general efi questions

[Brian Dessent]

I've been studying the wiring diagrams of my car's ECU and trying to
understand the workings in general of the computer control (I have a
background in computers and electronics but not so much in the
automotive area.)

I have a couple of basic questions.  The car I am studying is a 89 Mazda
626 2184cc 4 cyl turbo.

- Injectors 1 & 3 are wired in parallel as well as 2 & 4.  I think I've
read that at startup all 4 injectors are fired simultaneously, but soon
after it switches to firing them in two groups.  By my logic when #1 is
on intake #3 is on exhaust, and when #3 is on intake #1 is on
compression, please correct me if I'm wrong [firing order is 1-3-4-2]. 
Why in the world would you want to fire an injector during exhaust or
compression (it seems this would not do anything since the intake valve
should be [mostly] closed)?  Or, stated another way, why in the world
would you wire injectors in parallel, why not let each one have its own
control signal?  And, why fire all four at once on startup?

- Advance/retardation is ideally based on degrees but the computer
ultimately works with time.  Does the ECU just use the RPM data from the
crank angle sensor to convert timing from degrees to ms?  I can't recall
at the moment but I think the angle sensor sends out a single pulse for
every rotation of the distributer, or it could be 3 pulses per rev. 
Assuming one pulse per rev, this seems awful "course" for measuring RPM
if it is changing rapidly.  What I mean is, if you are trying to achieve
timing accurate to say a degree or so, if the rotation rate is changing
rapidly and you only have 1 pulse per rev [actually 1 pulse per 2 rev of
the crankshaft since distributer is geared 1:2 right?] then it would
seem that it would be hard to achieve the necessary accuracy.  Can
anyone see what I'm getting at and explain?  I guess adding more angle
sensors would fix this problem, and maybe that's how they get around
this problem.

- I want to scope some signals while driving.  Does anyone know of any
issues with using a (classic analog, not DSO) scope with a DC power
inverter?  I want a cheap-ish inverter but I'm thinking it would need at
least a moderately clean (modified sine wave?) output for the scope.

I have more questions but that's it for now.

Thanks,
Brian
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