sfrank2886

Ok to start off, I love the sound of my Npp at full throttle, but they are just to loud under normal crusing, even in the closed position. I also have uncatted kooks x pipe. What are my best options to make it a bit quieter.

1. Try to switch to high flow cats w x pipe.
2. Maybe try Corsa mufflers or another brand

Let me know if you have switched from uncatted to cats.

Almost wish I would have just added cut outs instead of Npp.

Does not seem like it would make sense to run the base mufflers with this set up either.

Opinions please.

Turbo6TA

I was in almost the exact situation as you ... I have a 2011 GS / A6 with the NPP mufflers.

I then installed a set of Pfadt headers with cats. During normal driving, this system was hardly any louder than what it was when stock ... However, at full throttle when the NPP valves would open, the car became [obnoxiously] loud.

While some folks might really like the extra noise at full throttle, I didn't at all.

My fix was to remove the NPP mufflers and install a set of Borla "Touring" mufflers.

The exhaust sound is now just very slightly louder at idle and during normal driving than it was with the headers and the NPP.

However, at full throttle it now sounds very nice and does not scare the crap out of other drivers around me.

I really hated to remove that expensive NPP option from my GS, but it's the only thing I could do if I wanted headers on the car and not alert every cop in town if I ever went full throttle.

Be careful when buying aftermarket mufflers .... the headers on your car may make most aftermarket mufflers drone on the highway.

I can tell you that the Borla "Touring" won't drone ... and I have heard that the quieter style Corsa mufflers won't drone either when used with headers.

Turbo6TA

BTW .... Just my opinion, but I think you made a mistake installing headers [without the cats] on you C6.

And there is very very little gain in HP when removing the cats

SnakeQ

I put ARH on my 427 vert and kept the stock exhaust from the headers back including stock NPP mufflers. Sound is louder with valves open, but about the same with valves closed (normal driving mode).

sfrank2886

Thanks for your comments and keep them coming. I have other people also claim that their system is about the same in the closed position. Mine is significantly louder. My NPP’s do not appear to be modified if anyone would do that at all.

I guess magna flows could be an option, or I could swap someone out for my Kooks non catted x to one with cats. Any takers. My system only has about 500 miles since new.

Turbo6TA

No comment on what info I provided to you ?

sfrank2886

Sorry I mentioned magna flows and meant to say borla’s as you suggested. Also, i was looking for those without cats to comment. If you have never run uncatted then your opinion is biased and I chose not to address your commment about wether or not I made a mistake by not buying an x pipe with cats. I researched it pretty well before I made my purchase and understood the pro’s and con’s. I also run headers without cats on my 74 but do not have issues with it being to loud at cruise.

I would like comments from those who have run uncatted and then changed either their NPP out or added an x pipe with cats, specifically anyone using Kooks as their cats seem to be pretty small.

Thanks everyone for your comments.

cam1969

The NPPs require manifold vacuum to keep the valves closed, wide open throttle drops the manifold vacuum and allows them to open.

One cheap fix that would be to plumb in a vacuum reservoir (basically an empty can that holds a reserve of vacuum) near the mufflers. This will make them react slower to full throttle vacuum losses. If you are just getting on the throttle for short periods, they will stay in quiet mode. Search "vacuum reservoir" on Amazon.

Another even cheaper fix is to wire the valves closed at the actuator (muffler). This will keep them closed all the time and is basically free.

Lastly, there are other options people have done to control the exhaust with a switched vacuum pump instead of the engine manifold vacuum. This would let you just switch them loud or quiet at will.

Not So Fast

My car has Kooks with 3"Xpipe w/Hiflow cats and 3"X2.5" Y pipe into the stock NPP.
I agree 100% of the loudness with them open, but closing the valve makes the sound very docile, we can carry on a conversation, listen to music, all in all pretty darned quiet,
So I don't know what you are talking about it being loud with them closed, it should be night & day difference
NSF

HOXXOH

Option 3 - earplugs

rwag

I had x-pipe with high flow cats and ARH headers noise level was not much different than stock until WOT then you could hear the difference.