How do AICs work?

[Daniel Houlton]

I have some questions about some AIC/AFC (Additional Injector/Fuel
Controllers) I'm hoping maybe someone here can answer.  I understand the
basis for these in that you somehow measure how much extra fuel you need
either through the MAF or MAP and some other sensors of your stock ECM
(temp, TPS, etc) and inject the required extra fuel through 1 or 2 extra
injectors.  This is presumably without the stock ECM knowing or caring
about it.

I'm wondering how this can be.  I've turbo'd my 4 cyl engine and I've
measured the duty cycle of my injectors with an automotive multimeter
and found that they reach 100% duty cycle @ 9 psi @ 5000 rpm.  Now supposing
I added an AIC that injected the extra fuel the boost requires.  Forget 
for now how the AIC figures out how much it needs to inject, just that it
does.

The stock ECM though is still reading directly how much air the engine is
ingesting.  My ECM has shown to be capable of injecting the correct amount
of fuel under boost up until the duty cycle of the injectors is maxed out.
It reads the air mass from the MAF and dumps the correct amount of fuel
for it.  My mixture at WOT (*above 3000 rpm) stays rich all the way to 
5K.

With the AIC also dumping fuel to make up for the boost I end up with an
extremely rich mixture.  At 3/4 throttle, closed-loop mode this would be
fine.  But once I hit WOT, the computer switches to open-open loop.

It doesn't know that my AIC is injecting fuel as well, and it isn't watching
the O2 sensor to know that the mixture is going rich.  So my WOT mixture is
going to be way too rich and my stock injectors duty cycle is still going
to max out at 5000 rpm assuming of course it'll rev that high from choking
on all the fuel.

None of the AICs I've seen seem to make the stock ECM compensate in open-
loop for the extra fuel being injected.  It seems like the AIC would also
have to either intercept and massage the signals from various sensors 
before reaching the ECM, or intercept the signal to the stock injectors
and chop them off so they don't fire as long.

So why is this apparently not a problem?  What am I missing?


thanks
--Dan