Compound turbo piping sizes

roachie

roachie

Taco Master
Joined
Nov 2, 2007
Messages
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Lets discuss hot and cold pipe sizing and thickness.

Most kits I see use a 3.5" or bigger hot pipe. I have seen some use a 4". At what point does the 3.5" become restrictive?

Also seen some using 18ga/16ga and a lot of sch 40 pipe. I do realize the thicker pipe retains heat much better. I have also heard that thin pipe will help on the heat retention due to air being a natural insulator (I really don't want to get out the thermo books, don't quote me on that.). Also what effect would jet hot coating and wrapping the thin metal have on its fatigue life?

Getting on the cold side, 4" turbo inlet. What effects would a 4" cold pip have vs. a 3.5 or 3".

I know the smaller the better the spool, I wanna know the limits though.

Anyone know a pressure limit (boost) on 16ga exhaust pipe?
 
I just used thin wall 4" had it coated and wrapped and seen some crazy egt's and all is still fine. material is easier to work with and mucheasier to get different radious bends.
 
Help me push said:
Lets discuss hot and cold pipe sizing and thickness.

Most kits I see use a 3.5" or bigger hot pipe. I have seen some use a 4". At what point does the 3.5" become restrictive?

Also seen some using 18ga/16ga and a lot of sch 40 pipe. I do realize the thicker pipe retains heat much better. I have also heard that thin pipe will help on the heat retention due to air being a natural insulator (I really don't want to get out the thermo books, don't quote me on that.). Also what effect would jet hot coating and wrapping the thin metal have on its fatigue life?

Getting on the cold side, 4" turbo inlet. What effects would a 4" cold pip have vs. a 3.5 or 3".

I know the smaller the better the spool, I wanna know the limits though.

Anyone know a pressure limit (boost) on 16ga exhaust pipe?
Click to expand...

On the hot side it's basicly what fits easiest between the top and bottom turbos. However, bigger IS NOT better here. The higher you keep the velocity in the hot pipe, the better you are at spooling the primary. Remember, the primary housing is the main restriction. If the hot pipe is too big, the gases expand and cool...not good for spool at all.

The the hot pipe should not be any larger than the inducer of the turbine on the top turbo...so in most cases, 3" is optimum!!! This is real easy to accomodate if the primary turbo has a T4 inlet flange!!!

Not only should you get out the thermo book, get out the Machine Design text also!!!!

Pressure limit on 16ga pipe??? You need to determine the radial and tangential stress distributions across the pipe wall. However, since this is considered a "thin wall" application it is reduced down to what is known as Hoop Stress.



Average Tangential stress = (p(d-t))/2t

Where

t = wall thickness
d+t = average diameter
p = internal pressure

Since we are dealing with steel, Average tangential Stress should be less than 30,000 PSI
 
On the hot side, look at the small outlet from the exhaust manifold, a divided T-3 flange has 3.9 sq inches of opening.

A 3" ID hot pipe has 7 sq inches of opening.


You lose efficiency when you allow the exhaust gasses to expand and then force them to compress again as they enter and flow through the primary turbo's exhaust housing.

In my mind, the hot pipe should be roughly the same size as the top secondary turbo's turbine cross sectional area + wastegate's cross sectional area.

A standard S300 turbine exducer 71mm calcs to 6.14 sq inches. A 50mm external gate calcs to 3.04 sq inches.

9.18 sq inches would require a 3.4" ID hot pipe.

Another consideration is the inlet size of an open T-6 flange. A T-6 flange comes in at 9.19 sq inches. There is no sense in allowing the exhaust gasses to expand more than 9.19 sq inches because you will lose efficiency compressing the gasses through the T-6 flange.

My conclusion is that 3.5" is the perfect sized hotpipe.


Hopefully a professional twin turbo builder will add some real world experience to prove or disprove my mathematical theories.
 
Just ran the numbers, and for the pressures we are considering, 12ga thick exhaust tubing is MORE than adequate...you just have to support the primary turbo on a pedestal.

12ga. Aluminum is also more than adequate for a cold pipe...3.5" being the best to use on a S400 or GT42 due to the outlet size of the discharge flange.
 
Help me push said:
Lets discuss hot and cold pipe sizing and thickness.

Most kits I see use a 3.5" or bigger hot pipe. I have seen some use a 4". At what point does the 3.5" become restrictive?

Also seen some using 18ga/16ga and a lot of sch 40 pipe. I do realize the thicker pipe retains heat much better. I have also heard that thin pipe will help on the heat retention due to air being a natural insulator (I really don't want to get out the thermo books, don't quote me on that.). Also what effect would jet hot coating and wrapping the thin metal have on its fatigue life?

Getting on the cold side, 4" turbo inlet. What effects would a 4" cold pip have vs. a 3.5 or 3".

I know the smaller the better the spool, I wanna know the limits though.

Anyone know a pressure limit (boost) on 16ga exhaust pipe?
Click to expand...

I think I remember reading that the thicker wall pipe was generally used if it was going to support a decent bit of weight as opposed to the thinner wall stuff.

Air being a good insulator or not, I would still wrap the pipe or have it coated. The temp difference between the pipe and ambient is going to be huge. The higher that difference, the more heat will be transferred by convection (or conduction for that matter).

What kind of pipe are you using that you are worried about fatigue life?
 
the flow conditions inside the pipe are turbulent, so the insulating qualities of the internal gasses are basicly nill...there is almost no fluid boundary layer
 
How about the heat transfer book as well.... I don't even want to go find ANYof those texts.

You guys are starting to make this sound like work instead of a hobby.
 
Most of this is geared for my triples. I want to make this as efficient as possible. All 3 chargers will likely be a T4 flange, unless I find a turbine I like that has a matching T3 housing for the primaries.

Thinking of using mild steel pipe just because its easier to work with. And things like this are more readily available:

http://www.mandrel-bends.com/catalog/collectors-megaphones-100/merge-collectors-117/2-1-merge-collector-3-00-inlets-3-50-outlet-mild-steel-1169.html

http://www.mandrel-bends.com/catalog/transitions-forms-45/turbo-forms-99/3-00-to-t4-turbo-form-666.html

The fatigue worries me on the set I'm doing on my ford. Space is limited and its a street truck that will see LOTS of heat cycles. The primary will be supported anyway.

And I need to figure the interstate piping on both.

Also fabing a IR intake for my 24V head, I was worried about the pipe being able to hold 100psi.

Any thoughts on the cold side stuff now? This is all extremely helpful info!
 
CSM diesel said:
You guys are starting to make this sound like work instead of a hobby.
Click to expand...

LOL but it is comforting to know that what we have learned can be applied outside of the classroom.

Generally speaking Roachie - there is no concern with fatigue with steel materials. If you were going to be building a hot pipe out of aluminum to help support your primary/primaries, then I would be concerned with fatigue.


Help me push said:
Most of this is geared for my triples. I want to make this as efficient as possible. All 3 chargers will likely be a T4 flange, unless I find a turbine I like that has a matching T3 housing for the primaries.

Thinking of using mild steel pipe just because its easier to work with. And things like this are more readily available:

http://www.mandrel-bends.com/catalog/collectors-megaphones-100/merge-collectors-117/2-1-merge-collector-3-00-inlets-3-50-outlet-mild-steel-1169.html

http://www.mandrel-bends.com/catalog/transitions-forms-45/turbo-forms-99/3-00-to-t4-turbo-form-666.html

The fatigue worries me on the set I'm doing on my ford. Space is limited and its a street truck that will see LOTS of heat cycles. The primary will be supported anyway.

And I need to figure the interstate piping on both.

Also fabing a IR intake for my 24V head, I was worried about the pipe being able to hold 100psi.

Any thoughts on the cold side stuff now? This is all extremely helpful info!
Click to expand...

As far as the cold piping goes, based on what was posted above - 12 ga. steel pipe would be more than adequate for the hot pipe, without running the numbers for some higher pressures, I would guess that it would also be adequate on the cold side.

IR intake - same comments. Are you going to mod the runners in the head? PM if necessary.
 
CSM diesel said:
How about the heat transfer book as well.... I don't even want to go find ANYof those texts.

You guys are starting to make this sound like work instead of a hobby.
Click to expand...


LMAO...

well, I got currious about the temperature affects on carbon steel.

Per ASME Section II part D table 1A (Boiler and pressure vessel code)

anything over 900F at pressure (pressure being much higher than we would see in an automotive use) you would be in violation.

However, it looks that at 1500F, allowable stress for carbon steel is about 3000 psi...fyi, this is just a guess
 
j-rod said:
LOL but it is comforting to know that what we have learned can be applied outside of the classroom.

Generally speaking Roachie - there is no concern with fatigue with steel materials.
Click to expand...

To a point...you will end up with a crack on a poor weld, and if you are worried, a post weld heat treat will perform miracles!!! Using a rose bud to anneal the welds works in a pinch
 
Yes, I plan on dividing the runners. Although I'm not sure how, short of JB weld forming a divider. And I haven't even began to start calculating the sizes on it.

I planned on anealing (at home) and getting all of the hot stuff ceramic coated. Although tht may change on the ford with my time constraints.

Getting a little more into this, how about splitting the flow?

I.E. My 3.5" hot pipe has to split, now would it be better to have it going to a smooth transition (see link above) or just cap it of flat and weld some ears on? Then what size do you run, 3", 2.75", 2.5"?

Same thing on combining the compressor outlets. My thinking is they should stay the same as the discharge on the compressor cover and merge as close as possible to the secondary inlet.

Or just merge them as close as possible and run X size pipe to the secondary. How do we determie X?


I'm just full of questions aren't I.
 
That is what I'm seeing, also seeing that I need at least a 2.75" to feed a T4 inlet.
 
Help me push said:
That is what I'm seeing, also seeing that I need at least a 2.75" to feed a T4 inlet.
Click to expand...

I ran into the same thing with my smaller build...I'm sure you've already seen the pics, but here's what I came up with for the hot-pipe and splitting to two (half area) hot-pipes

Maybe it will spark an idea...each "foot" started as a 2" x 3" section of rectangular tubing...I basically cut the piece lengthwise and took a section out to get the internal area and external dimensions right.

17074_1121825168223_1304880143_3-1.jpg
 
Diesel Freak said:
Pressure limit on 16ga pipe??? You need to determine the radial and tangential stress distributions across the pipe wall. However, since this is considered a "thin wall" application it is reduced down to what is known as Hoop Stress.



Average Tangential stress = (p(d-t))/2t

Where

t = wall thickness
d+t = average diameter
p = internal pressure

Since we are dealing with steel, Average tangential Stress should be less than 30,000 PSI
Click to expand...

t = 0.0747"
d = 4.00"
p = 105 psi max

(105(4.00-0.0747))/2*0.0747 = x
(105(3.9253))/2*0.0747 = x
412.1565/0.1494 = x
x = 2758psi

I'm looking at max yield strength of 8000psi with 6061-0?
 
CTDYoungGun said:
I ran into the same thing with my smaller build...I'm sure you've already seen the pics, but here's what I came up with for the hot-pipe and splitting to two (half area) hot-pipes

Maybe it will spark an idea...each "foot" started as a 2" x 3" section of rectangular tubing...I basically cut the piece lengthwise and took a section out to get the internal area and external dimensions right.

17074_1121825168223_1304880143_3-1.jpg
Click to expand...

more pictures:drool1:
 
Help me push said:
The fatigue worries me on the set I'm doing on my ford. Space is limited and its a street truck that will see LOTS of heat cycles. The primary will be supported anyway.

And I need to figure the interstate piping on both.

Also fabing a IR intake for my 24V head, I was worried about the pipe being able to hold 100psi.

Any thoughts on the cold side stuff now? This is all extremely helpful info!
Click to expand...


If you are worried about fatigue, you may want to add a flex joint.
 
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