Help me understand the IAC circuits.

On the LT1, there is the TB and then the IAC motor. The IAC controls how
much air is allowed into the engine/intake manifold, with the throttle
blades closed. Or to help more air get in, past the barely open TB blades
at idle, right?

From there, it goes through the TB's IAC passages to the intake manifold
(between the two openings of the TB) and down into the innards of the LT1
intake manifold, right?

From there is goes down a "plenum" and is diverted to each individual intake
port near the head?

Jeff Stevens had found on his intake manifold that the holes under the intake were of different sizes. The holes/passages up to the intake ports from the "plenum" under the intake manifold, behind the allen head bolts.

Jeff also tried to fix the fitment of the IAC port from his BBK TB to the opening of the intake manifold by using a brass/bronze tube.

So, I have a monoblade on my engine. This causes a problem, since the
intake manifold gets cut up to allow the monoblade to open. Now, I'm not
sure if this was neccessary or not, but my engine builder epoxyed over my
IAC circuit opening in my intake manifold (right at the TB.) I always
assumed this was needed because of the AS&M monoblade.

So now my IAC circut just dumps into the main intake manifold plenum and the cylinders basically fight for the air. I'd assume (never really checked)
that my block learns would be WAY off, since the front cylinders at idle
would probably get most all of the IAC air.

When Mark Montalvo helped design, build, and finally tune my car, we did so
in closed loop (I always get closed loop and open loop mixed up...) (closed
loop in this description means the same as cold start up... where the pcm
doesn't look at all the sensors to make changes.) He said we must do this,
because my block learns or LTFT's would be all wacked out because of my IAC circuit.

Well this turned into a PITA and never quite right tunes (for driveability.)
So now that I am going to yank the motor out and freshen it up, I might as
well change some things (hopefully) for the better.

I'm thinking about a stock LT1 intake ported to match my heads (as this one
already is) and going to a BBK 58mm TB and keeping my IAC ports... as well
as the two vacumn ports on the pass side of the TB, which the AS&M monoblade does not. The two ports for the EVAP purge canister solenoid, and the rear pass valve cover breather tube. I've had a cheapo little filter stuck in there since the motor has been running (since '01 or '02...)

 

More than likely, even if your IAC ports were fully open and operational you would still need to crack the throttle blades to allow enough air for a well build and cubed motor.

Open loop = initial start up
Closed = after its been running

 

Thanks for clarifying open/closed loop. But I'll forget it again. I don't know why, but that always stumps me.

I realize I will have to crack the TB's open, but at this point, the IAC air is ALL going into the manifold, and is just like opening the TB blades.

The stock configuration goes into the innards of the LT1 intake manfiold and gets dispersed directly into the intake ports right at the heads.

 

The open/closed thing i always used to get confused on too. One day it finally clicked.

Yes you will have some air going through the IAC ports in the intake, but IMO if you crack your blades you are totally disrupting the designed airflow anyways.

 

I was fighting with an IAC problem with my new motor. One thing that I found with the 58MM BBK is that opening the TB won't fully compensate for the IAC's inabilities. If you crack your blades, the idle will rise and then the cylinders will be fighting for that extra air rather than being fed the extra air. To compensate for the small IAC passage (relative to cam size) I did the drill mod (a hole straight through the allows more air past the IAC but into the IAC "system"). It helped with the whistle a bit and with the fluctuating idle. I do recommend starting with a small hole and working your way up as you see fit. I went with 5/32 or something like that...but I would go a step smaller if I did it again. Also the brass tube did help my split BLM problem a bit.

 

I don't know how this would apply in your case but on carbed engines with big duration cams the same thing can happen erratic idle can't adjust the carb enough to make it right. I would drill a small hole in each throttle blade near the idle transfer slot to allow extra air in without opening the throtle blades and uncovering the transfer slot and voila all of a sudden the car idles.
So maybe drilling one or two small holes in the throttle blades would help on efi cars with big cams also.

 

I guess you could back 1 of the screws out of the Blades and use that as a passage. If it works then drill a hole. I have a problem with mine idling when cold. after it comes up to temp then it idles fine. Anyone have any suggestions?

 

i just cracked the blades open a bit and it worked fine.

 

Anybody have a pics, or a write up on this "drill mod?"

 

Before you begin drilling throttle blades, try shimming the linkage to modify
the closed position.

For a carburetor, you don't want to open the primary throttle blades too much
because it will activate the transfer slots as well as the main circuit.

To get around this, you can shim the secondary throttle blades to allow more
air while keeping the primary blades about 0.040" into the transfer slot for a
nice idle.

With your LT1 Throttle body, there must be a similar method to shim the blade
as opposed to drilling into it? I realize there is a sensor that will read the throttle
blade angle, but I'm wondering if there are alternatives?

 

i cracked mine with the allen screw on the throttle cable side. just turned it in then reset my computer it 1% over for the TPS sensor