# Why do we need mufflers/silencers on cars? **URL:** \ **Category:** Factual Questions\ **Created:** [May 12, 2009, 5:23am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140 "2009-05-12T05:23:10Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Chickenwrangler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chickenwrangler/32/91_2.png) [@Chickenwrangler](https://boards.straightdope.com/u/Chickenwrangler)\ **Post date:** [May 12, 2009, 5:23am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/1 "2009-05-12T05:23:10Z")
Well not just on cars but all internal combustion engines. The normal ICE is 4 stroke with the ignition stroke being the the one that makes the bang, yet at that point of the cycle the valves are closed and the cylinder effectively sealed so how does the sound get out? From the point of ignition the piston travels down the cylinder and is on it’s way up again before the exhaust valve opens and lets the exhaust gas and sound escape so has the sound of the ignition bang been waiting to be let out? If it’s possible to contain sound this way the possibilities are endless, so why don’t we have tins of canned sound effects available? 😃 ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [May 12, 2009, 5:45am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/2 "2009-05-12T05:45:22Z")
The cylinders are sealed in terms of _gas_ not getting out, but that that doesn’t mean that _sound_ can’t get out. You do realise that sound can travel through solids, including the metal your engine is made out of? ---
**Author:** ![Crazyhorse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crazyhorse/32/31_2.png) [@Crazyhorse](https://boards.straightdope.com/u/Crazyhorse)\ **Post date:** [May 12, 2009, 5:58am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/3 "2009-05-12T05:58:11Z")
> [@Chickenwrangler](#): > > From the point of ignition the piston travels down the cylinder and is on it’s way up again before the exhaust valve opens and lets the exhaust gas and sound escape so has the sound of the ignition bang been waiting to be let out? It is the sound of pressurized gas that has been waiting to be let out colliding with the lower pressure gas in the exhaust pipe. > [@Chickenwrangler](#): > > If it’s possible to contain sound this way the possibilities are endless, so why don’t we have tins of canned sound effects available? 😃 I have some of those! These soda cans I have here go “Wshhhhhtt” every time I open one exactly the same reason. 🙂 ---
**Author:** ![ivn1188](https://avatars.discourse-cdn.com/v4/letter/i/e480ec/32.png) [@ivn1188](https://boards.straightdope.com/u/ivn1188)\ **Post date:** [May 12, 2009, 6:01am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/4 "2009-05-12T06:01:19Z")
I ran my car ('75 MGB) with a straight 2 foot pipe one time. It’s a tiny little 100HP four cylinder engine. The level of sound coming out of the engine was the kind of noise you _feel_ – it was unbelievably loud. Pulses of exhaust gas come out of your engine 1500 or more times per minute, and the gas is pressurized when it is ejected into the manifold. A good portion of the sound is a bunch of hot gas shooting out of the engine several times a second, not just the sound of the fuel igniting. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [May 12, 2009, 6:04am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/5 "2009-05-12T06:04:50Z")
I don’t think that sound escaping through the solid of the engine is the issue, since if it were putting a muffler on the exhaust wouldn’t muffle. The valves may be closed when each cylinder fires, but when they open compressed gas escapes violently. A series of violent escapes of gas is called “a loud noise”. ---
**Author:** ![brad\_d](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@brad\_d](https://boards.straightdope.com/u/brad_d)\ **Post date:** [May 12, 2009, 7:06am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/6 "2009-05-12T07:06:57Z")
I’m under the impression that the combustion itself is relatively quiet - detonation (which you can hear from the driver’s seat) is a sign that something is amiss. Rather, like **Princhester** and others have said: the loud sound out of the exhaust pipe comes from the exhaust valve opening. There’s still a considerable pressure difference between the just-combusted gases in the cylinder and those in the exhaust manifold, and each time the exhaust valve begins to open there’s a loud “pop”. Figuring on the fly… (Engine rotational frequency) \* (number of cylinders) / 2 = (frequency of exhaust note) …the factor of two is because (on a four-stroke engine) there’s only a combustion event every other revolution of the crankshaft. Just watch units - engine speed is usually measured in revolutions per minute, while sounds are typically quoted in cycles per second; account for this appropriately when performing the calculation and it ought to work. I think…? ---
**Author:** ![Tenebras](https://avatars.discourse-cdn.com/v4/letter/t/977dab/32.png) [@Tenebras](https://boards.straightdope.com/u/Tenebras)\ **Post date:** [May 12, 2009, 7:07am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/7 "2009-05-12T07:07:53Z")
Think about the biggest, loudest most annoyingly ear splitting motorcycle you’ve ever heard. The engine in that bike was smaller than the engine in a Volkswagon Beetle. Not the new sleek and plastic Beetles. The old ones that hippies drove around in. The engine that you and your friend Steve could take out of the car and carry into your parents’ basement to tinker with. ---
**Author:** ![Deereman](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@Deereman](https://boards.straightdope.com/u/Deereman)\ **Post date:** [May 12, 2009, 7:49am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/8 "2009-05-12T07:49:04Z")
Why do engines need mufflers? hee hee, here’s a good example of an engine with no restriction in the exhaust at all! :eek: WARNING…link to a short video > **[Photo Storage | Photobucket](https://s108.photobucket.com/albums/n8/weirddeere/DIproject/Plowing2/?action=view¤t=100_7739.flv)** > > Store your photos and videos online with secure storage from Photobucket. Available on iOS, Android and desktop. Securely backup your memories and sign up today! This engine is 501 cubic inches…the same size as high performance cars from the 60s/70s. I love earplugs… ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [May 12, 2009, 8:29am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/9 "2009-05-12T08:29:32Z")
> [@Chickenwrangler](#): > > The normal ICE is 4 stroke with the ignition stroke being the the one that makes the bang, yet at that point of the cycle the valves are closed and the cylinder effectively sealed so how does the sound get out? That’s right, when the ignition in the cylinder takes place both intake and exhaust valves are closed so there’s no sound coming out except what little travels through the solid which isn’t much since there is a lot of metal to absorb it and the elastic engine mounts will kill the rest. Around the time the cylinder reaches bottom dead center (BDC) the exhaust valves start to open. At that time the exhaust gasses still possess quite a lot of energy (pressure) and to make things worse they are forced out by the movement of the cylinder. For a comparison, piston-type air compressors make a helluva noise and there’s no combustion taking place. It is just the rapid movement of air that causes it. Also for the same reason there’s quite some noise from the intake side of the engine. Drive a car around with the filter box removed and it becomes rather loud. ---
**Author:** ![Deereman](https://avatars.discourse-cdn.com/v4/letter/d/a3d4f5/32.png) [@Deereman](https://boards.straightdope.com/u/Deereman)\ **Post date:** [May 12, 2009, 9:01am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/10 "2009-05-12T09:01:50Z")
I guess I could actually add info to this from my own experience… When cranking over an engine slowly you can hear the different sounds of intake, compression, ignition, and exhaust… Intake, you can hear the, well, sucking, intake of air into the engine…depending on the engine, you can make out this sucking noise as it runs. Well, you can’t really hear the compression… but ignition is audible…on a diesel it can be a very distinct “knock” or “rattle” inside…though it is loud, that is not the sound you hear out of an exhaust. When cranking I can feel the release of pressure as the exhaust valve opens, and that release of pressurized air makes a VERY audible distinct “POP” out the exhaust…when the engine is actually running on it’s own, the "pop"s all blend together into a smooth roar. ---
**Author:** ![Balthisar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/balthisar/32/2825_2.png) [@Balthisar](https://boards.straightdope.com/u/Balthisar)\ **Post date:** [May 12, 2009, 11:35am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/11 "2009-05-12T11:35:05Z")
> [@Tenebras](#): > > The engine in that bike was smaller than the engine in a Volkswagon Beetle. Not the new sleek and plastic Beetles. The old ones that hippies drove around in. The engine that you and your friend Steve could take out of the car and carry into your parents’ basement to tinker with. I wish Steve had helped me. I did it all myself. 🆒 ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [May 12, 2009, 12:24pm UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/12 "2009-05-12T12:24:56Z")
The sound of one valve opening and releasing gas is essentially the same as a fired gun, right? ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [May 12, 2009, 1:16pm UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/13 "2009-05-12T13:16:34Z")
Exhaust systems have other purposes besides reducing noise. Here’s a video of a [racing chainsaw](http://www.youtube.com/watch?v=L6gEKsCRzcU) which obtains extra power due to a large “tuned” exhaust system. ---
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [May 12, 2009, 10:38pm UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/14 "2009-05-12T22:38:57Z")
> [@Sapo](#): > > The sound of one valve opening and releasing gas is essentially the same as a fired gun, right? like a balloon popping. ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [May 12, 2009, 11:26pm UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/15 "2009-05-12T23:26:54Z")
> [@Xema](#): > > Exhaust systems have other purposes besides reducing noise. Here’s a video of a [racing chainsaw](http://www.youtube.com/watch?v=L6gEKsCRzcU) which obtains extra power due to a large “tuned” exhaust system. That’s true. Two stroke engines (like that chainsaw) are very dependent on the backpressure provided by the exhaust because they do not have valves to keep the fuel from escaping to the atmosphere. Without an exhaust the engine won’t work at all. There’s a nice animation showing the operation here: [Two-stroke engine - Wikipedia](http://en.wikipedia.org/wiki/Two-stroke#Different_two-stroke_design_types) 4 stroke engines are not that sensitive but still removing the exhaust completely will alter the power band characteristics. Usually it will hurt performance at low to mid RPM (due to fuel escaping but for different reasons compared to 2-stroke engines) but it will improve at high RPM. ---
**Author:** ![Chickenwrangler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chickenwrangler/32/91_2.png) [@Chickenwrangler](https://boards.straightdope.com/u/Chickenwrangler)\ **Post date:** [May 13, 2009, 2:45am UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/16 "2009-05-13T02:45:50Z")
Cheers! Another mystery fully explained in true SDMB tradition Thanks 🙂 ---
**Author:** ![sich\_hinaufwinden](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@sich\_hinaufwinden](https://boards.straightdope.com/u/sich_hinaufwinden)\ **Post date:** [May 13, 2009, 4:05pm UTC](https://boards.straightdope.com/t/why-do-we-need-mufflers-silencers-on-cars/496140/17 "2009-05-13T16:05:43Z")
> [@Sapo](#): > > The sound of one valve opening and releasing gas is essentially the same as a fired gun, right? “The correct answer is Mooooo, uttered in a sort of plaintive bellow.” > **[What is the sound of one hand clapping?](https://www.straightdope.com/21341215/what-is-the-sound-of-one-hand-clapping)** Sorry - first thing I thought of. But it got me wondering if a cylinder was fired in the forest and no one was around to hear it, would it make a noise. (For the purposes of this thought experiment I don’t believe it’s relevant whether or not the cylinder is on a treadmill.)