Ignition ECU operation, HEI
One of my customers sent me a photo of his GM HEI 4 pin module he plans to use in his DeLorean. That got me thinking so I took some measurements of the stock ignition voltage on the negative side of the ignition coil. Correct me if I say something wrong to my thinking.
FYI:
You drive current into the ignition coil to saturate the core (iron) magnetically. So more current or keeping the current on for longer time will only waste power and increase heating of the coil. The coil would take a fixed time of current to saturate it (lets say it takes 2.0 ms to saturate). So it would be best to have that timing at all engine RPM. So voltage on that negative coil side should be zero volts for only 2 ms and +12 volts for the rest of the time.
When current flow is stopped, the magnetic field collapses and that creates the high voltage spark.
So I measured the dwell on that negative coil signal. With the meter set on 4 cylinder dwell at 850 RPM I read 62 deg and at 2500 RPM I read 60 deg. Checking that signal on my oscilloscope, the power on time (zero volts) was 11 ms and 21 ms (you get two timings because of uneven fire PRV) at 850 RPM. At 2500 RPM I got 4 ms and 7 ms.
So it looks like the stock ignition ECU has no dwell change to speak off. The GM HEI claims to have a dwell change which would keep the ignition coil running cooler and therefor not require dropping resistors if the correct coil is chosen. So I'm going to do some playing with the GM HEI 4 pin module. I know Bill uses the Ford Duraspark unit with success but the Ford unit holds the dwell constant.