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Old 09-09-2020, 07:01 PM   #1
mr.nismo.
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S13 SR20 idle air flow path

Hey everyone!

So I always thought it was weird that the OEM intake path was 3/4" ID hose to the valve, but pretty much every A/M hot pipe was 5/8. I figured it was since the the opening into the lower plenum is only 5/8. My thing is why the 3/4 all way up to the plenum, then reduce it? Looking at the casting plug, I wonder if it was more cost effective to do 5/8 (since mass manufacturing needs to just be "good enough") or if there was engineering behind it. Im toying around with opening up the original hole, or adding another directly under where the iacv sits. Im more leaning towards the second hole as to provide cyl. 3/4 with a more direct path of air. I also was curious if the Greddy manifold is like this? Ive never had my hands on one, and I couldn't find pics of inside the iacv chamber. It almost looks like its not just dumped straight into the plenum. Either or, I wanted some opinions on opening up another flow path so the full 3/4 ID of flow can be used. And also if anyone that has a Greddy Mani can chime in. Looking forward to the discussion!


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Old 09-09-2020, 07:05 PM   #2
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On the greddy one, that area is completely open same as my jun manifold
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Old 09-09-2020, 07:07 PM   #3
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Quote:
Originally Posted by S14rebuild View Post
On the greddy one, that area is completely open same as my jun manifold
Do you notice a better/easier idle recovery? Especially with accessories loaded?

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Old 09-09-2020, 07:10 PM   #4
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I never had any idle issues always idled at 800-900rpm no surging. Even when i had ac always worked. Could be driving throw it in neutral and always caught smooth idle.

Was told the maf played a huge roll in idle quality...i always ran a blow thru maf setup since ive had an sr in my car eitjer that be stock or z32
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Old 09-09-2020, 08:09 PM   #5
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Quote:
Originally Posted by S14rebuild View Post
I never had any idle issues always idled at 800-900rpm no surging. Even when i had ac always worked. Could be driving throw it in neutral and always caught smooth idle.

Was told the maf played a huge roll in idle quality...i always ran a blow thru maf setup since ive had an sr in my car eitjer that be stock or z32
I'll be running draw through N62 along with speed density/IAT, so that shouldn't be an issue.

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Old 09-13-2020, 02:12 AM   #6
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Well to update...

I put a 1/2" hole towards the edge of where the iacv sits. Obviously that's more than 3/4" of total flow path, but it does give a more direct path of flow for cylinders 3/4. I also figured I'd port match the plenum to the runners. I found there was quite a difference with how rough the production casting is. I'm mad at myself for not taking a picture before I did the plenum. Cylinders 1/4 were opened up ~2mm, and 2/3 ~1.5mm. I also smoothened/polished the openings into the runners. I was surprised how much restriction of flow there was. I did find that exactly where the two halves mate were matched, and you could see the machined bevel that goes about 2mm into the pieces from the edge. However overall flow path was restricted. On the runners, it's a much better outlook. Cylinders 1/4 need about ~1mm of material opened up (sharpie marked), and 2/3 are good as they are. I decided to check the runners to the head and found out they're pretty off as well. On the runners, they all have excess material towards the "bottom right" (if you're looking into the runners). On the head there's excess material at the top of the ports. My concern on that is that's a literal wall as charge air comes up the runners, hits that wall, then pushes into the ports. I'm also wondering if that's maybe by design as fuel is getting sprayed into that charge air just before it hits that wall. So maybe by design it's meant to help mix the air/fuel before entering the cylinder. Thoughts on this? I know I wrote a fucking novel, but I figured since I have it apart, I might as well optimize it's efficiency.

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Old 09-20-2020, 08:20 PM   #7
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its fine if you port match, liquid fuel sprays to the closed intake valves and depends on warming from the valve and port to vaporize along with the suddenly flurry of events as the valve opens to fully atomize as it enters the cylinder.

Its probably okay to also 'port' some areas but I would not do anything to the roughness of the walls, mainly just focus on transitional regions and removing flash/burrs type of thing.

The design of the intake isn't going to benefit much from porting because of it's runner length and limitations of the head. Not to mention its alot easier to raise the boost pressure than to deal with fuel puddling issues as a result of improper porting.
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Old 09-20-2020, 09:21 PM   #8
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I decided to leave the head alone for the reasons you listed. I didn't polish up there either, as I know the roughness helps w/ mixture before entering the cylinder. All the work I did was just on the plenum to runner side. I also took material off the plenum at the throttle body mating as there was a huge lip sticking up from the bottom side of it. Otherwise everything is assembled and back on the motor.

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