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Old 05-05-2017, 09:48 AM   #1
Tfrasca
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Default Intake runner diameter

I have looked through a ton of posts here and on other forums about intake manifold design. I'm building one for my B23ft in a 142.

I've read about Helmholtz resonance, ideal runner length, plenum volume, and TB placement on a turbo car, but I can't decide between 38mm and 42mm runner ID. These are the two options available based on the bends I need.

I know velocity will obviously be higher and better for low/mid range with 38mm runners, which should suit my build. (9.3:1 CR, IPD turbo cam, TD04-19t, 3" DP).

But I'm wondering if the 38mm will be too restrictive at higher rpm. The 42mm runners would obviously flow more at higher rpm, but I don't know the speed at which flow will become an issue. It's unlikely that I'll be revving past 6k.

Does anyone want to flip a coin for me?
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Old 05-05-2017, 09:49 AM   #2
JohnMc
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Since you've artificially goal posted this decision, pick the arbitrary middle - 40 mm!

JK, I really have no clue.
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Old 05-05-2017, 09:56 AM   #3
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Quote:
Originally Posted by JohnMc View Post
Since you've artificially goal posted this decision, pick the arbitrary middle - 40 mm!

JK, I really have no clue.
You know, I would go for 40mm if I could find .095 aluminum 60 degree bends with that ID. The flange that KL Racing sells has 40mm port holes, and I've heard of at least one other person using 40mm ID tubes for his manifold. I just can't find bent tubing.
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Old 05-05-2017, 09:30 PM   #4
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Anybody else? I'm now leaning toward the 38mm runners, because the intake ports in the head are 40mm, and it would seem that avoiding an abrupt step from manifold to head is the right move.

Sound logic?
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Old 05-05-2017, 09:36 PM   #5
thelostartof
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The head will be more of a restriction than the 38mm runners will be at higher rpm. The fact that you already have a custom intake means you will be outflowing a stock head very quickly.
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Old 05-05-2017, 10:36 PM   #6
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The head will be more of a restriction than the 38mm runners will be at higher rpm. The fact that you already have a custom intake means you will be outflowing a stock head very quickly.
Good point. That said, I'd like to keep the door open for head work at some point. I haven't looked into porting the head too much, but from what I understand, you'd never be increasing the port diameter, right?
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Old 05-16-2017, 12:55 PM   #7
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With a larger diameter runner, you can shorten the length to have the resonance frequency be at the same rpm as a longer runner with a smaller diameter, if you have a certain rom where you want a positive pressure pulse. The benefit of this is that you will have more reflective positive pressure nodes throughout the rpm range, but they won't be as strong as a longer runner with a smaller diameter since they lose strength each time the pressure wave reflects.
Grape Ape Racing has some very good theory and information in the subject.

As far as enlarging ports, you can definitely do that.
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