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Old 11-10-2015, 10:25 PM   #1
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Exhaust modification questions answered.

I know we discuss exhausts on here quite frequently, and i actually just stumbled across this video that does a fantastic job of explaining a lot of what typically gets brought up in threads, such as increasing exhaust diameter too far, and different header designs. Enjoy.

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Old 11-10-2015, 10:31 PM   #2
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But what about all of my backpressures?

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Old 11-10-2015, 10:52 PM   #3
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Glad you asked lol, because i really like the way he described what happened when you go too big, like with a header primary.

When volume increases velocity decreases, which causes pressure to increase (because the air is slowing down and stacking up). So basically going to big just causes a massive pressure bubble in the middle of your exhaust ... so simple when you hear it explained that way. Thats why he's the professional. Funny to think that bigger can cause more back pressure haha.
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Old 11-10-2015, 10:56 PM   #4
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Stuff gets confusing real fast.

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Old 11-11-2015, 08:58 AM   #5
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Glad you asked lol, because i really like the way he described what happened when you go too big, like with a header primary.

When volume increases velocity decreases, which causes pressure to increase (because the air is slowing down and stacking up). So basically going to big just causes a massive pressure bubble in the middle of your exhaust ... so simple when you hear it explained that way. Thats why he's the professional. Funny to think that bigger can cause more back pressure haha.
Excellent video!
What he didn't bother getting into was that how sudden the increase size is impacts the pressure losses and actual recovery of pressure. I realize it was beyond the scope/intent of the video.
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Old 11-11-2015, 11:20 AM   #6
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Quote:
Originally Posted by Voltwings View Post
Glad you asked lol, because i really like the way he described what happened when you go too big, like with a header primary.

When volume increases velocity decreases, which causes pressure to increase (because the air is slowing down and stacking up). So basically going to big just causes a massive pressure bubble in the middle of your exhaust ... so simple when you hear it explained that way. Thats why he's the professional. Funny to think that bigger can cause more back pressure haha.
Yep. This is why cars don't come stock with 4in exhaust. The exhaust diameter really shouldn't be opened up unless you are getting significantly more exhaust flow out of the engine or only optimizing for a specific speed range. Great discussion! I'm bookmarking the video to watch later today.
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Excellent video!
What he didn't bother getting into was that how sudden the increase size is impacts the pressure losses and actual recovery of pressure. I realize it was beyond the scope/intent of the video.
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This is true. Just to expand (and maybe stimulate some additional conversation)...designers can only do so much with the space given to them. You'd want to minimize losses from reversed flow but I suspect you don't want the system to exceed a certain amount of pressure recovery anyway as it would require more exhaust pressure to overcome the resistance in the pipes. I'm sure the designers figure out the optimum exhaust velocity (kinetic energy) at different rpm (or exhaust flow rates) and design the exhaust diameter to accommodate the operating range. I would love to see some CFD analysis of an auto exhaust system. I would find it very interesting.

Sorry for my diatribe... Fluid dynamics gets me excited.

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Old 11-11-2015, 11:28 AM   #7
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also to further folks ideas....how much effort is spent on sound. We know it matters (hence all the artificial sound that is becoming more widespread) I wonder if they would trade a bit of power/efficiency for good tone.
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Old 11-11-2015, 11:53 AM   #8
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Just saw the video. Great explanations. I loved how this guy worked through the ranks to get to where he is.

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Old 11-11-2015, 03:45 PM   #9
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Originally Posted by natestang07 View Post
Stuff gets confusing real fast.

This is why some of us spent many years of our lives and a lot of money on engineering school.

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also to further folks ideas....how much effort is spent on sound. We know it matters (hence all the artificial sound that is becoming more widespread) I wonder if they would trade a bit of power/efficiency for good tone.
In a passenger vehicle like the Mustang, sound is going to be favored over performance. Like it or not, these are image cars, not performance cars. Most buyers purchase them because of the sounds they make, and the way they look. Otherwise, there would be no back seat, a Cd of less than .37, suspension that was designed with performance in mind rather than copy and paste from a very non performance oriented vehicle, and would not have been built on a four door platform.
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Old 11-11-2015, 05:24 PM   #10
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In a passenger vehicle like the Mustang, sound is going to be favored over performance. Like it or not, these are image cars, not performance cars. Most buyers purchase them because of the sounds they make, and the way they look. Otherwise, there would be no back seat, a Cd of less than .37, suspension that was designed with performance in mind rather than copy and paste from a very non performance oriented vehicle, and would not have been built on a four door platform.
The marketing team behind this commercial may agree with you...

https://youtu.be/UEmoO95T4PU

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Old 11-11-2015, 05:37 PM   #11
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Originally Posted by Strange Mud View Post
also to further folks ideas....how much effort is spent on sound. We know it matters (hence all the artificial sound that is becoming more widespread) I wonder if they would trade a bit of power/efficiency for good tone.
I'm sure they want to make sure they get the right tone and they will probably trade some efficiency for comfort. But I doubt there is a ton left on the table when they are done. We can only speculate without talking to a Ford engineer.

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