Thinking of swapping mufflers but got a question pertaining to my 340

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71DodgeDemon340

71DodgeDemon340

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Hey guys i have a accurate exhaust system on my demon with the performance mufflers. I was thinking of swapping the mufflers out with a pair of gibson mwa mufflers to try and get a little unique tone as i know many people aren't running them. I have one on my single exit bronco exhaust and it sounds good, plus no drone while cruising. My main question is will a high flow muffler hurt me? Gibson says the mwa only flows 3cfm less than.a straight pipe, im running 2 1/4 size duals all the way back. This 340 is pretty stout, dyno was at 400/400 so i know she needs to breathe. Would the high flow muffler hurt my back pressure enough to affect it or will it be better?
 
I know more flow in theory should be better but is there a issue of reducing back pressure? I always thought back pressure helped low end torque but im not 100% certain. I guess a less restrictive muffler should help as it was dyno'd with open manifolds
 
needing backpressure is a myth, you maybe will need to fatten the fuel mixture up a bit as you might go a bit on the lean side depending on how much flow you gain, id run em

*edit* not to mention you said it was dynoed on open manifolds... im sure you have plenty of backpressure from them i doubt mufflers would change much other than noise, id definitely go for it if you are fond of the noise that muffler makes
 
71DodgeDemon340 said:
I know more flow in theory should be better but is there a issue of reducing back pressure? I always thought back pressure helped low end torque but im not 100% certain. I guess a less restrictive muffler should help as it was dyno'd with open manifolds
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If back pressure was good, then a good set of headers would lose power. The last thing you want is burnt exhaust in the combustion chamber.


The stupid backpressure myth has been around since I was in high school (read a LONG time ago) and it was ridiculous then. These old wives tales just won't freaking die.
 
71DodgeDemon340 said:
I know more flow in theory should be better but is there a issue of reducing back pressure? I always thought back pressure helped low end torque but im not 100% certain. I guess a less restrictive muffler should help as it was dyno'd with open manifolds
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OMG, I cant beileve I'm reading this.
 
The less back pressure, the better. If there was any truth whatsoever to that ridiculous myth, you'd see restrictors on the exhaust of every single type of race car on the planet.
 
Needing back pressure is a myth that needs to die. Along with:

Big valves kill torque.
Solid cam's need constant maintenance.
3/4 race cams.

And etc etc.
 
Thanks guys. I never really researched it until now. I heard years ago about back pressure being needed and never really researched it to find the truth.

 
Never experimented with overly large pipe but I know in HVAC, too large of duct work creates tumbling of the air flow which can cause more noise and slow down the flow (back pressure?). You wouldn't want to install 3" pipe on an engine that can't use pipe that big and you for sure wouldn't use a pipe that's too small. IMO, a smaller but correct diameter pipe would create a better quality flow through it but getting too small will create back pressure and that's not what you want to do. I think this is what many think is going on when they go with too large of pipe and call it back pressure.....
 
On a daily driver it's really all about what sounds the best to you. The back pressure myth does have some truth to it, especially with modern cars, but not really how most folks think. In order to meet sound and emmission standards, modern cars run very restrictive mufflers and catalytic converters. The exhaust system design and engine tuning is set up to run the *best (hp, tq, mpg, whatever manufacturers actual goals are) with these restrictions in mind. The R&D runs very deep and inludes pulse tuning etc.
In theory slapping on a higher flowing muffler "should" make more power, but not for every situation, just like tuning to a specific AFR won't always make the most power either.

The only way you would know for SURE, was to run the car on a dyno back to back with your changes. Everything else is pure guessing.

But like I said, none of that matters if every single time you drive your car, you are thinking about how it would sounds, and not trying to shave tenths of seconds. Go for it, you won't hurt the engine or your street performance. The car will still pull hard and if it sounds better to you, your smile will be even wider..
 
AJ/FormS said:
The bigger the pipe the better! The faster that used up hot stuff gets out, the faster the fresh clean stuff can get on thru. It's just like yesterdays supper!
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well, it doesn't' make much sense to put on a pipe larger than the collector diameters itself IMO.

In regards to sound I've always found smaller pipes to be louder and have more rappp
 
Cranky said:
Never experimented with overly large pipe but I know in HVAC, too large of duct work creates tumbling of the air flow which can cause more noise and slow down the flow (back pressure?). You wouldn't want to install 3" pipe on an engine that can't use pipe that big and you for sure wouldn't use a pipe that's too small. IMO, a smaller but correct diameter pipe would create a better quality flow through it but getting too small will create back pressure and that's not what you want to do. I think this is what many think is going on when they go with too large of pipe and call it back pressure.....
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Your thinking is correct regarding HVAC, but is off the mark with exhaust. We are dealing with heat, sound and motion and the motion is very high speed gases. There is no reason to run pipe larger than the collector as roccodart points out, but neither should the exhaust be smaller than the collector either. If it is smaller. It should be as little as practicable as possible.
 
Lustle said:
Needing back pressure is a myth that needs to die. Along with:

Big valves kill torque.
Solid cam's need constant maintenance.
3/4 race cams.

And etc etc.
Click to expand...



Damn. Lustle is on a roll!

Just wish you would have filled in the etc's a bit more. There are plenty of golden cows that need to be gored.
 
I ordered them last night. Just gotta make sure they will fit. Same size as the ones on there now so in theory they should work but we will see. If going the hogh flow route gives me a better sound and a little better performance than ill be happy
 
Any amount of the precious back pressure will cause exhaust to be pushed back into the exhaust port and into the cylinders to contaminate the next round of combustion.

Yeah, back pressure sounds like a good thing. It never was and never will be a good thing. Some of yall need to let go of the past.
 
YR, I think Cranky's post is correct on the H AC, but differ's because of the engine is or has a pulse and the HVAC system does not.

In exhaust, we're moving hot gasses and we would want to retain some heat as so not drop velocity and allow cooling gasses to tumble. And the same with to large of a pipe.
 
yellow rose said:
Your thinking is correct regarding HVAC, but is off the mark with exhaust. We are dealing with heat, sound and motion and the motion is very high speed gases. There is no reason to run pipe larger than the collector as roccodart points out, but neither should the exhaust be smaller than the collector either. If it is smaller. It should be as little as practicable as possible.
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I disagree . volume vs velocity is a very important consideration in an exhaust system, just like an intake system, just like HVAC
 
JoeSBP said:
I disagree . volume vs velocity is a very important consideration in an exhaust system, just like an intake system, just like HVAC
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Except for 2 things. HVAC doesn't have near the speed, nowhere near the heat, and don't forget the sound waves.

The new normal I high dollar crap is much smaller exhaust valves with much higher speeds. If you spend the $$$$$ then you have to go all the way back to the exhaust lobe and start there. For 99.99% of us, the exhaust lobes we get are not designed for small area, ultra fast port and pipe speeds, which also requires very small header pipes (relatively), merge collectors and a couple of other things.


This thing has been beat senseless over on speed talk. Big Joe Sherman and I had very similar results with merge collectors and crap like that. Joe wrote about it a bit. Neither he nor I ever worked with an exhaust lobe like some of the other guys are. Of course, when you start working with the exhaust lobe, you have to consider the intake lobe as well because one affects the other.


It's not a cut and dry as it might seem. Very complicated.

Calvin Elston has sone very good tech on his site. You just have to use google.
 
True! Exhaust gasses via our internal combustion engines are different. To much going on to nut shell excellence.

The HVAC comparison should be taken as "kind of like" to help get a message across.

The workings of the engine from air cleaner to exhaust tip are all involved in some way shape and form.

I order to make things easier, when asking an exhaust sizing question, the more engine info given along with vehicles intended useage is the information needed. You don't have to go nuts listing everything, the basic HP/Tq outputs with vehicles intended useage is enough to get a exhaust tube size.

When it comes to mufflers, there's only two ways to choose a muffler.
Sound or performance. Performance should be the choice and goal.
Should the sound meet performance goals, then all the better.
 
Basically from what i read you have to find a happy medium on exhaust sizes. Too large will reduce scavenging which will make it seem like the low end torque has been affected. And then too small of a pipe which will increase back pressure reducing high end performance. From a couple articles ive read says you have to find a happy medium. You want some scavenging at lower rpm but want to flow as quickly as possible at higher rpm.

"I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application."
 
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rumblefish360 said:
True! Exhaust gasses via our internal combustion engines are different. To much going on to nut shell excellence.

The HVAC comparison should be taken as "kind of like" to help get a message across.

The workings of the engine from air cleaner to exhaust tip are all involved in some way shape and form.

I order to make things easier, when asking an exhaust sizing question, the more engine info given along with vehicles intended useage is the information needed. You don't have to go nuts listing everything, the basic HP/Tq outputs with vehicles intended useage is enough to get a exhaust tube size.

When it comes to mufflers, there's only two ways to choose a muffler.
Sound or performance. Performance should be the choice and goal.
Should the sound meet performance goals, then all the better.
Click to expand...


Exactly.

Now where did AJ go? He may be out playing with hockey sticks. Who knows?
 
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