Dwell and HEI's

[Clare Snyder]

bruce plecan wrote:
> 
> Me thinks I might be able to shed a glimmer of light on something.
> After looking reading this thread, the coil is "on" for a time
> determined by a R/C timer, so the dwell looks to change as the
> rpm do, well ya, but the coil on time is the same it the matter of
> how many crank shaft degrees occur during this time that gives
> the illusion that something has changed.
> Bruce   Murphys law may have started in 1949, with a rocket
>             sled pilot, who made a run only to find out all the
>             sensors were hooked up backward,  Geeeeez
This is a good point. Untill the on time becomes so long there is not
enough off time for the field to collapse, you get the best possible
spark. On a point fired system, the on-time decreases with engine speed,
causing spark output to drop off at high speeds. The coil does not care
how fast the engine is firing - it takes the same length of time (within
limits) to saturate regardless of speed. If this is indeed the
situation, at low rpm we should expect the dwell reading on a meter or
scope to be quite low, and at high speeds relatively higher. This would
explain why higher primary current (lower resistance) coils are
generally used with HEI systems (current limited modules) The limited
current dtermines the maximum magnetic field strength which is lower
than it would be with a set of points driving the coil. With a short
dwell and this limited (relatively high)current, we still get an
incredibly hot spark at tick-over, which stays the same at max RPM.
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