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Discussion Starter · #1 ·
:4-dontknow: I've read through this forum and maybe I missed my answer. I've seen where several people have preached free flow exhaust, less restriction, better hp gains, better mpg, etc... (sounds reasonable to me).

What really got my attention was when I read some members warning of too little restriction resulting in a reduction of mpg.

How does back pressure effect mpg?
How does back pressure effect hp?
Where is the happy medium on pipe size?

My "later down the road when I have some cash project" is to put an after market muffler and keep the original exhaust pipe"

The factory pipe is 3+ inches. I've read optimum is under 3". If that's true, would reducing the pipe down to 2.5 - 2.75just prior to the muffler inlet solve this problem? I'm wanting to have a DEEP rumble and I don't want anything to disrupt the sound as it travels down the 3" pipe, but I don't want to reduce my mpg as one member posted with surprise.

Some one help me out here. Explain the science behind it.:thk:
 

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Don't believe the myth about "you need some back pressure". Back pressure and exhaust restriction is always bad for power output. Some people say there is a loss in low end power if you reduce back pressure, that's simply not true.

What is often the cause of the loss in low end power is too large of an exhaust pipe diameter. The smaller the diameter of a pipe, the faster the velocity of the gas flowing through it. The larger the diameter of a pipe, the slower the velocity of the gas flowing through it. High exhaust velocity is desirable as it can help "suck" the exhaust out of the combustion chamber.

There is an upper limit of "too big" just as there is a lower limit of "too small". The stock exhaust is 2.5" diameter off the manifolds, 3" at the merge, and if you have oem duals it is 2.25" duals at the tailpipes. All of these diameters are just about perfect for stock to fairly heavily modified engines. Going larger or smaller would cause less overall power at certain rpm's. The diameters they engineered them at are optimum for power at lower and upper rpm's.

Hope this helps.
 

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Interesting, i just got off the phone with Magnaflow techs(2 of them in fact) They told me not to replace the y pipe on my 2500 hd hemi. There reasoning was, on hd trucks if you replace the y you will loose a noticeable amount of power/torqe.And only at high end -other wise higher speeds would you gain maybe 1-3 hp at most.They both told me the trade off if your going to use the truck for what it was built for toweing/hauling it is not recomended to loose the back pressure you do by useing there replacement y pipe.And that replaceing the muffler will give you much more in all ranges.

Infact i told them about all the post of guys claiming all the gains they get from just replaceing the y pipe-they said there is nothing in there testing that proves that at all-but has showen the loss of low end power, and bare minumal gains at high end with no mpg gains. If a new and complete build is done they said yes use it with the propper size pipe all the way from headers to tips. And yes a bit of sound is added.

In a nut shell they told me not to waste my time or money, unless i dont have much else to do-lol.

Just thought i would post what i was told by Magnaflow techs.
 

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I'm very surprised that engineers at a company like magnaflow would give you that advice, it is not accurate.

For proof, look at exhaust junkie's new mufflex y pipe that is now for sale on this forum, he posted the gains on a dyno and it has gains at all rpms (low and high) and max gain of ~10hp.
 

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To be honest, i was very surprised myself. I have replaced a y pipe in the past and thought it did some good. But that is exactly what they told me today, the first tech on the phone was named Rich. So i am putting off the y pipe for now and maybe makeing one or two more calls. I did how ever run this by my brothers-one is a machanic of 27 years and the other -the oldest a master of restoration and also owns and runs a bike shop-Harleys is his main ofcource and mine-lol.

Both told me it would be a 50/50 chance on any gain that would matter, that there are manny factors involved per truck that could make or breake that mod. But also agreed with the magnaflow techs about looseing low end power/torq, again depending on each truck and manny factors.
 

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I'll agree there are nearly countless factors that effect how an exhaust modification will change power output...

Not to pat myself on the back or anything, but just to help, I can say that what they told you is not correct. I have a civil engineering degree and spent a good amount of time studying flow through pipes. I've also spent a lot of time on the 1/4 mile growing up and have never seen a more restrictive exhaust outperform a free flowing exhaust.
 

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To be honest-i dont know why Magnaflow techs would try to talk me out of it, i agree with what you have stated, and both my older brothers statements would make me beleive that adding the Y and a new Magnaflow High performace muffler would be a nice plus to the truck, i most likely will take the 50/50 chance and do it, like i said i did it in the past and beleived it made a difference.
 

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Discussion Starter · #8 ·
WOW! I felt like I should have been at a bar having a drink with you two while arguing about exhausts.... lol. Thanks for the info guys. It helps a lot
 

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lol, I don't think we were arguing....

I was most likely having a drink though while typing :)
 

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Discussion Starter · #10 ·
Pro's / Con's to ONLY replacing the muffler with a less restrictive muffler: Would my performance on the high or low end be affected by maintaining the factory single exhaust? I'm trying to imagine the accelerated flow as described above and how a less restrictive muffler would perform.

Also, any thoughts on the appearance of a factory single exhaust pipe with the sound of an after market muffler (goofy, silly, WTF, okay, I'm going to hell for even thinking about it....)?
 

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Whether you go with a simple muffler swap leaving the single 3" exhaust in place or do duals after the muffler with say 2.25" or 2.5" pipes, the results will be very similar. Technically I'd say the duals will give more power if you use say 2.25" pipes because you have enough pipe diameter to move the same volume of gas as the single 3" pipe, but since you split it into two smaller diameter pipes you should have higher velocity. The results will be mildly different though. Plus, if you have a local shop do dual tailpipes they will most likely not use mandrel bends (which the factory pipe has) and these crush bends that they use will probably negate any improvements that you gain. The pipe will also likely not be stainless like factory.

I've seen dyno comparisons of a properly sized y pipe setup vs a properly sized x pipe setup and there was less than a 5hp max gain with the x pipe setup. IMO, the couple hp gained is offset by the extra weight of the extra pipe. Not to mention the added cost. IMO the only reason to add duals is for looks.

There is probably some change in sound as well, since it's coming from two different spots (think about mono speaker vs. stereo), but this is so technical there's really no way to "calculate" or figure this out without simply doing it and seeing if it sounds good to you...
 

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Snrusnack, I have to argue you some on a couple of your points. Having a free-er flowing exhaust does actually lose some of your low-end torque. The thing is, it will only be noticeable in situations where you are towing though at lower speeds, like maybe a camper through the mountains. Some back-pressure is needed for heavy torque, but most people don't find themselves in an environment where it's really needed. You mentioned spending time on the drag-strip, that's a high-rev (i.e. horsepower) environment more than torque. Unless at your drag strip a lot of guys don't get the tires spinning at take-off, then they definitely need torque!
I grew up on a farm, everything we do requires excessive torque, I will however qualify this by saying a lot of my info comes from diesel uses, but a lot of the physics are the same.
For the o.p. I would still lean toward the free-er flowing exhaust and I doubt you will notice any significant torque drop with the y-pipe mod. Unless this is a towing vehicle, a lot of towing and you may notice some. I still plan to do it to mine.
One last thing, Snrusnack, you mentioned that there would not likely be any hp noticed from duals out the back of the muffler. I'd agree with that, you're talking very little actual area there between the muffler and tailpipe. At that point, the exhaust gases are cooling, slowing down and have already passed through the restrictive parts. That would mostly be for looks after the muffler, unless you've moved it further forward anyway...
 

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True that at the drag strip high rpm's are more often seen than low rpms. Still though, back pressure(which is another word for restriction, basically) is not helpful for making more torque at any rpm, even low rpm's, even idle. I'm not sure where this "myth" comes from, but just about everyone out there believes it is the case. It really doesn't make sense if you think about it, an engine is an air pump that will output more power (and torque) by moving a greater volume of air(and fuel) through it, how would restricting the exhaust flow help to move air in and out of the combustion chamber faster/more efficiently? It won't under any circumstance or at any rpm, it is always going to hurt power potential (and torque output).

What you may have experienced on farm vehicles, or towing, or whatever else you may have done that used lower rpms is that there is not much difference at low rpms with a restrictive exhaust vs free flowing exhaust. It's not nearly as critical at low rpms. For example, say with a stock restrictive exhaust an engine may make 100lbft at 1500 rpm and 200 lbft at 4000 rpm. Then say the same engine with free flowing exhaust may make 102lbft at 1500rpm and 220lbft at 4000rpm. Basically no noticeable difference at low rpm, but a fairly significant and noticeable difference at high rpm.

I'd say that this may or may not be true for other types of engines such as diesels, 2 strokes, rotary's, etc. but for 4 strokes it definitely holds water.

You don't have to believe me but I'm just putting information out there from what I've learned and from what I've seen in my experiences.

Diesels run at extremely low rpms and they still get power increases from free flowing exhausts. Just another example. And I think some restriction is required for power potential with 2 strokes but that has something to do with the fact that it's a valveless engine, and the rate of flow in the exhaust has a significant effect on the rate of intake flow, engine rpm potential, etc. In other words 2 strokes are more relient on "exhaust tuning". 4 strokes rely on cams and valve springs, the pressure in the exhaust really has almost nothing to do with it and is only a restriction.
 

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I've yet to see anyone prove me wrong by showing a dyno that shows a more restrictive muffler producing more torque at low rpm's than with a free flowing muffler, all else being equal...
 

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Discussion Starter · #16 ·
Thanks guys. When I get the $$$, I'm going to leave everything stock and just change out the muffler. I just want a deeper tone, but not so much that I have a droning headache 100 miles down the road. I'm not concerned so much about the + 1 or 2 hp gain. I'm more concerned about the loss of hp and torque.
 

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I've posted this a couple of other places already but I think it applies here so I will post it for you too. I swapped from a Flowmaster Super 44 (chambered and therefore atleast slightly more restrictive) to a Magnaflow 18" straight-through design muffler on my 2006 4.7L and immediately noticed a definite slight increase in low end pull. The exhaust was entirely stock otherwise with both mufflers. I personally like the sound of the Magnaflow better too. You say you want a deeper tone, no loss of power (maybe a gain), and no severe drone...my recommendation is Magnaflow all the way.

And to whomever it was that asked about a single exit, stock appearing exhaust with an aftermarket rumble....no, there is nothing wrong with that at all. It's an economical way to get a great sounding exhaust and lots of folks do it. I have had that on a couple of different trucks and always got compliments.
 

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I had the gibson catback dual swept side on my 04 Ram 1500 QC SLT (4.7) after 2k miles and I loved it. Perfect all around driving and noticeable power increase all around with the K&N CAI as well. I towed a 33 footer back and forth to Florida with it and it was a great puller for me. Good luck with yor exhaust choice, keep us posted on any increases and get some pics of the change if you can :)
 

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Discussion Starter · #19 ·
I've posted this a couple of other places already but I think it applies here so I will post it for you too. I swapped from a Flowmaster Super 44 (chambered and therefore atleast slightly more restrictive) to a Magnaflow 18" straight-through design muffler on my 2006 4.7L and immediately noticed a definite slight increase in low end pull. The exhaust was entirely stock otherwise with both mufflers. I personally like the sound of the Magnaflow better too. You say you want a deeper tone, no loss of power (maybe a gain), and no severe drone...my recommendation is Magnaflow all the way.

And to whomever it was that asked about a single exit, stock appearing exhaust with an aftermarket rumble....no, there is nothing wrong with that at all. It's an economical way to get a great sounding exhaust and lots of folks do it. I have had that on a couple of different trucks and always got compliments.
Magnaflow it will be. My mother-in-law has FM on her Mustang and it sounds like a high pitch popping - trying to enter the rumbling zone. Irritating as hell to listen to while driving.

I was the one asking about stock exhaust with aftermarket mufflers. Thanks for your input. I just wanted opinions. I like the appearance of my truck.

I do have one question for everyone. My factory exhaust sounds more throaty when I start it up. It did not sound like this when I first bought it. Some time around 5,000 miles is when I noticed it. It sounds cool. But then again, after having driven a V6 for 18 years, anything sounds cool. :LOL: Has anyone else experienced this?
 

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Discussion Starter · #20 ·
I had the gibson catback dual swept side on my 04 Ram 1500 QC SLT (4.7) after 2k miles and I loved it. Perfect all around driving and noticeable power increase all around with the K&N CAI as well. I towed a 33 footer back and forth to Florida with it and it was a great puller for me. Good luck with yor exhaust choice, keep us posted on any increases and get some pics of the change if you can :)
Yes Sir, I will post some pics, but it will be some time down the road.
 
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