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Exhaust 4.7L Exhaust Discussion


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  #1  
Old 03-13-2012, 10:33 AM
micar71 micar71 is offline
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Default Restriction vs Freeflow

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.
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Old 03-13-2012, 09:47 PM
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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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Old 03-14-2012, 03:15 PM
fishhawk fishhawk is offline
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Default

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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Old 03-14-2012, 03:51 PM
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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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Old 03-14-2012, 08:52 PM
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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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Old 03-14-2012, 09:04 PM
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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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Old 03-14-2012, 09:45 PM
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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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Old 03-16-2012, 09:06 PM
micar71 micar71 is offline
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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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Old 03-17-2012, 08:20 AM
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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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Old 03-21-2012, 11:05 AM
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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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