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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 01-29-2013, 03:55 PM
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Default Out of curiousity why does everyone use 2.5" on the crossover pipe

Why does everyone use 2.5" tubing for the crossover in almost every turbo kit done on here for a setup designed for 600-1000hp?

JMO.... but the headers, crossover, merge section need to be sized accordingly based on velocity of the exhaust based on what the motor will do at max boost, peak airflow.

Discuss...
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Old 01-29-2013, 04:14 PM
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I was waiting for a thread like this, one person used 2.5" and now everyone thinks its the ONLY way to do it. I should have some back to back testing in a couple weeks.
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Old 01-29-2013, 04:21 PM
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My cross over is 2.25" ID...probably only at the 600 WHP mark at this point but stepping it up this spring to 750 WHP.

I do know that there was a 3" cross over that was tried with a member on here with a T6 large turbo and he has since reduced his pipe to 2.5".

When I did build my hot side, I calculated my cross-sectional areas to keep the volumes of the pipes "equal"

Area of 2.25" ID pipe = 3.97"
Area of a 3" ID piper = 7.065"

If two 2.25" pipes merged into a 3" pipe, they would supply the turbo with less of a pressure/area difference then a 2.5" crossover.

Is this an answer that you were looking for by chance?
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Old 01-29-2013, 04:21 PM
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TracyRR did some testing awhile back....

Ended up going to smaller pipes that worked better in the calculator...
Boost response was better to the point he wanted to put a larger turbine housing on
With the smaller pipes and the larger AR turbine housing the combo made boost down low quicker than the big pipes and the small AR


http://www.yellowbullet.com/forum/sh...=turbocharging
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Old 01-29-2013, 04:26 PM
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2.5" crossover will support 2300hp+

So why would you use a 2.5" crossover that will support 2300+hp in a 700hp application?

Velocity can't exceed the speed of sound(11xx FPS) inside the pipe
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Old 01-29-2013, 04:38 PM
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Originally Posted by Chicago TDP View Post
My cross over is 2.25" ID...probably only at the 600 WHP mark at this point but stepping it up this spring to 750 WHP.

I do know that there was a 3" cross over that was tried with a member on here with a T6 large turbo and he has since reduced his pipe to 2.5".

When I did build my hot side, I calculated my cross-sectional areas to keep the volumes of the pipes "equal"

Area of 2.25" ID pipe = 3.97"
Area of a 3" ID piper = 7.065"

If two 2.25" pipes merged into a 3" pipe, they would supply the turbo with less of a pressure/area difference then a 2.5" crossover.

Is this an answer that you were looking for by chance?
In a 370" assuming about 20# of boost your looking at about 470-500fps at the crossover and 525-550fps at the merge. That velocity is almost equal to the exhaust gas past the exhaust valve. It would be interesting to see the cross section of the throat of the turbo where it starts to open up at the turbine.

On my setup the velocity at the throat of the turbo is ~ 1100fps.
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Old 01-29-2013, 04:41 PM
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I cut up my truck manifolds, and those suckers pinch down to a 2.25"! That's the size crossover I'm running in my 67. 4.5" of exhaust (combined) is more than enough for 1000whp.
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Old 01-29-2013, 04:48 PM
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I think everyone is used to putting dual 3" exhaust on their pick-ups..
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Old 01-29-2013, 04:50 PM
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Originally Posted by Chicago TDP View Post
I think everyone is used to putting dual 3" exhaust on their pick-ups..
That's the damn truth! That's exactly what my dad said when I told him I'd be running 2.25" crossover, and having a 5" downpipe. He said I was trying to be a 'rice rocket'. I love my dad. lol
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Old 01-29-2013, 04:51 PM
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So what would you suggest for a the average 1000rwhp build? 2.25? 2?
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Old 01-29-2013, 05:00 PM
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I would vote that if you have 1 5/8" primary tubes that either feed a log or forward mount turbo header, you should use 2.25" cross over that merges into a T4 flange.

If you are upping the ante and have a T6, you should have a 1 3/4" primary (it does make a difference....) to a 2.5" crossover which then will go into the massive T6 flange.

Just my two cents...computed with basic math.
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Old 01-29-2013, 05:10 PM
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Ive used 2.25" since you and i talked about almost 2 years ago, and cannot help but think your math was flawless then as i came to a very similar conclusion.

For the record, i was considering 2 1/8" until i realized how hard it was to find.

Phil= Da man!

Also, on the t6 note, 2.25" pipe squares out perfect to fit inside a divided t6 flange.
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Old 01-29-2013, 05:25 PM
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Can someone provide the formula?
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Old 01-29-2013, 05:32 PM
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Ditto,glad to see some one posted this. I was going to use 2.5 but im now just gonna use 2.25 even though ill be going t6 but im not planning on over 700rwhp any time soon
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Old 01-29-2013, 05:34 PM
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Originally Posted by F110Mech View Post
Can someone provide the formula?
Which formula?

Cross sectional area, I used this formula many many times, each time it seems to be in an exhaust talk, lol

http://www.engineeringtoolbox.com/pi...ons-d_873.html
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Old 01-29-2013, 05:37 PM
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Originally Posted by JAX04 View Post
Which formula?
For what size cross over. I'm building a 347 with v banded truck manifolds. Any way someone could show me the formula using this setup?
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Old 01-29-2013, 05:41 PM
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Originally Posted by F110Mech View Post
For what size cross over. I'm building a 347 with v banded truck manifolds. Any way someone could show me the formula using this setup?
I guess that it makes sense to ask what parameters need to be known to figure this out.
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Old 01-29-2013, 06:35 PM
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Originally Posted by F110Mech View Post
So what would you suggest for a the average 1000rwhp build? 2.25? 2?
On a 370", setup to run x275, 88mm turbo, 25# of boost, I'd like to see 1 3/4" stepped headers to 1 5/8. 2 1/8 or 2 1/4 to 2.5" or 2.75" into the turbo. Velocity gradually increases to a max of 1000fps at the turbo. About 400fps at the valve
300-350fps in the header
600-650fps in the crossover
925-975fps in the merge
1050-1100 at the turbo

Out of the turbo is another story. Usually 120% of the exducer and external wastegates is a good start.
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Old 01-29-2013, 06:56 PM
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What about the ever popular 370, manifolds and up to a 80-88mm turbo?

That seems to be what most people here run in a street/strip car.
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Old 01-29-2013, 07:00 PM
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Phil what about same turbo on a bigger 408/427
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