Removed NPP actuator and spring holds valve normally open. Replaced actuator with a spacer nut under each attaching bolt. Now valves always open. See if I like it and easily reversible if not. I like long tubes, no cats and minimal mufflers so unless it drones like crazy it'll stay this way.

 

Curious on an update.

 

Won't that throw error codes and generate a cel?

 

Why not just put exhaust sound all the way up in z-mode, my-mode, or track?

 

Nannies are closing the valves to differing degrees in all modes at lower rpms.

And no CEL. Computer still thinks valves are being actuated. No downstream sensors to signal any difference.

On c7s and earlier you could unplug the valves and get same result. C8 needs actuator to remain connected and operating.

 

So, have you road tested it? Did seem to be louder? You are right that the valves may close under some conditions, even in track mode, but what are those conditions? Definitely not at idle - nor revving the engine while stopped - you can switch between exhaust settings and clearly hear it louder under those conditions. With exhaust setting in track mode, I dont beleive it closes with any spirited acceleration either, nor does it seem to close when I am cruising leisurely on a back road, because I can clearly hear the difference in the settings then too. It leave me wondering whats left. I believe there was video where someone else kept them open (with a different method) and in that video reported no perceived difference. Have you notice any difference yet?

 

Wife drove her car today and tripped the following codes P31D0 & P31DE. I returned the valves to operating function and found one of my spacer nuts had slipped out and was sitting on the actuator. Guessing that jammed the actuator and tripped codes. Drove about 40 miles before cel came on. Gonna leave em alone for a bit. Wife doesn't like extra lights on her 4 day old car. It did seem louder after the cold start cycle wound down but didn't measure it. On hold for now.

 

That was my logic and experience as well. In track or any mode set to track sound, I hear big differences immediately in idle, revs, even low throttle. Can’t imagine there is any possible difference locking them open.

 

There are state (and Federal?) standards for drive by noise and this has been discussed before. The valves NPP will close in certain conditions due to drive by noise standards, even in Track mode. These are usually set for certain speeds (not accelerating) and allow more noise as speed increases. GM must comply with the most restrictive standards if they want to sell in all 50 states. During idle and acceleration the valves are wide open in Track mode but they will close during certain lower speed steady state conditions.

Noise standards by state:
https://www.semasan.com/resources/ex...%3A%2072%20dBA.

 

So the system eliminates drone. Still sounds louder than Touring even at part throttle, so curious to know what percentage it closes. Id think idle and acceleration are where you want sound, not a buzzing while just moving at constant speed.

 

I did not think it had variable positions (if that is what you meant by percentage), just on, or off. And I just read in another thread that Z06 will have "infinitely variable" NPP valve positions for its exhaust. I have been wondering about this for a while, because the description and the schematics in the 2020 Service Manual show that it uses a "pulse width modulated" signal to control the valve, which to me implied that it has, or at least is capable of having variable positions. So, perhaps it was set up that way to accommodate the "future" Z06.

As for when the current one closes, based on what was said above, it must be at speeds somehwere between 0 and maybe 30 or 40. I know on those back roads I travel, at speeds around 40, I can hear difference between the settings. I can't say I have tested it at all the speeds between 0 and 40 though.

 

I have a Borla exhaust that still uses the AFM and NPP valves. Using MyMode and just changing the exhaust setting from Tour to Sport to Track, there is an appreciable difference between each setting and also between all of them and the Stealth setting in Weather mode. Based on that it seems that the valves must have variable position - not just open and closed.

 

I won't disagree with the perception of others regarding a volume difference between sport and track settings of the exhaust sound, but this excerpt from the 2020 Service Manual describing the operation of the valves seems to describes only two "commanded states (open or closed". It also mentions the pulse width modulated control signal, which, based on what I know, could support variable positions by varying the pulse width, if the valve actuator was designed to respond to that. But that method of signal modulation would of course also support only open and closed states as well.

 

I'm supposed to have an appointment at the Chevy dealer later this week because one of my exhaust valves is throwing error codes. While I am there I'll see if the Corvette mechanic can answer this question for us.

I will say that when he originally installed the exhaust he showed me the valves going to the opened and closed positions on command. He didn't send them to any other positions that I know of. I will say, however, that I don't think the different tones/volume that I hear when switching modes can just be my perception. They are clear and noticeable.

 

Are you switching between the Sport and Track drive modes, or using Z-mode and just changing the exhaust sound? When I do the comparison, I use Z-mode, and just switch the exhaust sound setting - leaving all other settings the same. I have the engine/shift set to track in when I use Z-mode. But if you switch between the Sport and Track drive modes, that also changes both the throttle progression and the transmission shift mode, according to the table in the table in my 2021 Owners Manual. From the table, throttle progression in Sport Drive mode is set to tour, and it is set to track, in Track Drive mode. Shift mode is set to sport in Sport Drive mode, and it is set to track in Track Drive mode. These two items (throttle progression and shift mode) don't show up as separate items in the menu in the car - on mine its called engine/shift (it may have a different name depending on the year of your car). But the point is there may be changes between Sport and Track drive modes that affect how loud the exhaust sounds, especially the throttle progression. I don't know that - just pointing out there may be other things going on. I edited this to add, that of course, the exhaust sound also changes between the drive modes (as we know) from sport to track, so there must be some difference there.

 

I am just changing the exhaust setting in MyMode. I can hear a definite difference between Weather, Tour, Sport and Track modes. I hear the difference at idle, normal driving and high accelerations. As I said above, I don't see how the exhaust can sound different in all those modes if the valves just have two positions - open and closed. That's why I'm going to ask the dealer's Master Corvette mechanic about it when I next take my car in.

 

Interesting- that does say it’s either open or closed... I have a feeling differences in sound at part throttle that people are describing is the AFM kicking in to 4cyl when not in track mode. I can noticeably hear difference in drone with track mode at all times compared to Tour, along with louder idle and louder WOT. I don’t hear it suddenly just quiet down when cruising, unless AFM kicks in like in Sport mode.

 

Ask whoever BUT there are two positions open and closed! Even for the V4 pre muffler valves. Was the same for the C7. See pic of valves. I was not sure in the C7 if the V4 valves were fully closed BUT the pics below of the C8 valves make in clear! Yep when V4's close all exhaust goes through those small holes. BUT in V4 probably using max ~40 hp vs 495! No need for large pipe.

Now for the C8 Z06 they state the amount of NPP valve opening can Vary! Frankly was a bit surprised GM whet from a two position vacuum operated NPP in my C6 (had no V4 in C6) to stepping motor operation in my 2 C7 NPPs. BUT they still were only open or closed! However they could check IF the motors got to the closed position, probably done for V4 as it affects performance. IF not they tripped a CEL! So understandable they can made variable in the C8 Z06.

Be surprised if the average Corvette Tech knows or frankly cares. Car repair is their business NOT a hobby for many! When they go home they read about football and watch sports like most folks! Don't want to think about cars!

 

Jerry, you are most probably correct - I have no reason to doubt you. I just haven't seen any explanation yet for the difference in volume/tone that I experience in different exhaust modes. I have Borla S-type exhaust (louder than OEM NPP) and am changing the settings by adjusting the exhaust setting in MyMode (so other performance settings remain the same).

When I first got the exhaust I set MyMode to the Track exhaust just to enjoy the incredible sound. That soon became a little too much, however, so I turned it down to Sport exhaust setting. I have tried Tour exhaust setting a couple of times and it just isn't quite loud enough. Keep in mind that this is all in MyMode and not changing any of the other settings.

So I'm not arguing that the valves have more settings than open/closed. I'm just saying the exhaust sounds different in different exhaust mode settings while leaving all the other performance settings the same.

 

^^^
Since no one has been able to fully set the OEM NPP valves to fully open as they did in the C7 and were able to validate what I "guessed" was happening from my tests, i.e.
In Track after starting the valves closed until about 1700 rpm then opened above that.
In Turing is was about 2500 rpm and in between in Sport

For lack of other input I'll assume the C8 acts about the same way. So it's throttle and rpm dependent (perhaps other inputs.) In all Modes expect past ~2500 the NPP valves are open. Does that help define what you hear?