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Out of curiousity why does everyone use 2.5" on the crossover pipe

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Out of curiousity why does everyone use 2.5" on the crossover pipe

 
Old 02-06-2014, 04:10 PM
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Originally Posted by engineermike View Post
Haha thanks for that. So you know what a 5M', 5RN/T, 6B, 7EA, and 7FA are...
Yep...GE gas turbines...

My experience with GE was with frame 3 MS 3002's.

So...Since we are talking numbers and letters...

How about TB-5000 or TB5400...?
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Old 02-06-2014, 05:04 PM
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Sorry, no experience with those, just the big single-shaft ge's.
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Old 02-06-2014, 05:10 PM
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Originally Posted by engineermike View Post
Sorry, no experience with those, just the big single-shaft ge's.
Cool...

The biggest thing we had was a frame 5.

The TB's were Rustons...
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Old 02-07-2014, 05:13 AM
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Originally Posted by engineermike View Post
The first time the concept was introduced to me was when it was explained that the energy contained in a gas (enthalpy) is a function of temperature only (NOT pressure). In other words, 1000 deg F gas has the same amount of energy at 14.7 psia as it does at 100 psia.

Edit: Sometimes when trying understand the science, it's best to start with an open mind. I had been rebuilding engines and racing before going to engineering school. I disagreed with the professors and books many times because of what I thought I knew and what I had been told by other racers. In the end, I was wrong on many accounts, including this one.
I remember my thermo class a bit differently from you on this one, enthalpy is a combined measure of all of the parts of energy. The engines are described as heat pumps because they create power by using pressure changes to drive air pumps - those pressure changes are caused by exothermic chemical reactions, and huge heat transfers take place - gaseous expansion largely. Again - this is at least, what I was taught.


Enthalpy (H) - The sum of the internal energy of the system plus the product of the pressure of the gas in the system and its volume.

Pressure is inherent to the enthalpy change that drives a turbo. In the case of a car having lag and suddenly spooling a turbo with more RPM (and obviously exhaust gas volume), its a combination of temperature and pressure change, but I'd wager the volume*pressure component is significantly larger than the E component in the enthalpy equation on turbocharged internal combustion engine.


Edit - more directly. Air at 14.7 psia on either side of a turbine, with it 1000 F on the hot side, would not spool the turbine, but air at room temp on either side and 14.7 psia at outlet, 25 psia at inlet, would spool it no problem - hence why I say its because it allows for a faster pressure buildup. If you let pressure drop - of course temperature will drop, they are obviously interconnected, but temperature itself is only one factor - the cooling of the exhaust gas in this case is because of expansion through pressure change.

Last edited by Bimmerdude; 02-07-2014 at 05:18 AM.
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Old 02-07-2014, 12:46 PM
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Great topic. Back in November when my fabricator got my car to build my turbo set up I was all set on going 2.5" and right as he was ready to go he asked why not 2.25". I had to make a decision quick and did some research, made some calls but it was hard to find any kind of definitive answer due to the variations from combo to combo. Ultimately I choose to go with the 2.25" mainly because whether or not it increased velocity, pressure or spool time it wasn't small enough to cause a restriction in power up to the 1000RWHP or so.
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Old 02-08-2014, 10:56 AM
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Originally Posted by Bimmerdude View Post
I remember my thermo class a bit differently from you on this one, enthalpy is a combined measure of all of the parts of energy. ...

Enthalpy (H) - The sum of the internal energy of the system plus the product of the pressure of the gas in the system and its volume.....
You made me dig out my old c1996 Thermo book. Turns out that enthalpy is, in fact, internal energy + (pressure x volume). However, MY book points out on the very next line that PV = RT, therefore enthalpy = internal energy + RT, bringing it back to a function of temperature only. It's almost humorous the lengths my book goes to drive this home:

"...for an ideal gas, the internal energy is a function of temperature alone!"

"...the internal energy of an ideal gas is a function of temperature only is known as Joules's law"

"We can also show that enthalpy of an ideal gas is a function of temperature only."

"For an ideal gas the definition of enthalpy, h=u+pv, gives us h=u+RT"


Relating this to the turbine, the first law of thermo is:

q=w+h2-h1+(V2^2-V1^2)/2+g(z2=z1)

Since you're not adding heat to the turbine, q=0. I did some quick and dirty math and found that V2 and V1 are close enough to cancel each other out, and potential energy change is negligible. Therefore, it simplifies to

w=h1-h2

And as stated above, h is a function of temperature alone. Therefore, the work done by a turbine is a function of the temperature change of the gas passing through it only, or

w=f(T1)-f(T2)

This is bringing back bad memories of homework sessions extending to 4 am.

Originally Posted by Bimmerdude View Post
Pressure is inherent to the enthalpy change that drives a turbo.....
It just so happens that it's pretty easy and cheap to design something that extracts the heat energy using the mechanism of dropping the pressure, but the pressure drop is not required. However, it's important to understand the concept so you can also understand that high temperatures can get things done at a much lower mass flow rate and pressure drop, which I believe was my original point that lead into this. If you get caught up in the "pressure and velocity drive the turbine" idea, then you miss some important things like the ability to achieve <1:1 drive to boost ratio and the importance to keep turbine drive gas hot.

Note that if all this weren't true, a gas turbine could not operate. Even though the pressure change and mass flow rate through the turbine and compressor are identical, the turbine produces about 1.5 times more power than the compressor absorbs. It is able to do this because the temperature change across the turbine is much greater than the temperature change across the compressor.

Originally Posted by Bimmerdude View Post
...air at room temp on either side and 14.7 psia at outlet, 25 psia at inlet, would spool it no problem....
Know that the air would actually come out colder than it went in.

Think of this. . . you can extract work from a hot gas without undergoing a pressure drop (though possible, it's not typically practical). You can't extract work from a hot gas without undergoing a temperature drop.

Last edited by engineermike; 02-08-2014 at 11:08 AM.
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Old 02-08-2014, 04:47 PM
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I think we are arguing semantics - PV=nRT and nR is a pair of constants.

My point is that because the dynamic response of the turbo is related to pressure induced spooling (I understand that in an all out environment you wouldn't follow this logic - street car rules I think we can), maintaining a minimal volume will allow the pressure to raise more quickly, allowing for earlier and faster spool.

Everything you've said I would agree with for a jet turbine, but on an automobile we aren't going to design a 90% efficient system in our garages.

I also have been thinking that it makes sense to step the crossover (back on topic of the thread), I'd be curious if anyone has taken EGT readings at different points along a crossover.

This whole conversation makes me think it might make some sense to follow the header primaries rule and account for cooling but maintain velocity/temperature by reducing volume (cross section) as you approach the turbo?
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Old 02-13-2014, 02:26 PM
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Though I'd share. I'm putting together a mild combo and will hopefully be ready in the coming months. I went ahead and ordered/received 2" material for my up pipes and will merge it into a 10" piece of 2.5". I'll set the 2.5" about 1" passed the inlet flange into the exhaust housing.

I'll have an apples to oranges comparison at best, but I figured once it's done and I get a good datalog from the new set up I'd post what I find.
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Old 02-13-2014, 06:33 PM
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Originally Posted by Bimmerdude View Post
...maintaining a minimal volume will allow the pressure to raise more quickly, allowing for earlier and faster spool.
I would argue that pressurizing 1200 deg F exhaust gas to 45 psia adds heat of compression and increases the temperature to about 1800 deg F, thus giving it the energy it needs to quickly spool the turbine.

Originally Posted by Bimmerdude View Post
Everything you've said I would agree with for a jet turbine, but on an automobile we aren't going to design a 90% efficient system in our garages.
The same thermodynamic principals govern.

Originally Posted by Bimmerdude View Post
I also have been thinking that it makes sense to step the crossover (back on topic of the thread), I'd be curious if anyone has taken EGT readings at different points along a crossover.

This whole conversation makes me think it might make some sense to follow the header primaries rule and account for cooling but maintain velocity/temperature by reducing volume (cross section) as you approach the turbo?
I haven't taken these measurements, but I would bet that it doesn't cool much on it's way to the turbine. After all, the exhaust gas only spends about .008 seconds in the crossover pipe.
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Old 02-24-2014, 11:35 PM
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For you guys using a 2.25 crossover, how are you dealing with the merge, or more importantly the flange for a T4.

My original plan was to build the passenger side to bend upwards (manifolds down and forward) into the T4 flange by itself, and then build the drivers side and have it merge into the passenger side pipe wherever fitment allowed. 2.25 from the manifolds to the T4. The problem is the major size difference between a T4 flange and a round 2.25 pipe. So I was going to use a 2.25 to 3 inch cone and weld the T4 flange to the reshaped 3 inch pipe. Someone even recommended using a T4 flange that's made with a 2.25 round opening and bolting that straight to the rectangular T4 turbine.

Just curious how you guys are dealing with this. I would really like to find a decent 2-1 merge that goes from 2.25 to 3 and use that.
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Old 02-25-2014, 05:26 AM
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Originally Posted by HexenLord View Post
For you guys using a 2.25 crossover, how are you dealing with the merge, or more importantly the flange for a T4.

My original plan was to build the passenger side to bend upwards (manifolds down and forward) into the T4 flange by itself, and then build the drivers side and have it merge into the passenger side pipe wherever fitment allowed. 2.25 from the manifolds to the T4. The problem is the major size difference between a T4 flange and a round 2.25 pipe. So I was going to use a 2.25 to 3 inch cone and weld the T4 flange to the reshaped 3 inch pipe. Someone even recommended using a T4 flange that's made with a 2.25 round opening and bolting that straight to the rectangular T4 turbine.

Just curious how you guys are dealing with this. I would really like to find a decent 2-1 merge that goes from 2.25 to 3 and use that.
I usually do it like this:
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But also have done it similiar to what you are thinking of doing using a 2.25 to 2.5 Y and a 2.5 to T4 transition from Columbia River.
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Old 02-25-2014, 05:54 AM
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Originally Posted by HexenLord View Post
For you guys using a 2.25 crossover, how are you dealing with the merge, or more importantly the flange for a T4.

My original plan was to build the passenger side to bend upwards (manifolds down and forward) into the T4 flange by itself, and then build the drivers side and have it merge into the passenger side pipe wherever fitment allowed. 2.25 from the manifolds to the T4. The problem is the major size difference between a T4 flange and a round 2.25 pipe. So I was going to use a 2.25 to 3 inch cone and weld the T4 flange to the reshaped 3 inch pipe. Someone even recommended using a T4 flange that's made with a 2.25 round opening and bolting that straight to the rectangular T4 turbine.

Just curious how you guys are dealing with this. I would really like to find a decent 2-1 merge that goes from 2.25 to 3 and use that.
i have a divided housing and so i stayed divided all the way.

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Old 02-25-2014, 06:45 AM
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Could use a t4 to 2.5" flange and bump it up at the end. I think I paid $25 for mine, couldn't find a cheaper one this time....

http://www.ebay.com/itm/T4-Undivided-to-2-5-inch-Inlet-Turbo-Flange-Transition-1-2-thick-CNC-machined-/130912462220?pt=Motors_Car_Truck_Parts_Accessories&hash=item1e7afda58c&vxp=mtr
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Old 02-25-2014, 11:47 AM
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Some guys slip the piping as far as they can into the exhaust housing, then weld it.
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Old 02-25-2014, 11:50 AM
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Originally Posted by slow67 View Post
Some guys slip the piping as far as they can into the exhaust housing, then weld it.
I've always wanted to try this, had i not re-used an old flange i wouldve.

Probably would've taken the time to form the ends into cones to match the ID form of the turbine housing. no gap, no turbulence when it enters, has to be worth something, maybe damn near unmeasurable, but gotta be worth something.


Im talking, slitting the pipe, knocking it in there, to form its own shape, weld it up, and grind it smooth... inside and out.
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Old 03-02-2014, 10:46 AM
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Any testing data?
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Old 03-03-2014, 10:31 AM
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Originally Posted by JAX04 View Post
I've always wanted to try this, had i not re-used an old flange i wouldve.

Probably would've taken the time to form the ends into cones to match the ID form of the turbine housing. no gap, no turbulence when it enters, has to be worth something, maybe damn near unmeasurable, but gotta be worth something.


Im talking, slitting the pipe, knocking it in there, to form its own shape, weld it up, and grind it smooth... inside and out.
I've seen a few references to doing this but no data either way.

Probably helps with the sealing at the flange as well for those running higher drive pressures.
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Old 03-03-2014, 11:00 AM
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Quote:
Originally Posted by HexenLord View Post
For you guys using a 2.25 crossover, how are you dealing with the merge, or more importantly the flange for a T4.

My original plan was to build the passenger side to bend upwards (manifolds down and forward) into the T4 flange by itself, and then build the drivers side and have it merge into the passenger side pipe wherever fitment allowed. 2.25 from the manifolds to the T4. The problem is the major size difference between a T4 flange and a round 2.25 pipe. So I was going to use a 2.25 to 3 inch cone and weld the T4 flange to the reshaped 3 inch pipe. Someone even recommended using a T4 flange that's made with a 2.25 round opening and bolting that straight to the rectangular T4 turbine.

Just curious how you guys are dealing with this. I would really like to find a decent 2-1 merge that goes from 2.25 to 3 and use that.

these were bends, 2-1/4" to 3" merge and 3" to t4 adapter I bought and used to make the merge I have. I know the wastegate placement isnt ideal or the greatest but it works for now and next time I build one I know better. since the 3" round to t4 was slightly bigger I had about 1/8" of cutting to do and it was a smooth transition into the flange
Attached Thumbnails Out of curiousity why does everyone use 2.5&quot; on the crossover pipe-screenshot_2014-02-02-16-48-40.jpg   Out of curiousity why does everyone use 2.5&quot; on the crossover pipe-screenshot_2014-02-02-16-47-40.jpg   Out of curiousity why does everyone use 2.5&quot; on the crossover pipe-screenshot_2014-02-02-16-46-42.jpg  
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Old 02-23-2016, 07:17 PM
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Phil,
This is one he'll of a thread.

How would you tackle a rear mount setup? Say 5.3-6.2L, T4 mount, 1000hp goal in a first gen Camaro. The wheel base is approximately 108".
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Old 02-23-2016, 07:52 PM
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Slip fitting the pipe gives pipe extra structure support. Over but joints. I do this to the pipes. Have access to a pipe expander. Any professional muffler shop will have one. And not charge much to expand one end of the pipe for a slip fit. Not only that. It will be easier to rotate the pipes if got bends and easier to mock up the pipes.
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