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I can verify that using the suggested manipulation of selecting weather mode before starting the engine has NO impact on the first 15 to 20 seconds of startup exhaust volume.
The different results are possibly related to differences in engine temperatures during start. The first start in the morning is always louder than subsequent starts as the computer adjusts the air/fuel ratio. To be meaningful the comparisons need to be done on the first start on different mornings.
The problem is that Weather mode doesn't "stick". The only modes you can restart in are Tour, Sport, and My Mode. It seems silly that they didn't make Weather mode stick.
Will your car start in Sport mode? My manual states:
So in order to start in a quieter mode ie Weather you must use the Service mode as described in the OP. My question is does the loud start actually perform some function other than noise and it seems not. So if the car can start in a quieter mode, it would be relatively simple for GM to have the car start quietly via a setting in Tour or the highly customizable My Mode.
Will your car start in Sport mode? My manual states:
So in order to start in a quieter mode ie Weather you must use the Service mode as described in the OP. My question is does the loud start actually perform some function other than noise and it seems not. So if the car can start in a quieter mode, it would be relatively simple for GM to have the car start quietly via a setting in Tour or the highly customizable My Mode.
The manual may be incorrect. I know my C7 would remain in Sport mode and I thought my C8 did also (but I never use Sport mode so I don't know for sure).
Part of the reason for loud start is to get the cat up to temperature for emissions. But the non-NPP C8s meet emissions so having the valves closed during start should still be acceptable.
The different results are possibly related to differences in engine temperatures during start. The first start in the morning is always louder than subsequent starts as the computer adjusts the air/fuel ratio. To be meaningful the comparisons need to be done on the first start on different mornings.
I did the test this morning in my garage, it was the first start since yesterday, and temps were in the 50s overnight, definitely a cold start. My wife's office is right above the attached garage, and she served as another tester. She has frequently commented in the past that it scares the crap out of her when cold-started which has always been in Tour or Sport mode.
I tried the Weather mode cold start procedure without saying anything this morning, and I thought it was noticeably quieter. My wife was in her office and also thought it was quieter, but still not "quiet." There is no question in my mind that it doesn't have nearly the same "bark" upon cold startup in Tour or Sport.
Our C8 does remain in Sport mode if shutdown that way, which is contrary to what the manual picture posted above says. I'm thinking that is hold-over language from the C7, which did always revert back to Tour on the next start after shutdown if left in another mode.
The different results are possibly related to differences in engine temperatures during start. The first start in the morning is always louder than subsequent starts as the computer adjusts the air/fuel ratio. To be meaningful the comparisons need to be done on the first start on different mornings.
This is exactly my criterion. Morning cold starts. One manipulated to weather mode the other in (default) tour. Same-O-Same-O for the first 15 seconds, at least.
I have my-mode set to the loudest exhaust, but I switch over to tour mode when I park at home. Doesn't seem to quiet the start by much though...
Are you saying that weather mode starts are noticeably less loud than tour mode starts?
Yes. Don't get me wrong, it still doesn't sound like a Honda Accord, but it's definitely quieter.
Although I have heard that this might not work on 2020 MY C8s? It definitely works on mine, and on at least one other 2021.
Originally Posted by RandelB
I can verify that using the suggested manipulation of selecting weather mode before starting the engine has NO impact on the first 15 to 20 seconds of startup exhaust volume.
I can verify that while it might not work for you, it definitely works for others. Even on a first thing in the morning cold start.
^^^
I was of that opinion thinking why would GM be so foolish for a few seconds on starting. Then someone posted a GM statement from an electronic service manual saying that is why it is done.
I had thought it related to getting the CATs hot quickly to reduce pollution. They apparently do things to help that but the wide open NPP is to make noise! Perhaps it's both.
I watched a video a while back where one of the powertrain (engine) engineer said the valves open during start to quickly heat up the cats which helps to reduce emissions.
Since the car reverts back to My Mode, Tour, or Sport after shutdown, that would satisfy the GM design goal of having the valves open on cold start to heat the cats as quickly as possible for emissions reasons. However, if one does the work-around procedure in the OP, it may defeat that feature.
Good thing the one neighbor I like is hard of hearing... And the other neighbor I could care less about because they leave their dogs barking all the time.
Several mentioned that the valves are open at start up to help warm up the Cats. And I see from a post above that it was stated by a GM engineer. I could not find confirmation of that in the 2020 Service Manual, but I did find a description of the cold start strategy for warming them up. It does not mention the Exhaust Flow Control valves as part of the strategy, but that does not rule it out.
Honestly, I can't see how having the NPP valves open would warm the cats any faster, but I keep hearing that too. It seems like the opposite would be true, which is that the cold start algorithm would warm them up faster with the two valves closed. Maybe there is some other reason for not having them closed during that sequence.
Honestly, I can't see how having the NPP valves open would warm the cats any faster, but I keep hearing that too. It seems like the opposite would be true, which is that the cold start algorithm would warm them up faster with the two valves closed. Maybe there is some other reason for not having them closed during that sequence.
In elementary school that's what I learned "maybe" means.
Obviously, I was simply stating I don't understand how having the two NPP exhaust valves open can heat the cats up faster hoping someone might explain the mechanism underlying why that would be true. Equally obviously, I understand how the cold start fast rev and high idle sequence produces that outcome, just not the role of the valves.
I was pondering that perhaps the GM engineers didn't want the NPP valves closed during that fast rev and high idle sequence for some other reason.
Several mentioned that the valves are open at start up to help warm up the Cats. And I see from a post above that it was stated by a GM engineer. I could not find confirmation of that in the 2020 Service Manual, but I did find a description of the cold start strategy for warming them up. It does not mention the Exhaust Flow Control valves as part of the strategy, but that does not rule it out.
To be clear, I wasn't trying to state that reason for the valves being open was for warming up the catalyst. The air/fuel mixture to warm up the catalyst quickly is the reason for the cold start bark. It happens with or without NPP, it is just louder with NPP because the valves are open.
How is the non-NPP exhaust system plumbed anyway? With the NPP valves closed, two of the 4 tail pipes are blocked off, forcing the exhaust through the path with baffles. But what happens in the non-NPP system? I suspect that exhaust flows out of all the tail pipes but with baffling in both paths. Perhaps the non-NPP system is less restrictive than the NPP system with the NPP valves closed. Anyone know?
That is what I have observed as well. You get the initial roar regardless of mode when you start the car cold. It's awesome, but the neighbors may not think so.
The good news for me is there is a guy down the street with a Charger and aftermarket exhaust that is about 5 times louder so they see me as "not as annoying as him" at 6 am.
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Our C8 does remain in Sport mode if shutdown that way, which is contrary to what the manual picture posted above says...
Originally Posted by RKCRLR
The manual may be incorrect.....
My apologies for believing the manual. I stopped the car in Sport mode and it restarted in Sport mode. Now that my faith in the manual has been shattered I have no idea what to believe. Maybe we need a sticky for "owners manual inaccuracies"? Is there one?
I've noticed that there is a lot of carry-over language language in manuals even across a generation change. The "always starts in Tour mode" language was true of C7.
There are also frequent edits and updates in manuals across MYs in the same generation, as errors and necessary clarifications are identified. I've noticed from this forum that someone with a MY '20 may refer to information on a certain page, but when I go to that page in my MY '22 manual, it's not on that page, it's somewhere else, or worded differently.
