1000 apologies if this has been asked and answered. I did a search and found nothing.
Is there a flow diagram(s) that show the exhaust flow with valves open vs shut. I'd like the track mode exhaust sound at all the times, without having to select track mode or Z mode or my mode, every time I start the car. My C6 427 Vert has a mild/wild aftermarket switch. I put it in mild when I'm pulling in out of the neighborhood at early morning or late hours, other than that, its in wild mode. Pulling fuses is another option. But what can be done to keep the exhaust open on the C8? While not getting check engine light?
Bee Jay

 

 

I’m not sure what the video was selling or showing, but looking at the picture, I think I’ve figured out the system. The first pair of valves is to introduce restriction only. So I would want these open at all times. The second pair of valves redirects exhaust flow. When closed , the exhaust flow is forced out the small holes, into the muffler cavity, then out thru the inside tailpipe. When the valves are open, the flow is directly out the muffler to the outside tailpipe, so I would want that open at all times also. Am I right?

 

Looking at this makes me want to buy a new exhaust…NOW…that I cannot afford right now.

 

The 1st set of valves are the AFM values, they only shut when the engine is running in V4 mode to cut down on the drone and resonance. The 2nd set of valves are your NPP values; these are the ones that adjust your exhaust from quiet to loud. When these NPP values are shut, it forces the exhaust flow through that small hole into the muffler chamber and then out the inner exhaust tips. If you want these open/most of the time, you can either use Track mode, or edit your Z-Mode and MyMode accordingly.

The factory muffler is actually a very good design when you study it. With NPP valves open, the exhaust flow is basically a straight-through design with internal X-pipe. When the NPP valves shut, that is when restriction is introduced to calm down the volume. With the NPP valves open, the biggest restriction is when the exhaust flow passes through the open AFM valve section.

 

Wounder what the non NPP exhaust looks like inside the muffler? As you would not have the straight through design on the outside tips but possibly some equalized flow between all four.

 

It likely contains some form of internal X-pipe, but all exhaust flow would become baffled in some fashion afterward.

 

Based on the GM 1.8 HP-.8 TQ documents, I'm guessing almost exactly the same as the NPP just missing the valves.

 

I just cut mine open to see exactly what is going on. The middle chamber is completely sealed from the two outside chambers. There is an X Pipe in that chamber that is just tacked together. It doesn't matter if it leaks because it's in a sealed chamber. I welded mine up just because.
Each main pipe simply crosses from one side to the other and exits through the outer tip. It's a straight through 3" pipe. The four holes in the main pipe just allow the sound waves to exit into the outer chambers. (Corsa does this in a lot of thier mufflers) There is no restriction from these holes whatsoever. I think any claims of increased flow and HP from aftermarket muffler manufacturers is, at a minimum, highly exagerated. I actually think it is total BS. What can flow more than a straight 3" pipe?
We're not talking about a header that uses scavenging and tuned tube lengths to fit certain RPM ranges to suit big cams with lots of overlap. This is a stock motor and the only thing that really matter is restriction.
When the valves close, then the exhaust gases are forced through the holes, into the open chamber and then out through the inner pipes. In this mode, of course, we do have some restriction.
I made a curved plate and welded it to cover two of the four holes on each side. It sounds awesome compared to stock. I'd put it against most of the aftermarket exhausts and it;s basically free. I added a set of Carbon Fiber Akropovic tips from my M5 and it looks great as well.

 

Photos if you get a chance to post some!

 

Yes please!

Do you have pics to explain what you did? I always love an excuse to fire up the machine.

 

I am working on the same project, I woul love to see photos of your exhaust tips as I am having trouble sourcing them as they need to be short and the inner and outer exhaust tubes are different diameters. I also cut out the valves as they take a lot of space in the tubes and must cause a fair amount of turbulence and resistance.

 

Andy by covering two of the four holes on each side doesn't that cause some restiriction even when valves are in the open position? I ask this becaue with the valves open some exhaust must flow thru those holes and exit thru the inner exhaust pipes. Or are you saying with the valves open all exhaust flows thru the outer pipes?

Also, probably the only difference between the oem and aftermarket system is the aftermarket has some weight saving benefits.

 

That’s my concern as well. I am running the cicio NPP valve delete so my valves are always open and even at idle there is a fair amount of exhaust coming out the inner tips. Which is the reason I cut out the valves, to try and reduce any internal drag/turbulence. The OEM exhaust is 2.75 inches O.D. for most all the length except where the hangers are welded on, they are 2.625 at that point. I am sure smarter folks than me can calculate the optimal pipe size for the volume needed for a 6.2 liter engine at 7,500 RPM? Seems most aftermarket manufacturers feel 3 inches is the number?

 

FYI, these are the butterfly valves in the exhaust, the one with the holes is the AFM.

 

multiple sources and calculators all point to needing about a 2.75 dual pipe system, looks like using the modified OEM to a two pipe system at 2.75 should be optimal.

 

You can see what I did inside. It's very simple. It is considerably louder and has absolutely no drone. I only covered two holes because I don't have the exhaust module kit yet. If you covered all four, and the valve closed, it would be like sticking a potato up the exhaust until you hit the "Z" button or changed the mode.
For those that are worried about reduced flow because the holes are covered, At this horsepower level, we're not passing enough exhaust gas to be an issue. If you go back to C5, nearly all the mufflers had a single pipe in the muffler. I've made over 800 RWHP on the stock 2 1/2" Ti mufflers. I never saw any significant difference between a 3" and 2 1/2" muffer on my dyno.
Anyway, if one was really worried about this, it wouldn't be hard to "Y" the inside pipe to the outside one inside the muffler. I may, at some point, dyno it before and after doing the "Y" inside, just to prove the point.
I'm just saying that this mod costs literally nothing, in comparison to $4000 PLUS a lot of labor for an aftermarket system. You don't even need to pull the muffler off to do it.
The tips were about $300, as I remember. I found them through one of the BMW Forums.

 

Yep agree. The pic that GMAC1 posted on this Thread shows 2 things I had not seen clearly before:

1. The X Pipe connection is clear, which does affect sound and power
2. That there are 4 holes not two per side on the NPP pipe that when the valve is closed cause all exhaust to go into the muffler outer shell and into the short sections of pipes that lead to the inner pair of exhaust tips.

Also agree that would be foolish to mess with the first pair of valves that are before the muffler as they are only "Partially Closed" when in V4 mode. In the C7 messing with those would cause a CEL. They are fully open when in V8 mode.

 

I will be curious on the non NPP exhaust how the internal is designed and if it can be easily modded? I think modded OEM and high flow CAT's would sound amazing. I will still go aftermarket but while having an non NPP might tear it open and look at it but this will be a good while (Still awaiting for allocation).

 

It is amazing how quiet Chevy has gotten the C8 with this system. It’s as mild as some caddikacs or Mercedes. But I want it to sound like the mid engine super car it is. Is $3000 and a computer relearn the only way? I ran muffler less on my C4, sounded great, but very mellow. I guess cats make good sound absorbers.