position in order to minimize exhaust restriction. To provide some control hysteresis, once the cylinder deactivation 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."

 

"The AFM valves only close when AFM is active. Give the car a little throttle and you will exit V4 mode."

Also moving into Manual mode on the DCT forces the var into V8 mode of course - another option

 

Great response @Andybump. Thanks.
I am wondering about the amount of flow through the closed off valves. If I'm cruising along in Tour mode the valves are closed. All of the exhaust on either side (right or left) has a certain amount of flow through the pipes. I thought all of the pipes had even flow at cruise. I now see that the Z51 NPP valves are closed while cruising in Touring forcing the total exhaust flow through the non-NPP pipe. With the NPP valves closed, just upstream of the valves are 4 11/16 holes for the exhaust to depart the NPP pipe. That exhaust gas exits the 4 holes and empties into a chamber (muffler) where the pressure forces the exhaust gas out of the non-NPP pipe. The non-Npp pipe is not attached to the exhaust system, it is short and mounts into the muffler.
As stated earlier, I am interested in increasing the fuel mileage. I race cars on race tracks so speed on the street isn't a big deal. On the race cars we have minimal obstruction in the exhaust systems. I'm wondering how much fuel mileage could be gained if the exhaust used all 4 exhaust pipes and not the 4 holes for the exhaust to escape. I believe this is what the cat back full flow systems are about, High HP and Torque gains due to max exhaust flow.
I can cruise at 74mph at 34.6mpg. I am going to drive the car for the next tank of fuel in Track mode to see if the open valves will increase the gas mileage.
Thanks for the response everyone and I will report back with my findings.
Save The Wave!!

 

I think you might find that the your mpg goes down in Track Driver mode with the comparison you propose, because Track Driver mode changes both the engine/sound setting as well as the engine/shift setting, and both affect gas mileage. I assume you know that under the conditions where AFM activates, you actually get slightly better (not worse) mpg. Preventing AFM won't increase your mpg. But...keeping the tailpipe exhaust flow control valves open all the time might....if everything else was unchanged. In fact, the reason that the the rated hp increases 5 hp with the NPP muffler (standard on Z51 and available on the non Z car) is because of the reduced backpressure of the the muffler design when the tailpipe exhaust flow control valves are open.

Keep in mind that when you use Weather, Tour, Sport, or Track Drive modes, both the engine sound setting and the engine/shift setting will change, and each of those affects fuel mileage. The engine/shift setting affects the way the gears will be automatically selected and also affects the condition where AFM can be active. In Weather and Tour Driver modes, the engine/shift setting is Tour. In Sport and Track Driver Modes, the engine/shift setting will be Sport and Track respectively. Both Sport and Track engine/shift settings will favor higher RPMs for the same speeds, with Track being more aggressive then Sport. It will downshift quicker/sooner, hang in lower gears longer, and not upshift as soon. And with engine/shift set to Track, AFM is inhibited. The change in gear selection and inhibition of AFM alone will affect your gas mileage. The way the engine sound setting affects the tailpipe exhaust flow control valves is covered in an earlier post.

My point though, is if you try to compare gas mileage in Track Driver Mode with gas mileage in Tour Driver Mode, you will not know whether any observed change in mileage is because of the valve behavior, or because of the engine/shift setting. And in fact, I suspect you might find that in Track Driver mode your mpg is actually lower for two reasons. 1) AFM never engages - so you never get the (small) increase in mpg that you would get with AFM active, and 2) the more aggressive gear selection (and also throttle response) with engine/shift set to Track will likely reduce your mpg.

So.....a better comparison would be this. Use Z-drive mode. Set all the parameters to Tour and measure your mpg. Then, change only the engine/sound setting to Track, while leaving all of the other parameters set to Tour. This will cause the tailpipe exhaust flow control valves to remain open almost all the time. But during periods of benign cruising, the AFM will kick in. And its an apples to apples comparison where you can estimate/measure the change in fuel mileage caused only by the change in behavior of the valves.

You could also repeat the test iin Z-mode to determine the affect of the valve behavior with AFM prevented. In this case, the first condition would use Z-mode and set everything to Tour except set engine/shift to Track. Leave the engine sound setting on Tour. Measure the mpg. Then repeat, but this time set everything to Tour except set both engine/sound and engine/shift to Track. Again, you can compare the mpg while AFM is prevented (engine/shift set to track) but valve behavior is changed (engine/sound set to Tour, then Track).

Using manual shift mode will also inhibit the AFM, but I think it would be difficult to get a controlled comparison since you now must manually select the gears and probably won't do it the same way every time.

 

AFM valves are only closed in 4 cyl mode. The reason for them is that exhaust sound in V4 mode is not pleasant to listen too.

 

That's right. As explained in post 3 and several other posts, AFM is only active during very benign conditions with very low power being produced by the engine, and that is the only time that that Cylinder Deactivation Exhaust Flow Control Valves in the muffler are closed. Those valves are open all other times.

Attachement one shows the exhaust flow through the system with the tailpipe valves open. It is essentially and effectively a straight through pair of pipes, via an H-pipe crossover, that exit through the open tailpipe valves through the outermost pair of tail pipes. There are a couple of small holes in that path that exit into the muffler shroud (or chamber), but since the tailpipe valves are open, not so much flow through that path. And, AFM valves are open so there is no restriction imposed by those when AFM is not active. AFM can become active in automatic shift mode if the Drive Mode is Weather, Tour, or Sport (or with engine shift set to Tour or Sport) when cruising at steady speed with very low power demand- but not Track Drive Mode.

Attachment two shows the exhaust flow through the system with the tailpipe valves closed. AFM is not active so the AFM valves in the muffler are open - no restriction there. But, since the tailpipe valves are closed, preventing any flow through that path, the exhaust must exit through the small holes in in the pipes and into the muffler shroud'chamber. Then they exist back out through the inner pair of tailpipes. This is a bit more restrictive. This condition will exist in Weather Mode, or if engine sound is set to Stealth, and also in Tour under benign driving condition - even when AFM is not active. In other words, whatever restriction if imposes has nothing to do with AFM.

The third illustration shows the case where the tailpipe valve are closed, and AFM is active as well. It looks the same as attachment two, but now the AFM valves are closed. When AFM is active, it commands both AFM valves and tailpipe valves to close, so the flow is the same as in attachment two. So, this condition can exist anytime AFM is active, which can happen in Weather, Tour, Sport Drive Modes* (in automatic shift mode), or any time engine/shift is set to Tour or Sport (in automatic shift mode) as long as the engine power demand is very low - benign driving. The exhaust restriction is minimal because the power demand is low. Fuel consumption in this condition is slightly lower than if the AFM were not active.

*some owners of Eray report that Sport Mode also seems to prevent AFM from occurring. But it does not on Stingray.

 

Thanks again @Andybump and others.

I have had the car for a short time. I cannot figure out the settings for each driving mode. I don't get the Z and MY MODE at all. I don't care what driving mode I am in, I want to see certain parameters of the car while it is driven.
I usually drive in Tour mode and I want the following info on the dash. (This is from my racing experience). Oil pressure,Water temp, tire pressure and RPM. That's all I want but I can not get them all at the same time.
I have watched some of the Youtube videos but I just can't see how they work. To me, the owner's manual is a joke, but I'm not laughing.
Any help will be appreciated and recommend a Youtube video would be great!

 

Well you wont fine me defend the way the manual is laid out. But what you want is possible. Z-mode lets you customize all of the parameter setting (steering, brakes, engine sound, engine/shift, suspension), and also you can indvidually set the display theme and HUD display. MyMode lets you cusomize all the paramters with the exception of the engine/shift setting, which stays on Tour, but it also allow to customize the DIC display theme and the HUD type seperately. The "display theme" is what you see on the DIC - and there are Weather, Tour, Sport, and Track Display Themes. The HUD display has different "types" - Tour, Sport or Track. You can always cycle through the HUD types using the info button on left of the steering wheel on the lower dash.

Now, the Track Display Theme allows you to select 4 info tiles - so you can get tire pressure, time and outdoor temp, and various other operating parameters. All the other display themes only allow two info tiles. For that reason, I prefer, and use, the Track Display theme. Below is an example of what I use. I have Z-mode set to show the Track Display theme.

You can see in the image that the icon at the center bottom shows I am in Z-mode. On the right of the screen you can see I am using the display design menu, and you can see I have selected Track the DIC, and Sport (in this case) for the HUD ( I normally set it show Tour for the HUD because I want the speed, gear, and speed limit up there but this is a handy photo I had). On the left you can see the 4 info tiles I selected (you can pick other) and they are transmisison fluid temp, engine oil temp, tire pressure, and oil pressure. Then I get water temp on the bottom left. Pretty much everything, I do a similar think with MyMode, with the exception that I have replaced tire pressure with time and outside temp. I shut the car off in MyMode and comes on in MyMode. If I want Z i simply hit the Z on the steering wheel. If I want to exit Z, I use the **** on the console and give it left twist back to MyMode. Edit: Oh, and also the Track Dispaly theme shows RPM that you mentioned. You can also get a digital RPM if you want - I dont like it.

Of course, I also set the other paramters in MyMode and Zmode to what I want. I use mostly Tour settings for those - especially MyMode. But in Z, I sometimes use a more aggressive engine/shift setting, and set engine sound to Track. I don't like the heavier settings for Steering, Brakes, and Suspension so I mostly use Tour.

In Track Driver Mode, you get the Track Display Theme but even there you can select the 4 info tiles that you want. The problem though is that in Track Drive Mode you are stuck with all of the other parameters being Track so loudest exhaust, stiffest ride, heaviest steering, aggressive shifting, and heavies brake action. Thats the beauty of Z mode - you an get just what you want most of the time if you are not actually Tracking.

 

Thanks for the very detailed reply! One more thing, How are the info tiles set?

 

I think its in the display design menu. I had a series of photos but I don't know if I still have them. But it there. I'll look for it tomorrow.
Its under the Options Menu on the DIC - see next post.

 

You do that with thumbwheel controls on the steering wheel on the right, and that controls the menus on the DIC on right. Use the left-right arrows to scroll to "Options". Under Options you will see Display Design (that you use to set up display theme and hud type you want, Info Tile Selection, Tachometer, and Speed Warning. Use the thrumbwheel to scroll to Info Tile Selection and press the thrumbwheel. This puts you in the menu for selectin the info tiles. To select a different tile, you must first de-select one (or all of the tiles). You use the thumbwheel to scroll through the choices, and press it to select or deselect it. Options you have are Voltage, eLSD Coupling, Fuel Economy, Lateral G, Oil Pressure, Oil Temperature, Time and Outdoor Temp, Tire Status,
Transmission Fluid Temperature.

Back to the Options menu, if you click on the Tachometer, it gives you the option to display the digital or the analog version. I use the analog one, but I included the digital as an example.

Edit: Notice the "redline" on the Tach shows just over 4500 RPM. That is because the engine is cold. As it warms up, the redline will move to the normal redline. If the car has not been broken in yet - less than 500 miles it will show redline at 4500 all the time.


Thumbwheel Controls

Options Menu. Select Info Tiles Selection

Into Tiles Selection Menu

InfoTiles Selection Menu

InfoTiles Selection Menu

Options Menu. Select Tachometer

Tachometer Menu. Select Analog or Digital

 

I just saw all of this after my giving up post. I see what I was missing and I'll try again.....

 

Thanks @Andybump, got it sorted,