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Discussion Starter · #1 · (Edited)
First, a question: Who sells stainless steel flex sections with the smooth internal passageway (double-braided, inside and out, so that the exhaust flow sees a relatively smooth ID)? I've seen 'em before, but can't remember who offered them.

I'm thinking that the standard flex section like I have on my car (claimed 3.5" ID, but it actually opens up to about 3.9" on the inside of the braid part) may significantly hinder exhaust flow at higher RPM. I'm thinking that this, combined with the step-down in piping ID at the GT Spec front muffler can and then another step-down at the entry of the rear muffler can may result in relatively poor exhaust flow. (I've already replaced the 2.9" ID GT Spec front flange with a 3.5" V-Band to match the 3.5" DP.) Does anyone here have any real-world testing or scientific calculations to support or refute this notion? I have some anecdotal (but I believe, strong) evidence to support this, but I won't bother to go into it here because it doesn't really prove anything regarding flex sections.

Try this: Take a drinking straw that has the flexible "accordian" section in it. Leave the accordian section collapsed (straw in the shortest position) and blow through it as hard as you can. While you're blowing through it, extend the straw by expanding the accordian section. On the straws I've tested, you can obviously tell that the straw flows less air when expanded. When expanded, the accordian section creates a much more uneven, undulating wall than when it is collapsed and relatively smooth. Interesting....
 

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Discussion Starter · #3 ·
Those appear to be about the same as mine -- they still have the internal ribs, and if you have to go to a 4" you are in a worse situation than I have now. (BTW, I got mine from a company in Kentucky that will make 3.5" or any other size, but I think all of theirs are ribbed and not double-braided, therefore not smooth internally.)

I'm looking for a double-braided section where the inside is also braided, and MOST IMPORTANTLY, maintains the same ID -- a smooth wall -- throughout. I think the ID change is very bad for flow.
 

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I understand about the ID size remaining the same, for me , it was either a flex or no flex, I choose to go with it, couldnt find a 3.5 flex in stainless, kinda figured they are not available so went with 4" stainless, which was a very tight fit. My DP pipings are 3.5 from turbo all the way back to the rear mufler... I am sure and hope that 4" flex will not slow the flow too much.
try BOOST LOGIC / Kean, I know their 3" flex is the type you are looking for, they "might" be able to help.
Food for thought - the factory flex is about 3.25, with ribs.
 

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Discussion Starter · #5 · (Edited)
kazumaTRD said:
Food for thought - the factory flex is about 3.25, with ribs.
That's true. We know that the factory DP with that flex that you described AND the factory catalytic converter hinder flow even at 300-ish rwhp levels. What we don't know is just how much of that (if any) is the flex section. I think we just assume that the cat is the main, or only, culprit.

Here's an anecdotal example of how a 3" ID non-mandrel bend cat-back which included a louvered-core 3" ID muffler (one of those goofy things with raised louvers inside the straight-through muffler tube) will KILL exhaust flow: On stock twins, my car, with the above-described cat-back and RMM DP would make less than 13.75 PSI boost (to clarify, this is 100% stock, WG hoses and all, other than the cat-back and RMM DP). With the ONLY change being the addition of a GT Spec cat-back (replacing the non-mandrel-bend, louvered-core cat-back), it would make 19.9 PSI. That really started me to wondering how various exhaust configurations (internal geometries) affected flow.

EDIT: For got to add "RMM DP" in the parenthesis above.
 

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At this point and until my car is running, i will not be able to tell whats going to happen, I tried to minimize any restrictions , now I hope the 4" flex dont restrict much of the flow, other than it, I am free flowing!
interesting example! I had a similar real world experiance, where an aftermarket high flow rated muffler (from a porsche shop) was installed, the bpu car had dynoed poor #s (36x / 18 psi)... so it did turned out to be restrictive, had to dump it. after the change, car put down 400s at about 18psi
 

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Discussion Starter · #7 ·
kazumaTRD said:
At this point and until my car is running, i will not be able to tell whats going to happen, I tried to minimize any restrictions , now I hope the 4" flex dont restrict much of the flow, other than it, I am free flowing!
interesting example! I had a similar real world experiance, where an aftermarket high flow rated muffler (from a porsche shop) was installed, the bpu car had dynoed poor #s (36x / 18 psi)... so it did turned out to be restrictive, had to dump it. after the change, car put down 400s at about 18psi
That, along with my experience and some others I've heard, really makes me wonder if there isn't a LOT more to exhaust flow than just the ID of the tubing and muflers......
 
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