"mandrel" bend in exhaust

[rjlichau]

ok so i have a noob question and i couldnt find anything when i searched it but what is "mandrel" bends in a n exhuast system? i see proven members talk about it all the time in exhaust threads, but i have no idea what it is. 1 person could probably answer this.

thanks

 

[romeen]

After rereading my last post I thought of a simpler way to verbalize my thought.

Minimize the amount of exhaust gas energy lost to heat so there is more available to spin the turbine.

 

[MyÜberFastGSX]

Romeen: Your a smart guy.

P.S. Thanks for the oil pan in advance.

 

[Rice Over Wheat]

I just replaced a 3" fabbed exhaust system, magnaflow muffler, cat eliminated. Most sections were 3", however the downpipe flange to flex section was 2.5", there were a couple rippled 90 degree crush sections of 3" pipe, and over the rear axle it narrowed to 2.25" into the muffler which was 2.25" at the actual exit (ignoring the large tip).

New system: Megan downpipe with 2.5" entry to 3" after flex section. N1 replica system 3" mandrel bends all the way to the 4.5" exit (cut and welded 3" connectors to eliminate the two neckdowns).

There was a VERY clear difference in power delivery after the install, demonstrating that a poorly designed system with 2.25" to 3" sections and crush bends is a restriction even on a BT28 at 20 psi. I should have overhauled my exhaust system a long time ago as I had no idea it was this restrictive.

 

[romeen]

Romeen: Your a smart guy.

P.S. Thanks for the oil pan in advance.

1) Thanks Darrel.

2) Don't mention it.

 

[DSMunknown]

FINALLY some time to respond...

okay, this is about to be REALLY long to make this as through as possible. Going to go into manifolds a bit as well.

When you are designing an exhaust there are several factors to consider to create the best system for your needs. The thing here is to put yourself into the mindset of 'optimizing' your creation. In a perfect world, you would have a straight tube, of a certain diameter and of ideal length to create the perfect balance for maximum power at a given rpm. But like the Walgreen's commericals, we dont live in a town called perfect.

You're okay with me so far...

The first thing to think about is where you want your peak power to be. Basically this is predetermined by the design of the intake manifold due to the complexities involved in creating one. (which i may comment on my disapproval of many "sheetmetal" manifolds for anything outside of drag racing). Lets say that we want out peak power at eh, 7500 rpms, which is more likely the general target that everyone is after. We could then calculate a proper runner length for our manifold.

I must disagree. There is more than one part that dictates peak horsepower. The T-Too-Small can only flow a certain amount (I forget the actual numbers) and at a certain threshold it can't possibly flow any more, no matter what fancy schmancy intake manifold (IM) you have bolted on.

For natural aspiration this is easier. For turbochargers, the restriction of the turbine wheel complicates this only a little but you can use the same rules as the non-turbo cars.

NO. The turbine housing has extremely significant influences on the flow of exhaust gases (both before and after the turbo) that completely change the equation. You should know better than to say something like that. Exhaust theory for NA applications cannot be applied to forced induction applications.

anwho, exhaust gases are not a continuous stream, they are made up of pulses, like electricity. For these gas pulses to move through the header, the leading edge must be of a higher pressure than the surrounding atmosphere. The "body" of a pulse is very close to ambient pressure, and the tail end of the pulse is lower than ambient, approaching vacuum. The high pressure end of the next pulse is attracted to the low pressure section of the previous wave.

I would love to see where you got the information that exhaust gas pulses are nearly equivalent to absolute pressure found at sea level.

You're assuming an exhaust system that is utilizing exhaust scavenging. An exhaust manifold (EM) or exhaust system can unintentionally be designed (and often is) from the factory so that these exhaust gas pluses collide with each other, creating turbulence, and etcetera. You need to step away from the NA realm and step into the forced induction realm.

Furthermore, in my understanding, there is only a "low pressure section" at the end of one pulse because that pulse is on its way out, thus creating a small vacuum. The high pressure end is not "attracted" to anything but the vacuum, the very slight negation of pressure in front of it, and thus expands and incidentally fills this gap/vacuum of space.

Why? Higher pressure moves to lower pressure regions. Its a fundamental law. Best way ive heard this described is that the pulses are "sucked" through the tubes. This is what makes those 4 to 1 headers work so nice for high rpm power production.

Ahhh, so you finally get to it. You probably should have left out the section claiming that certain sections of an exhaust pulse are "attracted" to another exhaust pulse. The wording in that section is just poor and may confuse people.

Additionally, I think the term you are looking for is that in regards to NA exhaust scavenging, the exhaust gas pulses are "pulled" through the exhaust piping.

And way stock manifolds are they way they are. Aside from cost, they make the best low end power, which is what most car manufacturers are after. (0-60 times...) but this does come at the cost of having the engine to work harder and it leaves more spent gases in the combustion chamber that the latter of the two of have sucked out (allowing for more fresh air, better combustion, blah blah blah). While pondering over your latest Summit catalog, you obviously wont be able to know what the piece is tuned for. But generally "the best high-revving horsepower can be had with headers utilizing larger diameter, shorter primary tubes. Headers with smaller, longer primaries will get you
slightly better fuel economy and better street driveability".

I'll just leave that alone, because you're still discussing NA applications. Most of us aren't interested in that discussion.

cliff notes: if anyone says you need backpressure for an engine to run properly, smack them with your ring hand.

That really depends on what your definition of "properly" is.

So this then brings us to the topic at hand. The exhaust AFTER the turbine housing. That nice little chunk of metal does create backpressure. This way the larger A/R ratio housings create more power at the high end. They have low restriction to the exhaust pulses. At the same time this is always a compromise as the turbo's compressor side is dependant of the design of the turbine, again based on desired power. But ill leave that to google if anyone wants to learn that, as i would prefer to not turn this is into a book.

But with turbos, you get a nice built-in restriction type muffler.

Okay, so now we're finally on to turbos. Hopefully you'll stay on this path.

So, post-turbo exhaust.

Im sure everyone has heard that "bigger is better". Well that is not the case. Just for clarification for the numerous people who will skim this artictle. BIGGER EXHAUST PIPE IS NOT BETTER!!!!

Stop feeding misinformation.

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But why you ask? Thermodynamics!

Just saying one term isn't going to get you out of this hole you've dug yourself in to.

We want our exhaust gases nice and hot. Cold air is considerable denser than hot air. Dense air is then considerably harder to push.

I agree. But you need to step out of the theoretical realm for a second. When are exhaust gases "cold" in regards to an automotive combustion engine? My guess... never. You cannot dismiss the fact that the gases coming out of the head will always be hot - it just depends at what degree this "hotness" is. I would love to see the rate of molecule movement at 1000 degrees Fahrenheit versus 1600 degrees Fahrenheit.

This means the larger pipe, which creates a slow flow of gases, will give plenty of time for the gas to cool.

Now that is just funny (but still asinine).

This means that the pulses wont line up correctly and all that tuning of the manifold from before is useless. Unfortunately, we know of no accurate way to calculate optimal exhaust pipe diameter. Variables like bends or kinks in the piping, temperature fluctuations, differences in muffler design, and the lot, make selecting a pipe diameter little more than a guessing game.

Wow. Okay.... I'm sure if technicians and engineers sat down and did the math pertaining to one specific application they would be able to find the perfect parameters of an exhaust system. Oh yeah, and stop talking about exhaust pulses that have next-to-nothing to do with turbocharged applications. Stop!!!! Didn't you label this section of your response as "post-turbo exhaust" ?!? You're going to confuse the noobs and turn them into catalysts for misinformation. And that's all we need around here - more of that BS, noobs spouting of misinformation to other noobs, and thus a domino effect of misinformation.

For engines making 250 to 350 horsepower, the generally accepted pipe diameter 3 inches. Power levels in the 500 range, about 4 inches and so on.

It depends on if you're talking about BHP or WHP. There's an important difference to be made here.

So then what matter do mufflers play? Well there are several types of mufflers. All of which manipulate the exhaust gases. Refkective mufflers bounce the sound waves and create a destructive wave pattern, which helps neutralize the sounds. These are also the most complicated. The simple ones are the glasspacks. These are absorbtion mufflers. They are packed with steel wool or fiberglass to absorb the sound created. This is the least restrictive on the exhaust gases but also the least effective. The other type uses restriction type mufflers, which OEM tends to use.

Wouldn't completely disagree, but the least restrictive would be no muffler at all.

So what of welds causing interference in the path of the exhaust gases? While flow seems important its more of a matter of pressure. If quickly looked at a scrap pipe with a weld protruding into the inner piping. It was a MIG weld for sure. Id est the protrustion into the pipe to be about eh. under 1/32 of an inch. in the worse places. A piece of stainless i had which was TIG welded, quite well actually. I could get anything to measure it inside it, but its almost feels smooth when you run your finger over it.

so worse case scenario, we have 3 hundreths of an inch of protrusion with the mig. (0.03 inch). I mean like, overwelding and too much heat allowed it penetrate stupidly far into the pipe.

The pipe has a 3" diameter. Which would be a 9.24 inch circumference. But i think area might be more important to look at here so lets do that. Area would be about 7.1 SQ. IN cross-sectionally speaking.

Now if the ring is 1/32 from the outer wall, the radius from the center of the pipe to the protrusio is 1.47 inches. (a 2.94 inch diameter pipe which is 98% of the orignial). In terms of area, you would have 6.8 SQ INs of area. So that means 96% of the original area. This occures for a section that is only a 1/4 long on the pipe.

So what does that mean? Any conceiveable power less from having a weld would actually be unmeasureable to due the variables on the engine from heat soaking on the block, coolant, intake manifold, air, etc, plugs, elevation (by 30 ft...) etc. In where are back in walgreens perfect town, i could estimate the powerloss to be in the range of 1/20th of a horsepower.

so yup, that one 1/20th HP was well worth that 500 dollars extra you paid for.

I would like to see where your account of exhaust gas turbulence is and how it exactly effects your example. Also, you're assuming one single weld protrusion. And are you discussing EMs here or did you skip back to the piping system after the turbine housing? There's a huge difference between the two, and trying to pass it off as essentially the same is a mistake.









Just to conclude, I would like to use a quote from one of our esteemed wisemen that was posted recently:

Whoever said you need backpressure

I'm not sure that anyone competent ever did. WTF

 

[gsx951]

A "mandrel" is an actual assembly of sectioned steel on a cable that goes inside a pipe before it's bent, keeping it from crushing.
I just saw on How It's Made today that they do the brass tubing for trombones by freezing water inside them before bending, then forcing a steel ball through the tube afterward to size it. Unfortunately, steel's not so easily worked, and it would collapse through ice. Copper tubing is sometimes bent after being packed with damp sand.
Here's enough to make your head explode:
http://www.bendtooling.com/l-n.htm

Good link sir.

Generally, stuff in low production runs, that is not required to be really precise (like a DSM exhaust) is done on a machine like a #3 Pines http://www.pines-mfg.com/34.asp
There is a long steel rod that attaches to the mandrel with segmented balls that is shown in the link Defiant provided. The tube to be bent, is placed over the mandrel and then a set of clamps that precisely fit the tube od, grab hold. As the tube is bent around the radius die, the mandrel is drawn through the tube, supporting it from within, while a pressure die keeps the tube in contact with the radius die. Behind the tube and nested in the back of the radius die, you will find the wiper die, who's job is to fill the void at the back of the radius and add further support the tube.
If the set up of the machine is correct, the result is a smoothly bent tube with only a very minimal reduction at the bends.
One of the reasons for the fairly high cost associated with good exhaust systems is the tool sets needed to do the bending and the costs of the benders themselves. The Addison DB75 we run was $500,000 with tools....

 

[gsx951]

Also, to argue that a cut and weld type of construction is inferior to a single bent piece of pipe on the idea that weld melt-through restricts flow or creates turbulance is just silly.
I have welded thousands of exhaust systems and can assure everyone that it is the degree of bend and not the type of welding construction that create restriction in pipes.

 

[xioca]

To add to what gsx951 is saying, the amount of differece would be minute. Also you die hard bolt on exhaust people... you know there is a little play in those flanges and there may be a extreamly small misalignment that would impede the flow.