I was inspired by your concept of the 555 fuel injection to develop an
engine controller.  Although I was unable to resolve the issues of
linearity and temperature stability to my satisfaction using the 555, I
developed a replacement circuit based on the concept of providing a
pulse whose length is linearly proportional to MAP.
There is, of course, more to a fuel injection control than this core
element, but the rest is pretty straightforward - decide when to sample
MAP and when to initiate the injector pulse, provide a mixture control.
This circuit creates converts the MAP voltage input to a binary number
which represents the number of clock intervals for one injector output
pulse. As shown, at max MAP the output pulse can be anything from 4.4
milliseconds to 8.8 ms, depending on clock frequency. As MAP goes down,
the pulse is proportionally shorter.


Victor Roberts

Did you try a CMOS 555? An entirely different animal as far as linearity is concerned compared to the old 555. The temperature in the cockpit must be benign.The CMOS 555 will do the same thing.