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Old 05-02-2009, 02:04 AM   #1 (permalink)
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Does Exhaust Noise Play a Role in Fuel Economy?

This has been bugging me for a long time. Does the amount of noise your car/truck make effect your fuel economy? I just installed a glasspack on my truck so it's louder now..

One thought is the energy. A lounder exhaust would me energy is wasted as sound, right?

My other argument is that a louder exhaust generally means a free-er flowing exhaust, which means more power. How does this translate to fuel economy?

It seems to me like a more restrictive, quieter exhaust would yeild better fuel economy. But that's just me.

Take spud guns for example, they are my other hobby. When you design a gun, you need to make the chamber and barrell the right size to have the right ratio and most efficent shot. A rant in a sentece, too short of a barrell means energy is wasted as sound and too long of a barrell means energy is wasted as friction from the potato and barrell after all the fuel is burned. You need the right balance of long-ness to use all the energy of the fuel but it can't be too long or energy will be wsted as friction.

What I'm getting at is if there is a balance of exhaust sound for optimum fuel efficiency. Can your exhaust be too long or short,, or too loud or quiet...

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Old 05-02-2009, 02:12 AM   #2 (permalink)
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My question is does exhaust noise attract hot babes looking for action? Many seem to think so.
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Old 05-02-2009, 03:09 AM   #3 (permalink)
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the spud gun idea is about right, it's about tuning your exhaust, only with your engine you can also get to large aroud, so your exhaust is kind of echoing around in the tube, instead of exiting exhaust gases have momentum and you want an exhaust system that allows that momentum to work with the engine instead of agenest it, if a large pipe was ideal then no pipe would be better, right? try running an engine without any exhaust and it will hardly run.
The only advantage that a glass pack can have is letting you know when you are stepping on the gas to hard, as it will get louder at that point, use a light foot that keeps the vehicle quite and you will quickly learn to avoid noise violation tickets and get better mileage at the same time.

Last edited by Ryland; 05-02-2009 at 09:55 AM..
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Old 05-02-2009, 07:22 AM   #4 (permalink)
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Wide open is the most efficient. Resistance is affected by the interacting factors of 1) the narrowness of the pipe and 2) the length of the pipe. More of either increases resistance.

However, a fat pipe is generally a pipe with more mass.

What I'm calling "pipe" here is anything that channels the exhaust once it leaves the engine.
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Old 05-02-2009, 12:34 PM   #5 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
My question is does exhaust noise attract hot babes...
Might be better (at least in Darwinian terms) to ask what effect your exhaust noise has on the whacked-out ex-Marine in the next lane.

Most of us, alas, never have hand grenades handy when we really need them.
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Old 05-02-2009, 03:48 PM   #6 (permalink)
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Yeah, tis a shame!
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Old 05-02-2009, 06:37 PM   #7 (permalink)
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The noise won't really influence FE at all. I say a statistic that the energy in teh soundwaves or someone yelling would take several years to produce the same energy as is in a cup of hot coffee.

dampening sound creation is probably the last place anyone will ever go to save energy(unless its only output is sound, speakers).

The trick to exhausts is you want 2 things.

1.) you want the exhaust to get away from the manifold without causing any backpressure.
2.) you don't want the exhaust to slow down in the pipes because of friction and create backpressure.

Issue one is avoided by making sure the pipes are not too large right out of the manifold. If they are the gas cools too quickly and decelerates creating(pretty marginal) backpressure.

Issue two is avoided by making the pipe as generally short as possible. On cars its got to go through cats and mufflers and then out from under the car somewhere, so its not very short.

Its pretty irrelevant unless you are running with the cat ripped out. If that is the case you want something thats just an open pipe.

Quieter doesn't necessarily mean more FE. Your muffler is a big box with baffles in it that dampens the sounds. . .but it also decelerates the air substantially. This is the same reason why silencers slow down bullets.

For a car your exhaust gases are accelerating for about 16-20 inches from the manifold. Any distance after that and they are decelerating and creating backpressure(pretty negligible). So unless you don't have a catalytic converter anymore don't worry about it as long as the exhaust gets out of the muffler.
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Old 07-11-2009, 01:53 AM   #8 (permalink)
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I'll add that not too much energy is needed to produce a loud noise, even really loud. Take a normal car stereo: you have two to four speakers, 25W-80W each, and at max you can't listen to it, unless you're a teenager. You can hear the thumping from teens' cars from hundreds of yards away, while only 100W-300W are going into making the noise (at that level it's not music anymore, whatever it is). That's only a fraction of 1hp, so no detectable difference in FE.

As was stated earlier, the FE gains come from relieving backpressure, not reducing/making more noise.
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Old 07-11-2009, 02:00 AM   #9 (permalink)
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Besides, the noise is there anyway... the original muffler didn't make less noise, it made the existing noise reflect and cancel itself.

It's not your exhaust making noise. Your exhaust directs the noise. The engine is the loud one.
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Old 07-11-2009, 06:29 AM   #10 (permalink)
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Exhaust noise comes from the positive pressure in the cylinder being suddenly released by the valve. To tune an engine as you do a potato gun, you can restrict the intake. Ideally, you'd want a short intake stroke and a long expansion stroke, but AFAIK, the Humpfrey pump is the only example of that, so we use things like early closing intake valves to minimize pumping losses. Once the sound is made, there's little hope of gaining on both silence and economy, except perhaps with an exhaust turbine.

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