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

 I recently did a Subaru Wrx engine swap into an earlier Impreza. The
 owner did not want to spend money on parts or the time it would take
 to install a late model fuel tank assembly that matched what came
 with the car the engine came out of.

 When it came time to deal with the fuel temp sensor, I added a
 resistor in the place of where the fuel temp sensor should have been.

 With the main swap completed, we took the car to a local smog test station
 to check if it would pass before taking the car to the Calif. BAR
 smog station to legitimize the engine change in the eyes of mama pelosi.

 The car easily passed the local tail pipe test with the resistor in place.
 Upon taking the car to the official state exam station, the car
 failed tail pipe. The reason? The temperature on the first test was
 about 90F; the second test the temp was about 65F. I was therefor
 paid to install a later model fuel tank system.

 Presumably, we will have to address the same issue, and other similar
 issues with the 555 system. Particularly because altitude temp differences
 can be far greater than what is regularly encountered while driving your
 car.

 Ed Carver

 Use a mass airflow sensor instead of a manifold pressure sensor.
 Since a mass airflow sensor has intake air compensation built in we can
 use the computer that holds the trim values to linearize the non linear
 mass airflow sensor.

 Paul Lamar

  If I understand correctly, the injector signal from a stock ECU
from Mazda as removed from a donor vehicle could be used for the
injector pulsing. The system can then be leaned with a pot input to
the 555 chip(s).

Dale Davies

Not exactly. Perry Mick has been flying a stock ECU for years
but some of the inputs had to be simulated.
The big problem is there is no manual leaning.

Paul Lamar


If you use four TLC555 chips. Install a resistor in the stock ECM harness to
replicate an injector. Tee the ground side of the resistor to the trigger on
the 555. It will go low when the injector is pulsed. Use a potentiometer on
the '5' terminal instead of the MAF sensor to lean it. This way the 555 should
reduce the injector on time to lean. 555 circuit as per your previous answer.

Dale Davies