Brad,
The importance of dwell is that it is a measure of the fraction of the
distributor rotation during which the coil is flowing current. When current
is flowing in the primary, the coil stores energy in the form of a magnetic
field, and when the current in the primary portion of the coil stops, the
magnetic field collapses, generating a high voltage in the secondary. At
high engine rpms, the coil does not have time to reach saturation and
voltage falls off. A capacitive discharge type unit like an MSD or similar
unit, drives the coil more effecively than a straight 9V DC resistive (not
capacitive) source. These units only use the points for timing purposes, so
dwell is of no importance, except as it indirectly affects timing. With a
standard ingnition system, dwell is a compromise between low rpm and high
rpm performance. For optimum high rpm performance, increase dwell to the
maximum (i.e., reduce the points gap) and also reduce the spark plug gap.
I've been a huge proponent of MSD systems and have been using them
exclusively in my Tiger since the '70s. I have an MSD-6 that's been in use
since around 1979. I use 8 or 9mm plug wires and gap the plugs to about 0.050".
The old Jaeger tachs sensed current flowing in the primary of the coil.
Ideally, the circuit should respond to the number of pulses, but it
obviously has a limited range of operation in terms of pulse duration and
magintude.
Hope this answers at least some of your questions.
Bob Palmer
>I've got the same problem and I've got a couple of basic questions.
>Does the old tach not self adjust for dwell?
>Due all new Tachs adjust for dwell?
>What is dwell angle? I know how to measure it, but I'm not sure what it
>is.
>Is calibrating the old tach against a "modern" new one a good idea?
>Will having both tachs hooked up effect the test negatively.
>
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