IAC Counts

[Bruce Plecan]

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.