Idle Air Control (IAC) air supply.

[robert presti]

I'm reworking a car. Greg Banish says the air supply to the IAC should not be on the boost side of a supercharger. There is a little box between the feed and the IAC, I don't know what it's purpose is.

The issue is if the car is blow through (MAF location) the IAC is then taking in unmetered air. If the car was draw through I see how one would simply have the IAC fed before the supercharger, but after the MAF. My car has never gone back to a stable idle well. I don't know if it is because the IAC air feed is subject to surge air or if the tune was just that bad. This is an air to air intercooled Procharged car. Any thoughts???

 

[Bullitt5566]

I have my IAC connected after the supercharger/aftercooler. I did this because I'm concerned the IAC may bleed boost at WOT and having the IAC inlet and outlet at the same pressure would fix this. I also know that the supercharger pressurizes the intake before the TB slightly at idle.

I've run the IAC source both before and after the supercharger and didn't see a bit of difference. No surging. I think as long as the Idle Air values are correct it won't matter. The black box is a noise suppression device some ford engineer spent time designing it and I'm not smarter than a Ford engineer so I left it.

 

[robert presti]

I have my IAC connected after the supercharger/aftercooler. I did this because I'm concerned the IAC may bleed boost at WOT and having the IAC inlet and outlet at the same pressure would fix this. I also know that the supercharger pressurizes the intake before the TB slightly at idle.

I've run the IAC source both before and after the supercharger and didn't see a bit of difference. No surging. I think as long as the Idle Air values are correct it won't matter. The black box is a noise suppression device some ford engineer spent time designing it and I'm not smarter than a Ford engineer so I left it.

I guess it didn't occur to me that yes you would have a small boost leak in most cases it opens and lets the throttle down slowly. There is probably a way to write that out of the tune, but not sure. I don't want to eliminate the IAC for colder fast idle temps, but it is tempting. I hate the way fuel injection cars free rev. They just don't sound as cool as a carb, or some thing like my Projection 4 fueled Ranchero which has no IAC only a cold throttle solenoid.

I just thought of this. You could probably use a brake booster one way check valve to prevent the boost leak. It should flow enough and I would think if it can hold 20+ pounds of vacuum it should hold 20 pounds of boost if installed backwards.