Paul,

An earlier post indicated that both mass air flow and rpm were needed to
determine the timer "on" duration.  It also appears that way to me.
Here's my thinking:

The timer "on" time determines the fuel flow per revolution, which needs
to be proportional to the quantity of air per revolution.

But the mass air flow sensor provides the quantity of air per minute.

To determine the quantity of air per revolution, we need to divide the
quantity of air per minute by the revolutions per minute.

Would a PIC chip be a simpler way to achieve this calculation than an op
amp circuit?

Philip

Not quite that way. The injector fires once per rev. With a fixed pulse width
(say 2 ms) that makes the amount fuel injected directly proportional to RPM.
Now it depends on how much air is entering the engine at any RPM. That depends on throttle opening. The engine can rev to 9000 RPM in the RX8 car if the engine is not in gear (no load) with very little throttle opening. However the aircraft prop load is increasing with RPM so the engine is going to need more and more fuel per revolution as RPM increases. It may need more than 2 ms worth of fuel. The mass airflow sensor now comes into play. At a fixed RPM the mass air flow sensor measures the amount of air entering the engine and adjust the pulse width and hence the mixture up or down by controlling the 555 through pin 5 which is what pin 5 is designed for. Probably up.  This is called "trim" in EFI and OBD II parlance.

It might work on the mass airflow sensor signal alone pulsing the injector at
a constant rate but that would not be "timed" injection.

Here is an OBD II mixture chart for an actual acceleration run on an RX8. "Lamda" is shown  in percentages of 14.7:1 air fuel ratio. In other words Lambda 1 (one) is 14.7:1. At 67 MPH and 6359 RPM Lambda is .76.  So .76 is full rich for acceleration. .76 times 14.7 is 11.71:1 air fuel. This is why you want to keep your foot out of the throttle in an RX8 :) The throttle position and load value are also shown but this must be taken with a grain of salt as the PC OBD II monitoring computer is not really fast enough to handle rapidly changing engine conditions.

We would set up the 555 pulse width in an aircraft engine to be rich all the time for engine acceleration purposes. No need for an accelerator pump. Many aircraft engine carbs don't have them. Once you reach cruise you lean the mixture to as little as 18:1 for best BSFC.

No need for a $1 pic chip with arcane and complex software  however it will surely work.

Paul Lamar