Read this first.
http://www.rotaryeng.net/simple-cheap-555.html

just had another idea.

Use a cheap CPU with a look up table who's only function
is to modify the voltage sent to the 555 as a function
of manifold pressure. This correction or "trim" in EFI parlance can
be edited in a spread sheet and downloaded using the USB.
Henry Nee's idea.

Atmel chips have a similar asm instruction set to a 6502 or 68K.
ZBasic sell's them on boards with USB software drivers.

http://www.zbasic.net/

The program would be so simple it would be easy and quick to do and maintain.

Main loop:
read the manifold pressure a to d
read the rpm.
Look up the correction.
Apply the correction to the d to a output pin
connected to the 555 pin 5, pulse width control pin.
Do it over again.

--------------------------------------------
The main reason EFI system use computers to trim the mixture is the air
actually getting into the engine is not  a linear function of RPM as one would
expect. The culprit is the intake manifold. How well it works is affected
by all the different resonant fluid conditions excited at different RPM's.
The intake air pulse frequency changes with RPM.

The other thing a 555 system does not do is compensate for air temperature.
It is a linear inverse function of temperature so it can be mathematically
achieved by a simple multiplication or division.

That can be done by the simple computer mentioned above but it could
also be done in hardware as Paul Lipps demonstrated in Klaus's early
analog fuel injection system. I just don't know how to do that yet :)

Here is a 3D surface that illustrates these points. A straight 555
system would have no hills and valleys. Just a simple smooth
surface.

BTW this can be plotted in a spread sheet program.
The Megasquirt tuning software does it for you.


Paul Lamar