[Camaro 79]

First off, thank you for everyone who gave me their insight in my other thread. I ended up deciding on the Flowmaster American Thunder NPP exhaust, and while I was initially happy with my choice, there now appears to be a major problem with the NPP functionality.

When I switch to Stealth, I get what sounds like a metal-can-like scraping or rattle. It is unbearable on cold starts and prevalent with normal driving.

One thing to note, when it is in "loud" mode, I have no issues whats so ever, so it's not due to improper installation. Does anyone have any insight?


UPDATE - Thanks to a very helpful Youtuber, I was able to find and solve the problem. It was actually the tension spring connected to the NPP Valve (the one that moves while you switch modes). It was too loose, so when it was in stealth mode the spring would rattle around. I fixed this by compressing the spring and putting a ziptie right behind it which completely solved the problem. A very happy ending and I am fired up with my exhaust now!

[ss2fast4u]

I have the same system with the same issues. I believe its the actual chevy actuator that is making the noise not the flowmaster. I also have the secondary cats removed and flowmaster midpipe and it sounds awesome.

[Camaro 79]

Originally Posted by ss2fast4u

I have the same system with the same issues. I believe its the actual chevy actuator that is making the noise not the flowmaster. I also have the secondary cats removed and flowmaster midpipe and it sounds awesome.

So I actually think that I will do that next, but I want to make sure I can still use the NPP functionality to quiet it down. Are there any solutions to valve? For what it's worth, this did not happen ever with the stock NPP.

Also, do you have any sound clips of your exhaust?

[enzia35]

You'll have to figure out if it's the flap on the muffler or the actuator that connects to the flap.

[Camaro 79]

Any guidance on that? I'm not mechanically inclined, but I would love to get this done asap.

[kropscamaro16]

hmm my afe power one with npp have no issues but i have longtubes so it doesnt make it any quiter just louder lol

[enzia35]

Hmm, there is a TSB for the actuator that some say fixes the rattle. You may try that.

https://www.camaro6.com/forums/showthread.php?t=556061

The other fix that came out shortly after the car was released made modifications to the valve in the exhaust.

[enzia35]

So no link to the video?

[bnguyen17]

I am having same issue. I called flowmaster and they are looking into it. Zip ties kind of worked for me but after paying 900 for an exhaust I wasnt expecting to have to use zip ties with install. Do you have the link to the video or have you contacted flowmaster?

[njweatherman]

I have the Flowmaster 817746 and the 81084 scavenger x pipe. Anytime the NPP valves would open and close, I would hear a sound similar to a mailbox slot opening. Once it was in stealth mode and the flap was closed, it did rattle. To eliminate both, I just pulled fuse 51 and kept it open all the time.

[97cavy22]

I was looking into this NPP flowmaster setup.....does everyone seem to get valve rattle once installed?

[njweatherman]

I had no problems with the 817746 when I pulled fuse 51 and kept the valves open all the time. However, when I installed 81084 (Scavenger X pipe with afm valves), I did hear a metallic vibration when I initially pulled into a parking spot and there were cars next to me. It only lasted a few seconds and went away. If I can do it again, I would just do the axleback and keep the stock chevy x pipe.

[KingLT1]

Originally Posted by 97cavy22

I was looking into this NPP flowmaster setup.....does everyone seem to get valve rattle once installed?

The later Axle back kits seem to have a updated spring that is very stiff and doesn't rattle. The drawback I find is that the excessive spring tension doesn't allow the valves to fully seal when closed and you get a light hiss that sounds like a leak. If I take a screw driver and push the valves closed then they seal and the hiss goes away. I think the spring tension is setup backwards on these exhausts. The Springs should assist in closing the valves so it helps keep them sealed and the NPP actuator should have to work against the spring tension to open the valves. Most that have these exhaust systems probably do not even realize this because they keep them open all the time. If I had cats I would probably do the same, but since I run catless, the exhaust is pretty loud and I like to keep the valves closed at times.

[97cavy22]

Originally Posted by KingLT1

The later Axle back kits seem to have a updated spring that is very stiff and doesn't rattle. The drawback I find is that the excessive spring tension doesn't allow the valves to fully seal when closed and you get a light hiss that sounds like a leak. If I take a screw driver and push the valves closed then they seal and the hiss goes away. I think the spring tension is setup backwards on these exhausts. The Springs should assist in closing the valves so it helps keep them sealed and the NPP actuator should have to work against the spring tension to open the valves. Most that have these exhaust systems probably do not even realize this because they keep them open all the time. If I had cats I would probably do the same, but since I run catless, the exhaust is pretty loud and I like to keep the valves closed at times.

Great info, thanks!! I really only close the valves when pulling down my street. Not too often otherwise. From what I've read the Borla setup doesn't seem to have any NPP issues I'm not just not crazy about the sound of the ATAK and I feel like the S-type will be too quiet and not worth the money. The AWE and Flowmasters sound the best in my humble opinion but AWE doesn't have the NPP option.