Add-on systems which affect output pulse width?

[Andrew Brownsword]

Hello again,

  After posting my project description yesterday I spent some time surfing
the web for add-on computers.  All the systems I have seen so far seem to
work their magic by modifying the airflow meter's voltage input to the ECU.
The ECU is then relied on to "do the right thing".

  It seems to me that this could be problematic if the ECU was designed to
expect a particular voltage range and the stock airflow meter generated that
full range ... in other words there is no buffer for going beyond the
expected range of values.  This seems fairly reasonable for an N/A engine
since it is extremely unlikely that the car will have to deal with >1
atmosphere.

  It seems pretty obvious that most add-on computers are designed to modify
input because it is easy.  The computer reads one voltage, maps it through a
table, and outputs another voltage.

  The alternative is to modify the ECU's output, which is a time dependant
pulse width.  It simply isn't possible to have full remapping control over
the output because by the time you know what the pulse width is, the time at
which you have to send it to the injectors has passed.  It occurred to me
this morning, however, that an add-on computer could fairly easily extend
(not shorten) the pulse width based on input from a pressure sensor.  It
would read its own MAP sensor to determine the amount of "extra" fuel needed
beyond the amount the stock ECU has calculated (being tuned for where the
ECU's airflow meter saturates), detect the trailing edge of the ECU's pulse,
and keep the pulse going for that extra time.

  So am I off my rocker here?  Is this possible?  Has anybody built a
product which does this?  Is this even an issue, or does a stock ECU program
ALWAYS allow enough buffer that input modification is a better approach?

  I hope I'm not reiterating common questions or wandering too far from the
list charter -- I haven't seen a FAQ for this list yet, so please bear with
me.  And if there is a FAQ, you somebody point me at it?

Cheers,
   Andrew