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Discussion Starter #1
So I'm kinda bored waiting for my spoiler to arrive from China so I need another project to work on while I wait. After browsing some of the performance car sites like Renntech, I noticed the module to manually control the exhaust valve. I'd like to have this option but I'd like to have a button in the dash or somewhere inside the car. I don't need another remote to carry around or lose. Also, I don't want to pay $580 for the option. Therefore, I decided to figure out how the exhaust valve works and how to control it myself. My first video explains (very simply) how it works and how the RENNtech module works. The system is not quite as simple as I explain in the video but the concept is correct.

While doing my research, I noticed that Wavetech has a video on how to disable the exhaust valve and force it to stay open. It is a good video and a good way to keep the exhaust valve open. Keeping the exhaust valve open let's exhaust gas flow out the passenger side tail pipe and gives you the performance exhaust sound 24/7. The downside of his method is that you are susceptible to throwing a CEL code / error code. There are some people who have experienced this when using his method. It is very common to get error codes when sensor, electric motors, accessories etc. are disconnected on the vehicle. You may not get an error on your dash but your ECU will think there's a problem with the item which was disconnected and will make adjustments because of it (like reducing power). In a future video, I will show you how to keep the exhaust valve open without disconnecting the connector. You just need to remove the metal coil piece which looks like a spring.

This first video is just a proof of concept and my next step will be to detail the process to implement the concept into my car. Hopefully you guys are willing to give me some feedback/opinions on how I should do it. For example, I need a place for the toggle switch. My best option is in the ash tray. I like that it will be concealed and that my wife won't ask me about it because neither of us smoke.

Anyways, take a look and let me know what you guys think.

Gruesse
 

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Discussion Starter #2 (Edited)
Here's what I've learned so far:
  • The exhaust system components for all AMG's in the same model year are identical. For example, a 2016 CLA45 amg without the U21 Performance exhaust is identical to a 2016 CLA45 amg with the U21 option. The only physical difference between the two cars is the sound generator which is connected to the air intake system and pumps out 'sounds' into the passenger cabin.
  • The sound generator is mechanical in nature but there is a sound generator valve located right next to the generator which opens and closes to allow sounds into the passenger cabin.
  • The sound generator, hosing and valve are located between the engine and firewall so it would be very difficult to add the U21 option to a car without it. Also, the car would need to be reprogrammed to use the sound generator and would also probably need a new wiring harness
  • The exhaust valve near the end of the tail pipe is found on all CLA/GLA amg vehicles. This information come from Mercedes original service and build manuals. It would be nice if the owners here could verify this for me. The valve is easy to see if you look through your tailpipe on the passenger side.
  • The exhaust valve is made up of essentially three parts: the valve inside the tailpipe, a metal part that looks like a coil spring, and an electric motor. The metal 'coil spring' is not really a spring because it doesn't hold the valve open or closed, it just connects the motor to the valve.
  • Removing the 'coil spring' will allow the valve to move freely but there should be enough friction to hold it in place (I have not tested this yet). I will look into a solution to be able to hold it open mechanically. The vehicle will not see any problems or get error codes because the motor is still functioning normally. The motor is just not opening or closing the valve.
  • If you disconnect the exhaust valve electric connector and run the vehicle without this connection, the valve will stay open assuming you disconnected the connector while the valve was open. It's unlikely you will get an error code on your dash but the vehicle may notice that the valve is disconnected and think it's jammed and therefore reduce power.
  • The opening and closing of the valve can be controlled via a switch. Opening the switch (and the circuit) will definitely open the valve but may not necessarily close the valve. I don't think the vehicle will detect anything wrong with the valve but I'm not sure and I need to do some more investigating.
I plan to supply photos, physical and electrical diagrams and more details in the near future. PLease let me know if you have any questions and let me know if you have a valve in your passenger tailpipe.
Jim
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Ive done a decent amount of research into the performance exhaust myself.

So if I follow correctly, the button on the dash turns on/off the valve to the sound generator (which works as a mechanical amplifier like Porsche’s system too). There is a separate exhaust valve (flap) in the tailpipe that is controlled via driving mode, RPM’s, temp, and probably a few other things. This valve is probably closed in Comfort mode and you want to open it 100% of the time?

One thing though, I’ve had two GLA’s.. one with the sport system and one with the performance and there is a difference in the two other than the sound generator button on the dash... I reallllllly wanted the performance exhaust on my first one so looked into it pretty extensively. The sport system has two pretty big resonators in the mid section and the performance only has one small skinny one.

From the rear end the sound of the two systems are quite different from each other, the sport system has quieter pops/burbles and if you rev it at park it’s like more deep/low tones. When under load and letting off it was a lot easier to get to burble but it’d be pretty low tones. The performance doesn’t burble, it pops and CRACKS. Revs at park sound a lot more loud and snappy and usually have like 2-3 loud cracks. Under load the performance exhaust drowns out the sport system (I’ve done pulls with a CLA45 w/ perf exhaust and my GLA45 w/ sport exhaust) and the crack on the upshift borderlines a gunshot.

I think more than changing the valve to be open all the time, getting a louder more “performance” exhaust note would come with a full res delete and/or backbox mod from a trusted exhaust shop. Of course you can always throw on a downpipe and get a light tune too.
 

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how do you find out if you have the sport one for performance one? is one with exhaust button and one without? any code we can identify via vin and lastvin.com? thanks.
 

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If you have a button on your dash below the display that looks like binoculars that is the sport exhaust button and you have it on your car. Absent that switch you do not have out. In 2017 in the USA market the code for Sport Exhaust was U21 (Performance Exhaust).
 

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Yeah if you see U21 on lastvin.com that’s the indicator for performance exhaust. It can be confusing because sometimes (like part schematics) the two systems are referred to as “non-sport” vs “sport” system and other times (sales/marketing) they get referred to as “AMG Sport” vs “AMG Performance” so on lastvin the “U21 - Sport Exhaust System” actually means you have the performance
 

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And why do we want the valve open again? i had it unplugged valve to keep open all the time for 2 years or so, no cell light on dash, but if you scan it , it will show up ,something up, and it pulls power down which is not worth it to me just for the sound. You can see in video how it works, so why mess with it, its more to it than just sounds.
Also valve is NOT gonna stay open if you remove the spring, it will close on its own due to exhaust pressure moving it. it did it on mine,so keep spring.
C
 

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for more sounds put down pipe, works! i have 200 cell and its louder, noticed right away. didnt even know its gonna be louder.
 

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Discussion Starter #12
finally finished my switches to toggle my exhaust valve and 'U21" valve on and off. I took the ashtray out and mounted the toggle switches there. It's a pretty easy job, just time consuming. I didn't get a chance to the test the U21 valve but I definitely heard a noticeable difference in the exhaust valve especially at 1k to 3k rpms. I put the car in manual mode, sport+ and cruise control at about 3k rpm. I have an audio recording of me opening and closing the valve. If you listen to the recording with ear buds, you probably won't even hear a difference. You need to listen on a system with decent bass. And that is exactly how I would describe the difference in sound - it's not really louder but it is like you turning the exhaust 'bass' from low to high if that makes sense. Anyway, it sounds a lot better.
sound file is on youtube:
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Didn't know about this post, but started another regarding the same subject. I was curious why the dash button seemed to not activate the exhaust flap like as if it was by wire, and wrong guessed it might be a function of intake vacuum. I see it is governed by computer now. Seems strangely overly complicated, and such a huge engineering effort for so little gains in sound and performance. A more reasonable approach might have been to have it open in constant state during sport+, and or over-ridden by the dash button. I do love the car as is, and down the road I may opted for a high performance exhaust system with higher flow and more control over the rear flap, but for now, the car continues to be a joy to drive.

Wayne, if you feel it would make sense to omit my post or add it to this one that would be great!
 

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noice, now need a wife proof ashtray cover......
finally finished my switches to toggle my exhaust valve and 'U21" valve on and off. I took the ashtray out and mounted the toggle switches there. It's a pretty easy job, just time consuming. I didn't get a chance to the test the U21 valve but I definitely heard a noticeable difference in the exhaust valve especially at 1k to 3k rpms. I put the car in manual mode, sport+ and cruise control at about 3k rpm. I have an audio recording of me opening and closing the valve. If you listen to the recording with ear buds, you probably won't even hear a difference. You need to listen on a system with decent bass. And that is exactly how I would describe the difference in sound - it's not really louder but it is like you turning the exhaust 'bass' from low to high if that makes sense. Anyway, it sounds a lot better.
sound file is on youtube:
View attachment 25618
 

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Hi sorry if I’m awkward my 1st post
So I have my 2017 GLA45 and wondering what exhaust I have - I don’t have the button in dash and I can’t see any U21 code in VIN decoder:does that mean I have the AMG exhaust not the Sport exhaust - I believe these are the official designations Thanks

also can I upgrade my exhaust what’s the best option for a more aggressive soundtrack
 

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Hi sorry if I’m awkward my 1st post
So I have my 2017 GLA45 and wondering what exhaust I have - I don’t have the button in dash and I can’t see any U21 code in VIN decoder:does that mean I have the AMG exhaust not the Sport exhaust - I believe these are the official designations Thanks

also can I upgrade my exhaust what’s the best option for a more aggressive soundtrack
Sounds like you have the AMG sport system, not the upgraded AMG performance system. Best thing to do for more noise would be downpipe imo. If you have specific questions feel free to PM
 

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Discussion Starter #18
I’ve never heard what gla45 sounds like with a high flow or catless downpipe but i previously owned a wrx in which I had two different catless downpipes on it. I didn’t like the change in sound. The exhaust sound wasn’t much different but the engine sound changed significantly. You could hear the turbo more but it really didn’t sound that great. I’d rather hear a deep exhaust tone that sounds coming from the engine.
My next project will be installing one of these (electric exhaust cutout). I want to wake up the neighbors!
 

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I had a renntech sport downpipe. It sounded great, a little louder, but with lots of pops and burbles. I got a CEL, so switched back to stock. The downpipe only adds about 10 HP, so not a significant difference.
 

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If you have a button on your dash below the display that looks like binoculars that is the sport exhaust button and you have it on your car. Absent that switch you do not have out. In 2017 in the USA market the code for Sport Exhaust was U21 (Performance Exhaust).
Hm, I was checking my car and it says I have U21, but I don't have the exhaust button on the dash though. This is a 2015 model.
Does that mean I have the upgraded exhaust or the normal one?
 
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