Date: Sat, 15 Feb 2003 21:34:50 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Gap of Sparks
In-Reply-To: <BA73B2FD.EE1E%ben@volkscafe.com>
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At 11:03 AM 2/15/2003, Ben McCafferty wrote:
>Well, yes and no. Points have a dwell angle, which is the amount of time
>(in degrees) they are close enough together to arc and send spark out the
>rotor and dist. cap to the plug. But if you consider that once set, dwell
>is a constant, then changing gap will in fact affect timing. I'm basing
Let me rephrase that a little -- points have a dwell angle, which is the
percentage of total time that they are closed. This is the time during
which current flows through the coil, ballast resistor and points and back
to ground, building up energy in the form of a magnetic field in the
coil. When the points open this electrically substitutes a capacitor into
the circuit instead of the points. The magnetic field in the coil rapidly
collapses, generating a high voltage in the primary circuit (the points
side of the circuit and a much higher one in the secondary -- the spark
plug side. The high voltage in the primary, call it 75 volts for ballpark,
charges the capacitor which takes up some of the energy that was stored in
the coil -- then discharges shoves it back into the coil again, expanding
the magnetic field and generating another fast spark. The energy bounces
back and forth between coil and capacitor, each time losing some into
another spark, until it's no longer enough to cause the plug to fire. A
while later the points close again to build up for the next spark. All the
parts -- capacitor, coil, plug wires, plugs -- have to be matched to some
degree for the system to work well. One of the problems with this method
is that at high rpm it's hard to get the points closed long enough to build
up sufficient energy in the coil.
In a mechanical distributor with points, the dwell is set by changing the
gap. In general the gap itself is of lesser importance here, it's the
dwell time that matters -- but since the angle that the points are open
changes along with how far they open, in a given distributor you can set
the dwell pretty well by setting the static opening distance. However,
making this adjustment will disturb the timing, so you have to re-time
whenever you adjust the gap/dwell.
In an electronic system, the "dwell time" is set electronically and the
distributor merely furnishes timing information, so it's a bit arbitrary --
could be either leading or trailing edge of whatever timing device is used.
david
--
David Beierl -- dbeierl@attglobal.net
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