Hi bros,
Here's my little concept~
http://i53.photobucket.com/albums/g61/ken12292703/My%20Exhaust%20Prototype%20Valve/1.jpg
Example of exhaust
http://i53.photobucket.com/albums/g61/ken12292703/My%20Exhaust%20Prototype%20Valve/2.jpg
Another view
http://i53.photobucket.com/albums/g61/ken12292703/My%20Exhaust%20Prototype%20Valve/3.jpg
Closer view
http://i53.photobucket.com/albums/g61/ken12292703/My%20Exhaust%20Prototype%20Valve/4.jpg
This simple plate is needed, don't have to fabricate the exact one. Similar that can be sticked by magnet. Use alluminium also can, make sure you attach a metal plate for magnet to stick on.
http://i53.photobucket.com/albums/g61/ken12292703/My%20Exhaust%20Prototype%20Valve/7.jpg
Idle or slow throttle.
http://i53.photobucket.com/albums/g61/ken12292703/My%20Exhaust%20Prototype%20Valve/8.jpg
Another view of it
http://i53.photobucket.com/albums/g61/ken12292703/My%20Exhaust%20Prototype%20Valve/5.jpg
This is how it works. The magnet at the bottom sticks the plate at a certain amount of force. When you open throttle more, back pressure increase that overcome the magnet force hence release the plate. For the top, just use a simple door hinge or flange. Magnet? Use those closet magnet or any magnet you can find. Hard disk magnet works as well~ The stronger the magnet, more back pressure needed to open it.
http://i53.photobucket.com/albums/g61/ken12292703/My%20Exhaust%20Prototype%20Valve/6.jpg
Full throtle.
This concept might look ugly, it's just a concept for cheap and simple noise control for "aahheemmm" that can save you $$$.
I don't have the time to do the real thing yet. These are my drawings, not copyrighted, copy or share with others, not a problem.