IAC Counts

[Walter Sherwin]

Your comments about the interaction of IAC counts, injector positioning, and
fuel spray pattern with a TBI were great!  I have also twiddled with these
parameters and have found the same results.  How many IAC counts do you aim
for (warm 5-8 ?)?   I have also found that the Rochester TBI injectors
respond positively to more fuel pressure.  The reward is slightly better
"shattering" of the fuel and a steeper, more regularly defined cone which
helps to reduce part throttle mixture maldistribution (especially with the
big 454 blades which tend to "paddle" the fuel towards the rear cylinders).
Of course, the injector constant has to be changed to match the new flow
resulting from the increased pressure differential.  Starting in 94, GM also
moved in this direction, with a target system pressure of 26-32psig.



-----Original Message-----
From: Bruce Plecan <nacelp at bright.net>
To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
Date: Sunday, October 11, 1998 12:24 PM
Subject: IAC Counts


>Now, that I think everyone has finished commenting on how wrong
>0 IAC is.
>  On a TPI, IAC counts are not as sensitive as TBI are in  this
>reguard.  On some TPI it doesn' matter if the idle warm IAC
>is 0, or 50.
>  But, at least in reference to 101, the comments are addressed
>to the 747, and it's calibration.
>  If you look at the discharge of the injectors of TBI's (gm) at idle,
>you'll notice the following.
>  Conical in shape
>  Depending on application the fuel discharge may fall entirely on
>    the butterfly, and slide off the lower side of the butterfly.
>  On some the fuel spray is right out by the vertical walls by the
>    butterflies.
>  On some, folks (myself included) space the injector head up some
>    so the fuel spray at idle is well above the butterfly at idle.  Yet,
>    close enough to the butterfly so that as the butterfly opens the
>    increase in air flow keeps the fuel off the walls.
>
>OK so what does this have to do with IAC.  The IAC bypasses the
>butterflies.  So that reduces the air around the butterfly and if you go
>to an extreme, you can imagine the fuel just splashing on the butterfly
>and dripping off of it into the plenum.   There is a noticeable difference
>in idle characteristics on some engines in this reguard.
>On my Comp Cam'd 270, 355 CID SBC, it makes a huge difference.  The above
is
>something I've worked on and is not theory,
>or reverse engineered, it something I seen with my own eyes.
>  Min. IAC leads to more butterfly opening, and a smoother idle.
>  Also, again, in my opinion, the engine comes off idle better, at/
>    during slow throttle movements.
>IAC Pintles.  It don't matter for taper, or not much.  Well, wrong
>answer.  Changing Pintles can make a difference of 2-4 MPH
>in top end in a stock 350 TBI chevy pick up 1990 running a 747,
>in back to back tests.  Testing was done within minutes, and
>on the same section of road, in opposite directions.
>  Also, the pintle does have a slight effect on the accleration
>characteristics of the engine.  The greater the taper, the leaner
>the engine as the engine acclerates, as the IAC retracts.
>
>