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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 01-30-2013, 11:36 AM
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Here is some data from real world applications that perform at the top level of competition.
From my import days I learned how important velocity was....


- 632 BB with twin 94s, 3.5" crossover
8300rpm, 35# of boost will make 3500hp
1100fps in the crossover

- 525BB with Twin 88s, 3.5" crossover
8500rpms, 42# of boost
1050fps in the crossover

(my turd which has gone 164 @ 3300 to the 1/8 mile)
438" small block with 106, 2.5" crossover
8500rpms, 40# of boost
900 FPS in the crossover

Lets say a 346 would make 450 at the tires NA, you would need 14-15# to double the rwhp assuming its built correctly

346", 15psi, 7500rpms
2" crossover yeilds 550 fps
2 1/8" crossover yeilds 480 fps
2.25" crossover yeilds 430 fps
2.5" crossover yields 350 fps
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Old 01-30-2013, 11:40 AM
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So have you calculated the velocity and/or have you been able to put some type of device in the pipe(s) to measure the velocity or flow rate?

From what I recall during my engineering classes when I was getting ME degree, you need to know the pipe size and velocity to get the flow rate OR the pipe size and flow rate to determine velocity. You have to have two of the variables to calculate the unknown.
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Old 01-30-2013, 11:53 AM
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Originally Posted by Phil99vette View Post
We will see how weak it is when it goes 175-180mph to the 1/8 mile.....

I think you might be able to go up on the turbine A/R.

Big pipes require more time and engine to develop the same drive pressure, once its up to pressure its going to take the same drive pressure to maintain boost and where the wastegate comes into play.

My motor makes about 1300hp through a single 2.5" OD pipe. Remember, pre turbo its about velocity.
Phil im joking dude...im read every page on the **** it turbocharging on yb

http://www.yellowbullet.com/forum/sh...d.php?t=308464

and have read all the pages on your build from day one! Ive talked to you many times on here and the bullet about my combo. I would love to have the drivetrain in my e46 bmw lol. But serious tho would 2" be to small for a 370/lsa heads, holley HR,and pt91.5 1k to 1.2k hp range? or 2-1/8 2-1/4 better?
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Old 01-30-2013, 12:34 PM
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Originally Posted by turbocamaro125 View Post
Phil im joking dude...im read every page on the **** it turbocharging on yb

http://www.yellowbullet.com/forum/sh...d.php?t=308464

and have read all the pages on your build from day one! Ive talked to you many times on here and the bullet about my combo. I would love to have the drivetrain in my e46 bmw lol. But serious tho would 2" be to small for a 370/lsa heads, holley HR,and pt91.5 1k to 1.2k hp range? or 2-1/8 2-1/4 better?
We are good. The calculator says 2" to a 2.5" combined merge section. I know it sounds small but remember 2.5" can easily handle 2000-2500hp.
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Old 01-30-2013, 12:42 PM
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Originally Posted by Chicago TDP View Post
So have you calculated the velocity and/or have you been able to put some type of device in the pipe(s) to measure the velocity or flow rate?

From what I recall during my engineering classes when I was getting ME degree, you need to know the pipe size and velocity to get the flow rate OR the pipe size and flow rate to determine velocity. You have to have two of the variables to calculate the unknown.
I am using a calculator. I know I know its not real world measured with a lab grade sensor but I can tell you this. Every combo I have put through the calculator that said it had low velocity resulted in slow spool, every combo that had high velocity resulted in excellent turbo response.

And yes, you can run a 3" crossover on a 346 or 370" and get good results.
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Old 01-30-2013, 12:50 PM
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Thanks for this!

I am about to order the piping for my crossover and would have bought 2.5" for my 76mm 1.0A/R cheap turbo, now I will buy 2.0".

Any suggestions on transitions from 2.5" to 2" with truck manifolds?
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Old 01-30-2013, 01:43 PM
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So to be devils advocate, why not run 1" crossover? I bet velocity on that would be INSANE!!
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Old 01-30-2013, 01:55 PM
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if 2.5" cross over will support 2300hp, how much will a 2.25" crossover support? sorry if i missed it.
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Old 01-30-2013, 02:17 PM
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Originally Posted by Taubr Unit View Post
So to be devils advocate, why not run 1" crossover? I bet velocity on that would be INSANE!!
Airflow would be faster than the speed of sound. Bad **** happens at that point.
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Old 01-30-2013, 02:20 PM
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Originally Posted by dudeman View Post
if 2.5" cross over will support 2300hp, how much will a 2.25" crossover support? sorry if i missed it.
About 500hp less, 1800-1900.
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Old 01-30-2013, 02:33 PM
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Phil....The reason people run bigger pipes is because they dont use your calculator...just saying Phil where do you get you calculator from....Walmart?
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Old 01-30-2013, 02:34 PM
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Originally Posted by Taubr Unit View Post
So to be devils advocate, why not run 1" crossover? I bet velocity on that would be INSANE!!
I would think backpressure would slow the exhaust down real bad
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Old 01-30-2013, 04:20 PM
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I did not design the formulas, Corky bell a washed up turbo guy came up with the formulas. I just put them into a spreadsheet.
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Old 01-30-2013, 05:05 PM
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I run 2.25 with a 370 n pt88 and the response is very nice

I wouldn't be scared to try 2" upipes
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Old 01-30-2013, 05:11 PM
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I ran a 2" crossover with a 390ci engine and S400-75mm T4 turbo. Response was good, did seem to drop-off a little early, which I expect was due to the small turbine wheel (never measured backpressure).
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Old 01-30-2013, 06:31 PM
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Originally Posted by SirNemesis View Post
I ran a 2" crossover with a 390ci engine and S400-75mm T4 turbo. Response was good, did seem to drop-off a little early, which I expect was due to the small turbine wheel (never measured backpressure).
An interesting swap would be a Precision x275-88mm unit. It would almost be a match in heaven. That turbine is alittle small with that unit.
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Old 01-30-2013, 06:52 PM
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Originally Posted by Phil99vette View Post
An interesting swap would be a Precision x275-88mm unit. It would almost be a match in heaven. That turbine is alittle small with that unit.
Thanks Phil, I was about to post up a thread asking for advice on a new turbo. I don't suppose anyone knows the physical size of the Precision x275-88mm compared to an S400? The S400 was tight, but did fit, I don't think I'd be able to find much more room.
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Old 01-30-2013, 08:39 PM
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After this thread and thinking about it a while I'm thinking about redoing my kit into 427/truck manis/2/2.25inch crossover to 2.5/2.75 into my 1.32 lf88
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Old 01-30-2013, 09:08 PM
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Id love to see some back to back testing with this.........
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Old 01-30-2013, 09:19 PM
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Haha funny you say that about Corky Bell, Phil. I just bought his Maximum Boost book. Are the formulas in there?
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