Restricting IAC passages

[c1500deathtrap]

I see what your saying but many a person has been down this road and it rarely works well. Go ahead and try the jb weld but id go ahead and order the bored out sbc tbi. But the motor wants what it wants to idle. And your issue is that to much air for your motor is slipping by those big throttle blades with even the tiniest crack. So if you crack them a turn or two to let fuel by it wont matter how much you restrict the IAC because the Motor already has more air than it needs so the computer will shut the IAC. You need to more accurately meter the air coming by the throttle blades. maybe try shutting the blades fully and adding tiny holes or slots a few at a time through the throttle blades like an old carb In addition to narrowing your IAC passages. Or just get a bored SBC TBI.

 

[Chewy1576]

I’m coming to the realization that more isn’t always better. You’re right, the big block TB is gonna be too much for my engine. Maybe if I was pushing 350-400 HP it would be a better match, but at 280 I don’t need that much air.


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[evilunclegrimace]

Here this might help. https://www.google.com/url?sa=t&rct...culator.html&usg=AOvVaw29sI8Qnn9C2qOxYzlDETot

 

[someotherguy]

I’m coming to the realization that more isn’t always better. You’re right, the big block TB is gonna be too much for my engine. Maybe if I was pushing 350-400 HP it would be a better match, but at 280 I don’t need that much air.


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The good news is someone will gladly buy that big block throttle body from you, offsetting the cost of the bored-out small block unit. Not everybody listens to the advice on this and insist on running that unit. Or, you may find someone with an actual big block that is missing the throttle body because someone swiped it for their small block.

Richard

 

[Chewy1576]

Well I ran my numbers with the CFM calculator. I haven’t had this engine above 4500, so I really only need 560ish CFM max. I’ll leave the small block one on for now.

Where should I start the bidding for all of these parts I’m not going to be needing now? [emoji38]


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[wheelman]

I have a big block throttle body and bored out intake manifold I want to run on a 383. I tried running the big block throttle body on my Edelbrock intake with stock throttle bores as an experiment, and I had to back the screw all the way out to get it to even try to open the IAC when controlling idle. I don't like doing this because then you're blocking fuel during idle also because the butterflies are as closed as they can be. So when I install the big block throttle body for good, I want restrict the flow of air through the IAC passages so I can get more air going down through the throttle bores to push fuel into the intake. Does anyone see a problem with filling the yellow areas with Moroso A&B or JB Weld? Anyone have a preference on which one to use over the other?

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[wheelman]

I thought the only difference in the throttle bodies.were the fuel injectors.they have a bigger flow injector for the 350 v 8

Change the injectors.the throttle bodies are the same right

 

[someotherguy]

I thought the only difference in the throttle bodies.were the fuel injectors.they have a bigger flow injector for the 350 v 8

Change the injectors.the throttle bodies are the same right

Nope.

 

[Chewy1576]

The small block throttle body has 1 11/16” throttle bores and the big block one has 2” throttle bores. The injectors are interchangeable, but you need the part number from the top to determine the flow. 350 injectors were 61 lb/hr, Caprice/Impalas “cop car” injectors were 65 lb/hr, 454 were 80 lb/hr, all rated at about 13 psi. Later 454 injectors were a lower flow rate, but the pressure was significantly higher.


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[Randyb2004]

The IAC is only a part of the problem, but that was the route I was going to take to try to fix it. Ok I’ll back up and try explaining what I was seeing again. Maybe I just have it screwed up in my mind...

The problem I had was that I couldn’t get the idle air adjusted with the big block throttle body so that the IAC counts stay at 5-10 when at operating temperature. To get the IAC to open few counts, I had to back the idle air adjustment screw all the way out. This created another problem because with the screw backed out, the throttle plates are almost completely closed and fuel wasn’t effectively getting down into the intake. Turn the screw back in to open the throttle plates slightly to let fuel by, and the IAC closes completely again and the idle quality goes south because the ECM has no IAC adjustment.

So my reasoning for restricting the IAC was to bias the idle air to the throttle plates so I don’t have to close them completely. More air (and with it fuel) through the throttle plates means less air needed through the IAC for idle, right? Or should I just stick with the small block throttle body?


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I would be looking for a gasket mis-match or other vacuum leak. Air is getting in somewhere, I doubt you will need to restrict the passages. Any leak in the intake, brake booster, or other hose will give you the problem you are having.

The IAC is the restriction. It's a needle valve (more or less). Restricting the passage will do nothing.

You cannot oversize a throttle body on a fuel injected engine, be it port or throttle body injected. When a Carb is too big, engines will not idle since carb's rely on velocity and pressure differentials to draw fuel into the air stream. When fuel is injected under pressure by an electronic injector the old rules do not apply.

This might be my first post on this forum, but I have been working on cars for 40 years.