Ignition coil charge time

[Bill Lewis]

The whole idea of dwell stems from the fact that the mechanical distributor
can't provide the correct coil charge time at all engine speeds.  I decided
I could do better by programming in a charge 'time' rather than a dwell angle.

Some years back, I experimented with a micro hooked to the HEI on a Chevy V8.
I took the input from the Hall sensor in the distributor, and triggered a
counter that waited until the coil-on point.  The output from that first timer
triggered a second counter that waited until the coil-off (firing) point.

When I was first testing the hardware and the code in the basement with a 555
simulating the pulses from the engine, I had to come up with a constant to
load into the dwell time counter.  I didn't feel like figuring out the correct
time, so I picked a number of of thin air and plugged it in.  Several days
later I found myself outside with a battery charger hooked to the car
wondering why the damn thing wouldn't start.  It acted like it wanted to, and
the timing light indicated I had spark at the right spot.

The punchline - the coil-on time wasn't enough to properly charge the coil to
get a good spark.  Again without thinking, I doubled the constant in the timer
and it ran great.

This time I'm planning to run a test circuit on the bench with my new coils
and watch the current ramp up on the oscope to determine the saturation point.

.../Bill

-- 
Bill Lewis - wrl at access.digex.net