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Variable Exhaust Outlet

 Control your 'Sports' muffler noise output
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Free flow exhaust systems come with a glass-wool filled muffler that has a wider opening and hence increases the sound output of the car along with performance (reduced back pressure). But there are situations when you could do with out the extra power or the noise - such as when your cruising back home or when you're at a 'decent' function.

My idea is to insert something that will help you control the opening of the end-can/muffler outlet. At wide open condition it will work at peak performance while at a restricted condition it would control the noise albeit reducing power.

—di1in, May 12 2010

Some Assembly Required http://www.autocarp...Exhaust__Silencers/
[DIYMatt, May 13 2010]

Muffler size and sound http://www.f2d.dk/rules/venturimufler.htm
Outlet size proportional to sound [di1in, May 13 2010]

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       This is a bit of bad science, glass packs aren't louder because the pipes are larger, glass packs are louder by design. If you add a restriction they still won't be quiet, you will just loose the performance gain. What you need is a switchable exhaust or even better an efficient yet quiet exhaust. (-)
—MisterQED, May 12 2010
   

       This is baked by some [very] old cars owned by Jay Leno - they have a valve that you can close to quiet the engine when driving through town, or make it louder to show off.   

       You can also buy exhaust silencers for modern cars but they have to be manually inserted in the tailpipe.
—DIYMatt, May 13 2010
   

       There is supposed to be an optional "Sports Exhaust" package on some Porsche models that allows you to show off more effectively in this manner.
—zen_tom, May 13 2010
   

       Check out Warlock exhaust systems. They offer a performance muffler with an exhaust bypass outlet. Jegs also offers exhaust bypass valves that can be operated from inside the car.   

       Also what Mr.QED said. What makes exhaust quite is wave cancelation.   

       [-], baked.
—MikeD, May 13 2010
   

       Check the link for f2d.dk There, they talk about reducing muffler outlet to reduce the noise.. how come?
—di1in, May 13 2010
   

       //how come?//   

       Probably because they cater to the likes of you, and don't want to confuse their clientele with the actual workings of a muffler ... assuming they know, themselves.   

       Welcome to the Halfbakery.
—MikeD, May 13 2010
   

       //Check the link for f2d.dk There, they talk about reducing muffler outlet to reduce the noise.. how come?// This is a complicated thing to answer, but let me give you some clues.   

       Sound is pressure waves in air. Those pressure waves are created when exhaust valve opens and vents the high pressure gas into the exhaust and then the exhaust vents into the environment. Mufflers work by either dampening those waves or adding those waves together is such a way as they cancel out, but a LOT more factors are at play. Any obstruction in the flow will create reflected waves inside the exhaust. Hot exhaust contacting the pipe walls will slow down due to friction and cool off due to convection. Any restrictions will cause back pressure which lower the pressure differential across the exhaust valve. To the extent that the restrictions do not limit the flow, they will necessitate increased velocity by obstructions. This is possibly key in this reference as humans only hear well in the 3k-20k Hz range. Motors in the sizes referenced are probably running at 10s of thousands of RPMs, so it is possible that the restrictions do not just lower volumes by lowering power, but also just raise part of it above the range that people can hear it.
—MisterQED, May 13 2010
   


 

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