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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 05-22-2013, 07:39 AM
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Just general thinking here, no specifics.....It's too easy to get caught up in conventional N/A thinking when doing the piping on a turbo. You HAVE to run smaller pipe on the hot side of the turbo. A turbine operates by pressure differential. You make it small on one end and big on the other (outlet). Reason being is not just velocity but also to keep the heat in the gas. The exhaust gas expands as it cools so you want that cooling to happen on its way through the outlet of the turbine which is why a big honkin' pipe on the actual exhaust is good on a turbo car. That's what gives you the spool. The higher the difference in pressure before the inlet and outlet (hot gas little pipe, cooing gas big pipe), the harder it's going to blow through that turbine and spin it. That effect also aids in PULLING the exhaust out of the engine because the higher the velocity of the gas pulse, the bigger the vacuum pulse is behind it that is yanking the next pulse along with it. At a certain point the bigger the hot side pipe, the more chance there is for the exhaust gas that you want driving your turbine to loose energy by loosing heat, pressure, and velocity. Yeah yeah yeah backpressure and what not but don't forget the cold side is generating vacuum to yank the hot side out which isn't there in near the capacity in a N/A setup.

The reason you have to go bigger on a N/A car with higher power is that the gas starts cooling and expanding as soon as it leaves the head so you've got to give it some place to go. Problem with a street car is tuning it correctly to operate acceptably at a wide operation range rather than do good in one spot while being a bitch at another. The problem all of us have is thinking we know more than the experts. Ha ha.
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Old 05-22-2013, 10:36 AM
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Originally Posted by SSellers View Post
]The exhaust gas expands as it cools
incorrect sir
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Old 05-22-2013, 10:45 AM
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SSellers, et al:

Interestingly, if you read up on Bernoulli's principle and the venturi effect, you will see that these things are not necessarily as you have described them here... Throw in the ideal gas law, and it gets really complicated (opposite of one thing you mentioned, SSellers).

I wonder how often it might be that the reason there is lower pressure in the hot side piping with smaller tubes is simply due to Bernoulli's principle alone, and possibly the addition of the higher flowing turbine housing just adds to the effect?....

Just thinking out loud about this complex topic. Maybe some of the actual engineers on here and the OP will chime back in about some of this stuff.
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Old 05-22-2013, 12:04 PM
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Originally Posted by SSellers View Post
Just general thinking here, no specifics.....It's too easy to get caught up in conventional N/A thinking when doing the piping on a turbo. You HAVE to run smaller pipe on the hot side of the turbo. A turbine operates by pressure differential. You make it small on one end and big on the other (outlet). Reason being is not just velocity but also to keep the heat in the gas. The exhaust gas expands as it cools so you want that cooling to happen on its way through the outlet of the turbine which is why a big honkin' pipe on the actual exhaust is good on a turbo car. That's what gives you the spool. The higher the difference in pressure before the inlet and outlet (hot gas little pipe, cooing gas big pipe), the harder it's going to blow through that turbine and spin it. That effect also aids in PULLING the exhaust out of the engine because the higher the velocity of the gas pulse, the bigger the vacuum pulse is behind it that is yanking the next pulse along with it. At a certain point the bigger the hot side pipe, the more chance there is for the exhaust gas that you want driving your turbine to loose energy by loosing heat, pressure, and velocity. Yeah yeah yeah backpressure and what not but don't forget the cold side is generating vacuum to yank the hot side out which isn't there in near the capacity in a N/A setup.

The reason you have to go bigger on a N/A car with higher power is that the gas starts cooling and expanding as soon as it leaves the head so you've got to give it some place to go. Problem with a street car is tuning it correctly to operate acceptably at a wide operation range rather than do good in one spot while being a bitch at another. The problem all of us have is thinking we know more than the experts. Ha ha.
And the thread is beginning to fall apart.
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Old 05-22-2013, 03:14 PM
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Originally Posted by jridenour31 View Post
and the thread is beginning to fall apart.
rofl! :-)
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Old 08-17-2013, 12:21 AM
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Old 08-17-2013, 08:14 AM
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Originally Posted by 1fast bird View Post
This thread makes me feel better about using 2" into a single 2.5" for my crossover.
Your ET/MPH wasn't enough?
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Old 08-17-2013, 08:23 AM
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Yes, virtually everything on SSellers post is incorrect. Sorry.
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Old 08-17-2013, 08:30 AM
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Originally Posted by Phil99vette View Post
Your ET/MPH wasn't enough?
This is a new project. Sold the T/A and am putting together a 99 s10 with a 5.3 and a little tc76. Just want something fun for the street. Would love to have a low 10, high 9 second street truck..... And no that ET/MPH wasn't enough for me, but apparently it was enough or my 402
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Old 08-22-2013, 04:45 PM
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Hey guys great information here, really appreciate it! Putting a 5.3 with 76mm turbo in my MKIII Supra and and working on headers. So I did a Solidworks design and used some of Pill99Vettes numbers from the first page to do a flow simulation. The manifolds will be log style due to my tight quarters and wanting to keep all the accessories low and headers high. Also the truck headers are heavy as ****! These will weigh about 7lbs each and do the job for 650rwhp. I am using 1 1/2" SCH 5 304 which are 1.9 OD, 1.77 ID. Anyways using Phil's 300fps at the header, flow simulation says I will have about 950fps at the exit of my manifolds. I am going with a 2" cross over so the setup should spool like crazy. Anyways here is a picture of the simulation:
Name:  LSLogManifoldFlowSim1Velocityf_s_zps43c900f6.png
Views: 415
Size:  317.0 KB

With all that said this does not take into account valve timing or anything else.

Thanks again,
Seth
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Old 08-22-2013, 04:56 PM
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Solidworks is awesome, but only 2 cylinders flow at a time and those two are 90 deg out of phase.
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Old 08-22-2013, 05:20 PM
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Hey Mike, I probably should have looked that before! lol Anyways it says 734fps with 1 and 3 and 772fps from 2 and 4. Not really sure what all this is determining for me but it is fun! Only one way to find out if it will do the job, run it!

Seth
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Old 08-22-2013, 06:30 PM
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I wonder how much power a 2 inch crossover will support on a 5.3L? At what point do you have to go larger? Or is it more due to engine size then power output when sizing the crossover?
Hope that makes sense.
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Old 08-22-2013, 06:30 PM
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Losses start stacking up over 150 f/s. 700 is moving, but ok for short distances, especially if the CFD shows no big problems. I was on SW today, by the way.
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Old 08-22-2013, 07:39 PM
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HAM.
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Old 08-22-2013, 07:49 PM
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Originally Posted by engineermike View Post
Losses start stacking up over 150 f/s. 700 is moving, but ok for short distances, especially if the CFD shows no big problems. I was on SW today, by the way.
Yea I based the original 300fps off Phil99Vette's numbers on a 407 at 20 psi on 88mm.... I am building a 347 with 76mm turbo and under 15psi so my numbers will be drastically lower if I were to guess. But for the design of the manifold the flow is surprisingly good. Not nearly as good as my normal equal length long runner manifolds but will work for the down and dirty setup I am building. I was on SW, Rhino, AutoCAD, and Mastercam today! And that is not even for my normal job! lol Thanks for your help!

Seth
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Old 08-22-2013, 07:58 PM
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That's funny...I was on 3 out of the 4 you listed, 1 for work and the other 2 for fun.
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Old 08-28-2013, 02:46 AM
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Curious. I plan to toss a 5.3 in my '68 LeMans A-body using truck manifolds and a GT45 turbo. Would it be ideal to:
Reduce the natural 2.5" diameter of the pipe (after cutting off the factory flanges) to 2" on both sides
Make the crossover pipe reduce into 2" and run it to the 2-1 merge
Have it all run to a 3" outlet, 2" inlet merge to the T4 Flange with a roughly 3" hole
What is the ideal intercooler size and piping to support 800whp on Ethanol?
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Old 12-10-2013, 02:31 PM
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What is the benefit of reducing the entire hotside and crossover piping diameter VS just dropping the diameter down at the turbo flange?

Assuming we are dealing with an automatic drag car and building boost on the line, is there any benefit to running smaller piping the entire length form the manifolds to the turbo flange?

Why wouldn’t necking the piping down at the turbo flange itself provide the same velocity into the turbo?
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Old 12-10-2013, 02:50 PM
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Originally Posted by SSZ View Post
I wonder how much power a 2 inch crossover will support on a 5.3L? At what point do you have to go larger? Or is it more due to engine size then power output when sizing the crossover?
Hope that makes sense.
On my setup, 383 ls1 with 88mm turbo, I run 2" hotpipes that merge into a 2.5 pipe at the turbo flange. Car goes [email protected] at 3400lb so it makes decent power. I think that the turbine wheel will choke it long before the 2" pipes will.
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