Home brewed cheap Dyno

[Robert W. Hughes]

The GM cars use a sinusoidal generator rated at 2000-4000 pulses per
mile with an amplitude of 6-10 volts pp. I tried attenuating this and
feeding it to the mike input of a small cassette recorder only to
discover that the recorder had a very strong low freq. cutoff at 30 Hz
which is about 30 mph. The ignition pulses have a much broader spectrum
so they are easier to record and recover.