Why round exhaust pipe?

[JeffMcKC]

I dont feel I hide a thing ,I get PMs and help as much as I can and take a lot of timeout to explain, some of it is just not worth arguing about. Its like the gear thing with you, ( I think it was you) some people defend instead of knee jerking and defending what they have always heard or thought. People that really know me in real life know I do anything I can to help them, it does not translate well on the net. I do tend to go against the normal train of thought from what you will read in a magazine

[SuperBuickGuy]

I dont feel I hide a thing ,I get PMs and help as much as I can and take a lot of timeout to explain, some of it is just not worth arguing about. Its like the gear thing with you, ( I think it was you) some people defend instead of knee jerking and defending what they have always heard or thought. People that really know me in real life know I do anything I can to help them, it does not translate well on the net. I do tend to go against the normal train of thought from what you will read in a magazine

we agree on that - magazine...

and it was the gear thing... hate to mention it (well, maybe not) I was right about the gear thing, and my issue with you isn't the gear it's that you called me a liar - that I take quite personally.

that said, whether I'd have a beer with you isn't the point, there are lots of people, who I've learned from, that if I found them drowning in the ocean I'd give them a glass of water.

You made a point and I wondered at the next question about back pressure - and you answered it.... where this thread went after that would require a compass, 3 day supply of food, and google maps.

[38P]

First, you're welcome for giving you an idea about an article.

Second, why are you guessing? you don't know either - and I really doubt you have any idea of what pressures are in an HVAC system; so to make a general assertion without anything support seems a bit.... let's see... what's word.... oh yeah, Chadish If anything, what happens at low pressure helps understand higher pressures....
* * * *

my bet - a single plane (meaning moves only in one direction) will have higher flow with less static pressure (fancy HVAC term for how much, snicker, drag is in the pipe vs. the air flow). I also bet the only real to square is its storage of water vapor in the corners - in short - it'd rust faster. The way air flows, it has a high pressure core then radially has less and less pressure because of the static pressure created by the enclosure. In a square tube, that means the air in the corners moves slower then the air in the center. In round pipe, it doesn't make much difference because the water can only collect in areas of high pressure, but that position (such as the inside of a radius) is untenable because of its mass - meaning it moves then gets sucked out at the pressure equalization point.

Given the elastic nature of air, I'm not ready to sign up for the proposition that low pressure/low temperature performance (i.e. HVAC system) is predictive of high pressure/high temperature performance (i.e. exhaust pipe). Certainly the slow-moving boundary layer is going to be influenced by factors such as the duct shape, duct wall finish, surface-to-volume ratio, density, flow turbulance and velocity.

But I also doubt that a squared section of exhaust "duct" of adequate cross-sectional area, positioned after most of the turbulent flow has transitioned to laminar flow and expanded somewhat, is going to have much measurable effect one way or another in a street car.

[Huskinhano]

A little off topic but it jarred something from the back of my mind related to round conduits, pipes, tubes, ect. I'm a NJ state licensed electrician which I have to take CEU's to renew my license every 3 years. The last time, an engineer pointed out a phenomenon about conduits and wire. There is a certain ratio of wire cross section to conduit where if you run 3 wires if the ratio is right, the wires will bind up when pulling. How ever if you run 2 or 4 or more, the wires won't bind. What happens with the three wires, they will lay side by side and bind up.

[SuperBuickGuy]

Given the elastic nature of air, I'm not ready to sign up for the proposition that low pressure/low temperature performance (i.e. HVAC system) is predictive of high pressure/high temperature performance (i.e. exhaust pipe). Certainly the slow-moving boundary layer is going to be influenced by factors such as the duct shape, duct wall finish, surface-to-volume ratio, density, flow turbulance and velocity.

But I also doubt that a squared section of exhaust "duct" of adequate cross-sectional area, positioned after most of the turbulent flow has transitioned to laminar flow and expanded somewhat, is going to have much measurable effect one way or another in a street car.

man there's a lot of jaw flapping without any numbers

HVAC "slow" as you call it, air moves between 60 and 100 mph. Bathroom fans, 10-20mph (okay, that's slow). There are high pressure systems (such as on aircraft) that operate with much higher speeds (air inside a jet moves close to 1000 mph, and that's where the tap is to pull air into the cabin... which is why it always smells like burnt jet fuel on the inside when the airplane starts its engines) - and those systems are still HVAC. how fast does one stroke of a piston moving +/- 4" go? does anyone know how fast exhaust moves through an exhaust pipe?

[JeffMcKC]

we agree on that - magazine...

and it was the gear thing... hate to mention it (well, maybe not) I was right about the gear thing, and my issue with you isn't the gear it's that you called me a liar - that I take quite personally.

that said, whether I'd have a beer with you isn't the point, there are lots of people, who I've learned from, that if I found them drowning in the ocean I'd give them a glass of water.

You made a point and I wondered at the next question about back pressure - and you answered it.... where this thread went after that would require a compass, 3 day supply of food, and google maps.

You tend to be right like TC is right, and post a link to this so I can see the context of my comment, I might have been right.

[SuperBuickGuy]

You tend to be right like TC is right, and post a link to this so I can see the context of my comment, I might have been right.

whether I prove to you or not that I was right doesn't matter to me because you don't matter to me....

back to the topic at hand.

so now you have a choice, you can either rant and rave like a little child, you can discuss the topic, or you can do what you promised on that gear build and not come to threads started by me.... your choice.

[JeffMcKC]

[SuperBuickGuy]

so something interesting I noticed today.
This is the original exhaust system for the GTO - it's stainless (and going to be the donor for piping for the 'vette) and flows from the left of the picture to the right



Even with the substantial abuse GM did to this pipe to "make it perfect" or whatever reason they did it - you'll notice there's lots of flat spots in the pipe - those are factory installed crimps.

What I found interesting was where the heat collected - right before the mufflers, as the pipe has no seams so while the shape is tortured, it's got the same square inches of flow, the shape didn't speed up or slow down the exhaust - but if you look at where the mufflers were, the pipe discolors (got hotter) in the 24 or so inches before the muffler...

[Barry Donovan]

I believe the backpressure is a side effect of pulse wave tuning - not the primary reason for torque gains. It all depends on the application. I've seen engines that lose power with more open exhausts. But it's a LOT harder to get a tuned, optimized exhaust than most people think. You have to test it, you can't just do math and copy others' work in hopes you nail it.

When exhaust is tuned right, it's awesome. At Spectre we had an EFI 383cid Camaro that picked up +50whp at peak just by moving the X-pipe backwards maybe 2 inches.

I agree.
and for shaping, it is common sense. As an example you got a scavenge timing smashing into a smaller opening, even if the 3" calc shows it good to 736hp...it is still a wall.

I have 90hp in a boxer that won't flow without 2 inch per 900cc bank. Volume it can hold is always calced smaller than reality of outcome. a 1.03 inch pipe according to a calc is asking for acts of god...not friendly ones.

the pulse wave thing...

[TC]

Backpressure should not be confused with pulse wave tuning, your talking apples and oranges..............

A shock wave moving through a gasseous medium is TOTALLY different than the movement of the same medium which would create pressure. Ideally you want 0 backpressure with ideal pulse wave tuning to assist in scavenging, but unless you're going to dyno tune a whole series of different primary lengths/diameters don't worry about it......

[JeffMcKC]

HUH?

Why a Dyno, cant I watch my MPH at the track?

You need to control the wave, what you have in back pressure should be a result of the proper exhaust and used thats why so much planning goes into it.

[dieselgeek]

Backpressure should not be confused with pulse wave tuning, your talking apples and oranges..............

A shock wave moving through a gasseous medium is TOTALLY different than the movement of the same medium which would create pressure. Ideally you want 0 backpressure with ideal pulse wave tuning to assist in scavenging, but unless you're going to dyno tune a whole series of different primary lengths/diameters don't worry about it......

You should cite your sources, instead of plagarazing.

[Bucky67]

You should cite your sources, instead of plagarazing.

You know what's funny DG, I found that same exact quote (by TC) on a Mustang Forum dated March of 2008. I shit you not, quoted word from word. The screen-name is 67mustang302 from California. I couldn't help but laugh and had to post it.... Ha Ha Ha!!!

http://mustangforums.com/forum/5-0l-1979-1995-mustang/423658-backpressure-i-might-need-to-change-some-things.html

Thought you'd get a kick out of it........ LMAO!!!

[BBR]

And a Mustang site no less.

edit... Treed!