"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]

It's a bending process in which the diameter of the pipe remains constant throughout the bend. This is opposed to press bent piping in which the diameter of the pipe is smaller in the bend which restricts flow through that portion. Mandrel bending is more expensive because of the equipment required.

 

[rjlichau]

thank you i appreciate it cause i am getting a custom exhaust put on and that is something that i wanted to ask about but if the shop didnt know what it was i was going to need to explain it to them.

 

[rst95eclipse]

Mandrel bends have the same diameter throughout the bend. Crush bends either are rippled or have a discontinuation of the diameter, crushing the pipe inward. Mandrel bends are virtually unobstructed.

 

[MyÜberFastGSX]

If you're going to have a local shop do your exhaust it can save you a lot of money, chances are, they don't do mandrel bends.

Crushed bends greatly reduce the flow of the exhaust, I saw somewhere that A crushed bent 3" exhaust is only 2.25 inches at the point of the bend. Since your exhaust is only as good as the weakest link, you just spent alot of extra money on 3" piping when you're only running a 2.25" exhaust system.

 

[romeen]

Most exhaust shops do not have a mandrel bending machine because they are rather expensive. Instead they may have a selection of mandrel bends in specific diameters, bend angles, etc which they can weld together to form a mandrel bent system.

 

[MyÜberFastGSX]

The only problem with having them weld together premade mandrell bends is that with 3 inch exhaust tubing, you usually don't have alot of room to work with, so you need exact bends. Also, with all those welds, there are more places for leaks to develope.

Just goes back to the rule of thumb. Don't try to save money by cutting corners, or in this case "bending" them. Pay to have it done right the first time.

You will thank yourself in the end.

 

[Eastwood95]

Dude, there's like a thousand choices to pick from when getting a cat-back, or turbo-back exhaust for a 2G GST. It might be a little more expensive but If you can afford it, it's worth it. Are you going with custom for the price?

 

[Defiant]

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

 

[Low Impedance]

Most exhaust shops do not have a mandrel bending machine because they are rather expensive. Instead they may have a selection of mandrel bends in specific diameters, bend angles, etc which they can weld together to form a mandrel bent system.

We've used this method numerous times and it works quite well. Considerably cheaper than renting or god help us, buying a mandrel bending machine. And the results are about on par.

 

[DSMunknown]

Not this topic again...

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

Google is your friend!

Most exhaust shops do not have a mandrel bending machine because they are rather expensive. Instead they may have a selection of mandrel bends in specific diameters, bend angles, etc which they can weld together to form a mandrel bent system.

True. However, because of these prefabricated sections of pipe, they need to be welded on to the rest of the exhaust system. Thus there will usually be penetration from the welding beads, and thus causing flow restrictions.

The only problem with having them weld together premade mandrell bends is that with 3 inch exhaust tubing, you usually don't have alot of room to work with, so you need exact bends. Also, with all those welds, there are more places for leaks to develope.

Agreed.

We've used this method numerous times and it works quite well. Considerably cheaper than renting or god help us, buying a mandrel bending machine. And the results are about on par.

I would beg to differ.

 

[Low Impedance]

Not this topic again...


True. However, because of these prefabricated sections of pipe, they need to be welded on to the rest of the exhaust system. Thus there will usually be penetration from the welding beads, and thus causing flow restrictions.



I would beg to differ.

If we are doing something that is very nice in appearence and quaility, we might tig the seams (especially with Stainless steel) which penetrates through, but have no protrusions through the inside wall of any considerable measure. for things like my rally car, we mig everything and use low carbon or generic piping. Even here you have considerably more restrictions from have 45 degree bends in your piping to clear various suspension components than you do from the protrusion from the MIG weld.

If you want to compare end prices of doing it this way, we usually use maybe 10 dollars in Argon, 15 dollars in rods (maybe a dollar in mig wire if we are mig welding it), about 200 in materials (including the flex pipe which is the most expensive part. So for under 300 dollars for a turbo back exhaust, with mandrels bends id say that is more than comparable to paying nearly 1000 or more dollars for something that is also turbo back, and one piece mandrel.

 

[rjlichau]

Thanks . I appreciate all the information.

 

[DSMunknown]

If we are doing something that is very nice in appearence and quaility, we might tig the seams (especially with Stainless steel) which penetrates through, but have no protrusions through the inside wall of any considerable measure. for things like my rally car, we mig everything and use low carbon or generic piping. Even here you have considerably more restrictions from have 45 degree bends in your piping to clear various suspension components than you do from the protrusion from the MIG weld.

What about the issue of porosity? Or incomplete joint penetration?

If you want to compare end prices of doing it this way, we usually use maybe 10 dollars in Argon, 15 dollars in rods (maybe a dollar in mig wire if we are mig welding it), about 200 in materials (including the flex pipe which is the most expensive part. So for under 300 dollars for a turbo back exhaust, with mandrels bends id say that is more than comparable to paying nearly 1000 or more dollars for something that is also turbo back, and one piece mandrel.

The issue is NOT about price. The issue is about airflow and maximizing its potential.

 

[Low Impedance]

by that logic then the turbo manifolds shearer and bushur sells shouldnt have as good flow as a CNC mandrel piece because they have ususally 2-3 welds in the tube sections. But realisitically, if all things are the same otherwise, you wont be able to measure the difference if the welds are professional, and not some first timer who is getting paid minimum wage.

 

[Gruppe-S1]

by that logic then the turbo manifolds shearer and bushur sells shouldnt have as good flow as a CNC mandrel piece because they have ususally 2-3 welds in the tube sections. But realisitically, if all things are the same otherwise, you wont be able to measure the difference if the welds are professional, and not some first timer who is getting paid minimum wage.

Exhaust gas flows along the exhaust walls first creating drag and friction. Welds are a necessary evil in any exhaust system - but the less welds the better. There's a reason why NHRA does not allow rolled / welded tube to be used in roll cages - only extruded seamless tube. That said, the difference between a few high quality welds versus no welds is marginal, but costs being equal the one piece system is superior in terms of both flow and material strength.

 

[Low Impedance]

Exhaust gas flows along the exhaust walls first creating drag and friction. Welds are a necessary evil in any exhaust system - but the less welds the better. There's a reason why NHRA does not allow rolled / welded tube to be used in roll cages - only extruded seamless tube. That said, the difference between a few high quality welds versus no welds is marginal, but costs being equal the one piece system is superior in terms of both flow and material strength.

yeah but im not relying on my exhaust system to protect me when i roll my car. im am going to have to start bringing out fluid dynamics forumlas to prove that the bead from a TIG weld plays effectively no part in restricting the exhaust?

 

[MyÜberFastGSX]

The welds don't restrict the flow at all, they're just more places for leaks to devolpe as time goes by.

What restricts the flow is the crushed bends. A 3 inch exhaust tube loses almost a full inch in a crush bend.

So really a crushed bent 3 inch exhaust is no better, if not worse than a 2 inch mandrell bent. Atleast with a 2inch mandrell bent exhaust, there is an even flow. Not a restriction everytime the exhaust takes a bend.

 

[DSMunknown]

Exhaust gas flows along the exhaust walls first creating drag and friction. Welds are a necessary evil in any exhaust system - but the less welds the better. There's a reason why NHRA does not allow rolled / welded tube to be used in roll cages - only extruded seamless tube. That said, the difference between a few high quality welds versus no welds is marginal, but costs being equal the one piece system is superior in terms of both flow and material strength.

Agreed. But while the difference may be "marginal", there is a difference. IMHO using the term "marginal" is relativistic, which is why I would be hesitant to use it in these terms.

yeah but im not relying on my exhaust system to protect me when i roll my car. im am going to have to start bringing out fluid dynamics forumlas to prove that the bead from a TIG weld plays effectively no part in restricting the exhaust?

Yes, please make your case in full. If what you claim is accurate, then it will not have much of an issue holding up to criticisms and critiques. BTW, it would help those who need some more background info.

The welds don't restrict the flow at all, they're just more places for leaks to devolpe as time goes by.

Welds don't necessarily "restrict the flow" of exiting gases, but they can cause unwanted turbulence (and thus causing unwanted restriction in flow).

What restricts the flow is the crushed bends. A 3 inch exhaust tube loses almost a full inch in a crush bend.

No. There is more than one factor that causes flow restrictions. And it depends at what angle the crush bent pipe is at. But you're bringing in the other issue which has already been discussed (mandrel bent piping vs. crush bent piping). The issue has changed to welding techniques and if/how they can cause flow restrictions. It should be clear to all of us that mandrel bent tubing trumps crush bent tubing in more ways than one.

So really a crushed bent 3 inch exhaust is no better, if not worse than a 2 inch mandrell bent. Atleast with a 2inch mandrell bent exhaust, there is an even flow. Not a restriction everytime the exhaust takes a bend.

Not necessarily true. Using your example, it would depend on where the piping was being crushed at in relation to the turbine housing, and how severe/subtle the collapsed section of piping was. You're also assuming that the two piping systems take the same general (or exact) degrees in directional change.

Again, there are other restrictions that need ot be taken into consideration, not simply that the inside diameter (ID) piping may say constant or not. One restriction you are overlooking is the bend itself - that is also a restriction when regarding airflow.

 

[Low Impedance]

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.

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. 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.

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. 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. 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".

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

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.

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!!!! But why you ask? Thermodynamics! We want our exhaust gases nice and hot. Cold air is considerable denser than hot air. Dense air is then considerably harder to push. This means the larger pipe, which creates a slow flow of gases, will give plenty of time for the gas to cool. 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. 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.

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.

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.

 

[romeen]

I am by no means an expert on fluid dynamics. But real world results has demonstrated that exhaust considerations in a turbo car are different from those of a n/a car. The pressure gradient pre vs. post turbo is what dictates the velocity of the gases traveling past the turbine. The greater the gradient (pressure difference) the greater the velocity.
This is why I heat wrapped my exhaust manifold and intentionally did not wrap my DP as so many do. Increased heat=increased pressure. The cooler the gas (although more dense) is under lower pressure. In theory I have hopefully increased the pressure gradient between manifold and DP. I don't have any actual measurements to prove this though.

I came up with this idea on my own. I welcome any counter arguments if someone sees a flaw in this thinking.

 

[Low Impedance]

its a clever thougt for sure. Honestly you could wrap the upper portion of the DP an the effect will remain. (and your alternator will thank you).

 

[DSMunknown]

I am by no means an expert on fluid dynamics. But real world results has demonstrated that exhaust considerations in a turbo car are different from those of a n/a car. The pressure gradient pre vs. post turbo is what dictates the velocity of the gases traveling past the turbine. The greater the gradient (pressure difference) the greater the velocity.
This is why I heat wrapped my exhaust manifold and intentionally did not wrap my DP as so many do. Increased heat=increased pressure. The cooler the gas (although more dense) is under lower pressure. In theory I have hopefully increased the pressure gradient between manifold and DP. I don't have any actual measurements to prove this though.

I came up with this idea on my own. I welcome any counter arguments if someone sees a flaw in this thinking.

An interesting proposal. Although I do not have time to fully respond to this, I would like to add some food for thought - the turbine housing "generally" absorbs a couple hundred degrees of heat when the exhaust gases are rushing through it. Think about how that fits into your example.

 

[romeen]

An interesting proposal. Although I do not have time to fully respond to this, I would like to add some food for thought - the turbine housing "generally" absorbs a couple hundred degrees of heat when the exhaust gases are rushing through it. Think about how that fits into your example.

I'll take a stab at where you might be going with this. As the exhaust gas loses heat it loses energy. Specifically, the more heat it loses the more it's rate of expansion slows.
Slower rate of expansion=less force driving the turbine. So perhaps you're implying that I should wrap the turbine housing as well. (?)

In summary, retain as much exhaust gas energy as possible until after it has passed through the turbine.

I actually wanted to wrap/insulate the turbine housing but with the 1" wide roll of wrap that I had I think it would have been rather difficult and frustrating, especially with the turbo on the car. Maybe I'll get one of those kits, basically a cover which fits over the housing.

I am open-minded and always welcome constructive criticism and new ideas. Looking forward to your response Anthony.

 

[Low Impedance]

An interesting proposal. Although I do not have time to fully respond to this, I would like to add some food for thought - the turbine housing "generally" absorbs a couple hundred degrees of heat when the exhaust gases are rushing through it. Think about how that fits into your example.

this is probably another contributing factor to way larger turbine A/R usually allow for better top end power. Seeing the exhaust gases flow with less restriction, there is less heat being built up in the manifold. The smaller turbos usually make good low end torque whichi might be similiar to the effect of a factory iron manifold as mentioned previously, on the exhaust pulses. But then it chokes out the top end.