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PostPosted: Tue Nov 30, 2021 12:55 pm 
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Joined: Mon Jan 05, 2015 8:45 am
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OK, thank you.

Does it matter? (asking for a friend...)

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Bill H
92 Saturn SL2 - SCCA F-Production


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PostPosted: Tue Nov 30, 2021 1:13 pm 
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planet6racing wrote:
OK, thank you.

Does it matter? (asking for a friend...)

depending on how large someone decides to modify their IACV, it could be significant enough to cause a power increase.

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Matt Blehm
Houston Region
EP 1st Gen RX7
STU Scion FR-S


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PostPosted: Tue Nov 30, 2021 2:10 pm 
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Joined: Fri Nov 08, 2002 8:29 pm
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Location: 138 miles north of Blight
Yep, I took my IAC off. And there is a plate there now. 'Cause that is how I read the rules.

Another fellow's father was telling me I should have the ECU open it during full throttle.

.


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PostPosted: Tue Nov 30, 2021 2:16 pm 
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If the throttle body ID is the same diameter on both sides of the butterfly, (and the IACV intake galley is located in the throttle body ahead of the butterfly) then it wouldn't account for any added airflow at WOT, only at idle and possibly at part throttle. If the IACV intake is not in the throttle body, then you would have additional airflow at WOT. Still, the rule would need to be clarified to avoid any tech shed drama.

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Jonathan Spiegel
Big Island of Hawaii Region and Colorado Region
FP/GTL Honda CRX


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PostPosted: Tue Nov 30, 2021 3:10 pm 
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That appears to be what's going on here.
Every fuel injected car I've worked on and paid attention to (a handful of Nissans over the years), the IACV is separate from the throttle body and sucks air from behind the MAF. So it's metered for fuel but not passed through the throttle body.

Newer cars like my Toyota that have a throttle by wire use the throttle body itself for idle control.

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Matt Blehm
Houston Region
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PostPosted: Tue Nov 30, 2021 4:22 pm 
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Jason, I'm not saying that your throttle body is illegal, just an example that I'm familiar with. The problem that initiated my letter to the CRB is my MK3 VW has a factory original idle air control motor that is not part of the throttle body. It is a real pain to get it to idle with a high duration camshaft without the idle control. It either idles at 3500 RPM or stalls if you close the throttle butterfly a small bit. The idle control valve controls it nicely. As Mike said in an earlier post, the amount of air that they bypass would be insignificant at wide open throttle.

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David Boles
HP MK 3 GTI #79
Boles Motorsports


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PostPosted: Tue Nov 30, 2021 6:30 pm 
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Joined: Wed Oct 28, 2009 9:24 am
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Image

Note the idle circuit of a DCOE Weber. The wording of the carb section of the PCSis slightly different to the fuel injection throttle body section, but nonetheless the Weber (and other carbs) technically violates it by allowing a small amount of air to bypass the venturi and throttle plate.

I'm sure that no one ever worried about this, same as the VW Mk1/2 idle bypass screw, since the systems are stock and the orifices tiny. I have a 1990s Ford V6 however that has an ECU controlled bypass with passages a good half inch in diameter.

If we are going to change the wording and then enforce it, let's make sure that the older FI TBs and carbs don't get caught up.


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PostPosted: Wed Dec 01, 2021 6:48 am 
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Here's the best photo I could find of a KA24DE intake manifold showing all of the guts and relative position of everything.

Note the idle circuit has a completely separate air tube- roughly 1/2" diameter- coming from the intake tubing. it completely bypasses the throttle body and everything about it. all of the idle hardware is at the back end of the manifold, same for the EGR. what's the total airflow capability from that? It's surely more than zero, but likely not 'significant'. Of course that's one case of many many engines eligible for Prod...


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KA24DE manifold.jpg
KA24DE manifold.jpg [ 125.76 KiB | Viewed 700 times ]

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Matt Blehm
Houston Region
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PostPosted: Wed Dec 01, 2021 9:49 am 
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RX7's have 2 separate valves that actuated to allow air in past the TB. In the ITS days we did dyno testing with them open to atmosphere and picked up nothing.


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PostPosted: Wed Dec 01, 2021 10:16 pm 
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Location: Portland, OR
If high velocity air is passing over the bypass (when wot) would the air flow not create a vacuum on the by-pass that is greater than the intake manifold vacuum? Fluids was not my best subject so I don’t recall. I would think some tests or calcs at least could shine light on any illegal benefit. If you wouldn’t just “have a bad vacuum cap” I wouldn’t think an internal vacuum leak would be beneficial either.

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Ian
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