The stock Corvette exhaust is way too quiet, and aftermarket mufflers are too expensive, too heavy, or too loud... Fortunately for us Chevrolet saw it fit to retrofit some of their vehicles with the clever NPP Bi-Modal exhaust that is loud when you want it, but quiet when you don't... Here is the easiest, quickest, cheapest way to get the NPP action in a car that was not fitted with it in the first place.

First you need NPP mufflers, of course. I bought a pair of C6 Z06 mufflers for about $350 and run them with my Kooks 1 3/4" headers and a 3" catted X-Pipe. Before them I ran GHL exhaust (intolerable drone), then Corsa Sports (raspy, and 22lbs heavier than the GHLs). Before today I was running them open all the time, and the drone was really starting to get to me.

For this, all you need is:

16 feet of 5/32" rubber hose ($15)
2 3/16" vacuum "T"s
1hour or so of your time

First connect the two butterfly actuators via a rubber hose into a 3/16"T". Make sure to loop the hose between the bumper beam an the muffler bracket, so it does not hang down:



Use zip ties to make double sure that these hoses can not pop off. You will have a vacuum leak if they do:



Connect a 15 foot long hose to the T, and loop it through the frame, heading into the rear driver side wheel well:



Loop the hose around the fuel tank filler line, then down towards the drivers side door sill. This ensures that the hose stays flush with the wheel well. N/A cars won't have the red Boost-A-Pump there, but fitment is the same:



Use a wire, or something similar, to snake the hose on to the front drivers side wheel well, then up along the windshield washer reservoir, and out by the brake booster:



"T" into a vacuum line and connect this line into it. Because of my supercharger kit, my brake booster line is "T"ed for the blow off valve and the Kenne Bell Boost a pump pressure sensor; I simply tied it in there.



THAT'S IT!!! $25 and about an hour of easy hose routing...
Now, how does it work?
It is very simple: The muffler butterflies are actuated by engine vacuum; when there is vacuum on the intake manifold, they close, quieting the muffler down. When the intake manifold vacuum goes away, they open, making the car loud.
Engine vacuum is proportional to throttle opening; open throttle = no vacuum (or boost in my case), closed throttle = vacuum.

On engine startup, they are open; the engine comes to life with them on loud then they close up and the car quiets down a split second later.
On cruise control, or light cruise, they are closed.
Around 50% throttle they begin to open progressively, and the car becomes louder with increasing throttle application.
Around 75% throttle they go full open an the car is as loud as it is going to be.

Want them open all the time? No problem: just pull the muffler hose off the "T" and cap it off:



Or simply crimp the line so the vacuum can't pass through

Here is a quick video showing the mufflers in action:

http://www.youtube.com/watch?v=1nx3nmWapgw

 

Here is what I like and what I dislike about this system:

Dyno testing shows that a supercharged C6 stands to lose as much as 40HP+ with the butterflies closed. For me, a simple system that has them all open or all closed is not acceptable because I need them fully open no matter what when I go full throttle, otherwise exhaust backpressure will be excessive.

PROS:
Car starts up with them open, all the time. I wouldn't want it any other way.
Butterflies are always open at high throttle settings, so the mufflers never restrict engine horsepower when I am trying to make power.
Light cruise is now QUIET!!!
No drone!
No need for me to do anything; they open when I want power, they close when I drive mildly.
Simple, light, reliable system.
CHEAP!

CONS:
Deceleration burble is just about gone, since the mufflers are always closed on decel, no matter what.
I have to get out of the car if I want them open all the time.
My car is supercharged, so at WOT, I am actually applying pressure to them.. I believe they will be OK with that, but that remains to be seen.

Overall I am very happy with the way it worked out, and just wanted to share it with the forum... Enjoy!

 

Nice effort , but why not just pop for the Mild2Wild control ? Sure its abou $90 , but a whole lot easier and convienient with the remote control .....

 

Sorry , take that back I see you did a retro-fit .

 

I just made a new video today that does a much better job of illustrating how the system works, and how quickly the flappers move... Enjoy:

http://www.youtube.com/watch?v=nhL6Y...e=channel_page

 

What I like about Sam's system, is operated based on the state of the engine. My question is, Is Mild2Wild changes automatically as the vacuum changes or do you have to manually turn it on and off with a switch?

Thanks!

 

Sounds real good... I leave it open all the time

 

No M2W has a switch so you can leave it either ON, or OFF, manually...
Still a very neat system, but at these power levels you just lose too much by leaving it off, and I wouldn't want to be switching it back and forth every time I wanted to romp on it...

 

I had them open all the time for 2 months, but the highway drone really started getting to me. I am so much happier with the system the way it is now... Its perfect!

 

One of the M2W systems as a RPM controller so they open automatically at a certain RPM or you can use the button to open them manually. But I like the start up wide open. sounds goooooooood.

 

Good post, nice job Sam. I think the jury is still out about a FI car losing 40HP on a closed system though.

 

Sure there are other controllers out there but they seem like overengineered solutions (hence needlessly expensive) for a very easy task.

This is simple and cheap and seems to work great. Thanks for sharing!

 

I am testing that, and the 1 3/4" vs 1 7/8" on my next trip to the dyno. I also find it VERY difficult to believe, but Rock'n'Blue08's test of that (open vs closed butterflies) looked pretty legit?

 

I'd be very surprised to see more than a 5hp difference with the mufflers open to closed (unless the car is tuned more aggressively). Looking forward to your results.

 

Sam,

I went from the SLP Powerflo to Corsas that I purchased from you. Without changing anything but the mufflers, my car leaned out enough for it throw codes and go into "limp mode." I could not believe the difference in just an axle back setup.

Also, I started with the 1 7/8" headers because I've learned the more air you can move through and out of the engine, the more power you can make (especially, boosted applications). I am still curious to know the difference, though.

 

Great post!!! I wonder if this would work on the 07 zo6 bi-mode mufflers i put on my 06 a6??? Not sure of difference between bi mode and npp.

 

Exact same thing. These are Z06 mufflers that I am using, as stated on the first post.

 

No kidding? They flowed so much more that your car leaned out?? Wow! Did you dyno it before/after? That would be really intertesting if you did...

 

Hind's sight 20/20, I should have dynoed it with the new mufflers, but I was under time constraints with getting the remainder of the exhaust installed.

Right before the dash lit up like a Christmas tree, you could hear a pinging noise from the engine. As soon as that happened, here came "limp mode." I had to pull over, shut off the car, and wait about 1 minute before I could restart normally.

 

run 14 closed 15 open. run16 closed Run14 and 16 are only ones where the flaps stayed closed!