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post #1 of 5 Old 10-14-2013, 07:24 PM Thread Starter
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Anyone here an engineer?

I've got a question about flow characteristics of an exhaust idea.
Some guys are hollowing out there DPF so it looks stock. What would it flow like with the downpipe being 4" then going to 8" for a few feet then to dual 4" exhaust with one 4"aero turbine muffler in each of the 4" duals. I've heard the aero turbine's aero foil design helps "pull" or scavenge the exhaust out but would the velocity crash once it hits the 8" section? I've also heard this will make the exhaust sound like stacks without having stacks. That sweet hollow grumble.
The more accurate and scientific the answer the better please.ill attach a crude drawing of what I mean.

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post #2 of 5 Old 10-16-2013, 04:19 PM Fanatic
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Mech Eng here. Problem with going 4" to a 8" area is you end up having lots of turbulence that will cause 1) poor flow and 2) a big echoing chamber. What I would suggest if you can do it is to figure out a way to put a 4" pipe straight through the 8" shell of the dpf. Maybe get a 4" delete pipe, cut your dpf along the length, peel it open, slide the 4" delete pipe in, and make sure the flanges come out of both ends. Might have to cut the flanges off the dpf. Then rotate the dpf so the cut/tacked up weld job is up toward the bottom of the cab. That way it looks like you have a dpf but it really is a straight pipe.

Running it the way you have it will work but I think it will sound funky. Just my 2 cents. I know it's not scientific but I am not about to do fluid dynamics and thermo again. One semester was enough for me
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post #3 of 5 Old 10-17-2013, 08:00 AM
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Originally Posted by james_de62 View Post
What I would suggest if you can do it is to figure out a way to put a 4" pipe straight through the 8" shell of the dpf.
This is exactly what I was going to suggest. Having the exhaust gas open up to that 8" area and then bottle neck back down into a 4" exit is definitely not great when compared to a smooth flow the whole way out.

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post #4 of 5 Old 10-17-2013, 09:00 AM
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Basically what your proposing is in effect a resonator. What that does is as soon as the gas enters the chamber it expands out and slows down and actually rotates back kind of and as the incoming gas impacts the rotating gases, thats what causes that low rumble sounds of stacks because the sounds waves are resonating back and dulls the sounds.

Problems is the gas is no longer traveling in a relatively straight path in the 8" section and when it gets forced into the 2 4" pipes its having to be compressed in a sense because unlike what common sense would tell you, 2 4" pipes have less cross sectional area than 1 8" pipe does. So that creates back pressure.

If you could slow down and see what the gases actually would do, once the truck fires it would travel down the pipe and when it reaches the 8" pipe it would proceed to fill the 8" pipe completely with gas before the gas even left the chamber out the 2 4 inch pipes. Thats why some compression happens.

I know thats probably a bit confusing, i could explain it better in person but oh well. I work at a company that builds high pressure/high temp vessels so i deal with fluid dynamics all day long(fun stuff). Feel free to ask any more questions and ill clarify.

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post #5 of 5 Old 10-18-2013, 03:32 PM
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welding engineer here. Make sure the welds are good...

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