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Purpose of mufflers #: 1472

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Posted: Wed Jan 9th, 2019 10:34 pm

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Eddie Money
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I read MadMaxx's maintenance log a while back (Disclamer: lol. it was a long while back and I've read tons of truck threads since so I'm not quoting him incase I remember wrong) and found the exhaust modifications interesting. It got me thinking....

Pre emissions vehicles had mufflers and very little of anything else. Post emission there is lots of high tech stuff to reduce vehicle pollution, but is a muffler part of that equation?

Besides reducing the engine noise to a legal decibel level, what else does a muffler do in regards to vehicle emmissions, anything?

The reason I ask is if you remove the 3 pre cat system and run dual exhaust with cats like MadMaxx did, do you need the muffler for anything other than the reducing the decibel level?

I think MadMaxx deleted his and added cherry bombs later. I would think they would have made it louder but I think he said it quieted it down a little and he likes it. I got the impression that if you warmed the truck up on your way to and in line at the emission testing site it would test fine as long as it wasnt too loud.

So what's a muffler do besides make it quieter?

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Posted: Thu Jan 10th, 2019 01:35 pm

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JAMMAN

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Moved to exhaust tech section.

Mufflers used to provide back pressure also, stopped backfiring during deceleration among other things, haven't really read up on it in a while.

Mufflers are noisy without cats trust me. Gutted all 5 cats in my oldest sons 02 and it was LOUD.

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Posted: Thu Jan 10th, 2019 01:53 pm

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Thanks JAMMAN. Thought I posted it there. Lol. I read up on mufflers last night out of curiosity. I have a bad habbit of posting then googling. Sorry.

Mufflers are used to reduce noise by baffling or acoustic chambers mostly. Some say they are also used to reduce and cool exhaust gasses away from passengers. But its main purpose is sound reduction which can result in an increase or reduction in back pressure as well.

After reading I discovered cherry bombs are a muffler but I've usually seen them used like a resonator to make the exhaust louder.

Do rangers have an exhaust resonator?

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Posted: Thu Jan 10th, 2019 02:21 pm

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I used cherry bomb glass packs without any other mufflers on my last Mustang & I liked the sound. My Ranger does not have any resonator in the exhaust.

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Posted: Thu Jan 10th, 2019 09:25 pm

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Cherry bombs were the go-to mufflers, back a number of years, but to me too loud, I always preferred the Turbo Mufflers, i.e. DynoMax is my choice !

There are a number of mufflers that reduce sound through acoustic silencing, but reduction is key... not eliminate the back pressure.

Back pressure is needed for an internal combustion engine to operate properly on the street and mufflers do that !

So yes, noise reduction is the main purpose of the muffler (that's why they are called mufflers) !

BUT decide carefully !

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Posted: Fri Jan 11th, 2019 10:28 am

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The concept of engines needing "back pressure" in the exhaust is erroneous - they will perform better with low restrictions in the exhaust system.

Basically the silencer is only there to quieten the engine to acceptable levels - straight through silencers present less resistance in the system than baffled silencers and are to be preferred.

System design is complex - the best systems use reflected waves of lower pressure at critical rpms to enhance cylinder scavenging during the exhaust cycle - these waves are generated wherever the exhaust path increases in volume - so a silencer / cat / end of pipe will generate a wave which returns to the ports in the head - ideally the wave(s) from one cylinder arrives back just as another exhaust valve is opening.

Motor manufacturers tend to get this right for their average customer but this may not suit tuned engines or those run at other than normal rpms. If you change the exhaust system or state of tune of the engine you may well upset the power delivery.

Silencers without cats don't have to be noisy at all - I run a single large straight through silencer at the very back of my MGB V8 and even with 3.6 litres & 200 bhp it has a quiet burble which is very pleasant!

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Posted: Fri Jan 11th, 2019 01:59 pm

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Gonna have to respectfully disagree about back pressure being unnecessary. Back pressure is precisely what creates the scavenging effect. A little, not a lot. Reflection of sound waves generated in the exhaust system is indeed a large factor in how a muffler can work without being too restrictive, but it is unrealistic to expect any of the waves to travel against the newly formed waves and exhaust pressure to reach all the way to the head in order to have anything to do with exhaust gas scavenging from the combustion chamber. Every exhaust designer that I have ever heard of will speak about the importance of some back pressure at the head being necessary.

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Posted: Fri Jan 11th, 2019 06:01 pm

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Plenty of articles on the web debunking that theory.

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Posted: Sat Jan 12th, 2019 02:45 pm

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- My only thought about back pressure is this... for a stock engine it is necessary to maintain the proper mixture of gases within the cylinder so not to allow raw gas to escape, while allowing full scavenging of the burnt gases. This is achieved by exhaust gas back pressure, how you get there I can't go into, too stupid !

For high performance, high rpm engines, that are running long duration, high lift cams, and do not have a care in the world about fuel economy, then the free flowing exhaust comes in play. There is a point that too much exhaust, too large exhaust valves, too much overlap in the Intake and exhaust valves, causes the fuel mixture to de-atomize, and flow out of the exhaust, not stay where it is needed.

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Last edited on Sat Jan 12th, 2019 02:47 pm by Scrambler82

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Posted: Sun Jan 13th, 2019 04:42 am

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I can't find any cam specs for Ranger engines except that 3.0 cams have an overlap of 47degrees.

Assuming that the overlap is symmetrical about TDC then it means that the inlet valve opens around 24 degrees before TDC as the piston is close to the head and the vast majority of the exhaust gases have left the cylinder - the momentum of the gasses passing into the manifold will continue to pull gas out of the cylinder even though the piston has pretty well stopped pushing.

Opening the inlet valve at this time does two things - it allows the exhaust gases to continue flowing under momentum and it allows the fresh fuel mixture to be sucked in under the now lower than inlet manifold pressure that exists in the cylinder.

As the piston passes over TDC the chamber is at its smallest and the last of the exhaust gasses is being replaced by fresh mixture - closing the exhaust valve is timed to coincide with the fresh mixture reaching it at around 24 degrees after TDC, just as the piston is starting its induction stroke.

So the valve overlap actually helps fill the cylinder with the mixture for the next power stroke.

The rate that the exhaust gasses leave is proportional to the difference in pressure between the cylinder and the manifold - anything that can be done to reduce the manifold pressure will help engine efficiency - absence of exhaust system restrictions and tuned lengths to generate reflected pressure waves both do this.

So "back pressure" is making the engine less efficient than it could be and is largely generated by poor manifold design (cheaper to produce than properly tuned ones) and by the need to quieten exhaust noise (efficient noise reduction causes restriction in the system).

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