Why Aftermarket Ignitions Advance Timing

[jlandry@domain.elided (John A. Landry)]

I think I've figured it out!

You may remember back earlier this year when we were discussing the
phenomenon of the way the ignition timing would be advanced when an
aftermarket ignition system is installed.  Those of us who have installed
an MSD, Accel, Jacobs, etc. system noticed that upon starting the engine
after installation, the ignition timing is advanced by 5 to 10 degrees
(typically).  After installation, the ignition is supposed to be returned
to the factory (or pre-installation) setting.

Since I recently retrofitted my 76 Traveler from the IH Holley electronic
ignition to an IH point distributor (as a trigger for my MSD system), it
got me to thinking about this question again.  In reading the MSD ignition
installation guide one day, I came across a trouble shooting tip to help
verify if the MSD system was working properly.  This particular test
involves grounding the MSD's trigger wire to cause the MSD system to fire
the secondary ignition system.  I tried it for fun and noticed that the MSD
system fired *immediately* upon the trigger wire being grounded.
Immediately is the key word here!

Contrast the way an MSD system fires (immediately) to the way a
conventional ignition system works.  In a conventional system, the points
ground the primary coil circuit when they close.  When the points *open*
they "break" this primary coil circuit causing the secondary coil circuit
to send a high voltage spike to fire the park plugs.  The difference
between the two types of systems then would be that the conventional system
fires later when the points open verses the MSD system begriming the firing
sequence sooner when the points close.

I called MSD and while the technician I spoke with hadn't experienced this
effect himself, he did acknowledge that the MSD system began it's firing
sequence when the voltage passing through the trigger wire rises to 3.8
volts, which means "immediately" in laymans terms.

When we were discussing various ignition subjects, the MSD tech also added
that each spark produced by the MSD 6A system were identical.  He explained
that depending on the rpm of the engine, there is a time window equivalent
to 20 degrees of crank rotation available for the ignition computer to fire
the spark plug.  At lower rpms the time window is larger than at higher
rpms.  The 6A ignition computer will fire the spark plug as many times as
it can during this window.  At idle it could be several identical sparks.
At 3200 rpms or greater it would be 1 spark.

The MSD tech also reaffirmed that a larger plug gap results in a more
intense spark.  At low rpms with multiple sparks, this isn't so important,
but at higher rpms were only one spark is present, a more intense spark is
desirable.  This is why it's a good idea to upgrade all the ignition
components (cap, rotor, coil, plug wires, etc.) so they can handle the
increased voltage requirements caused by a larger plug gap.

If the MSD system malfunctioned and one needed to revert back to the point
system for ignition, a large plug gap would result in point burning due to
the increased voltage requirements.  The point burning would not be serious
enough to cause a problem during a drive back to civilization.  But, upon
returning home, the points would have to be replaced and the plug gap
decreased if the MSD system was not immediately repaired.

Comments anyone?

Regards,

John
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jlandry@domain.elided             |
Conservative Libertarian        |  Scout(R) the America others pass by
Life Member of the NRA          | in the Scout Traveler escape-machine.
WA Arms Collectors              |
Commercial Helicopter - Inst.   | 1976 Scout Traveler, V345A, 727, 3.54
http://www.halcyon.com/jlandry/ |