Mailing List flyrotary@lancaironline.net Message #13427
From: Bill Dube <bdube@al.noaa.gov>
Subject: Re: [FlyRotary] Re: (unshrouded plug ?)required wrench
Date: Fri, 03 Dec 2004 09:15:27 -0700
To: Rotary motors in aircraft <flyrotary@lancaironline.net>

They should work fine.  I just felt that for a system set up for maximum spark power, I didn't want to have a big resistance in the plug.  I have no idea whether you would ever notice the difference.  I expect Paul Yaw would.

        The resistor is there as a "ballast" to limit the peak current. You need high voltage to strike the arc, but once the arc has formed, the resistance across it is low and it takes very little voltage to sustain it. Without the resistor in the plug, very high current flows, eroding the electrode and shortening the life of the plug. Also, the high current produces a high magnetic field, causing radio interference.

        The resistor also helps to increase the duration of the spark. There is a fixed amount of energy stored in the coil for each spark. Without the resistor, a high current flows. The energy is then quickly spent, resulting in a shorter spark duration.

        Additionally, you can "burn out" some types of spark plug wires by using non-resistor spark plugs. The wire can't handle the extra current and goes open-circuit.

        Most cars have "magnetic suppression" type spark plug wire these days. It is a long coil of thin wire wrapped around insulating fibers. It acts as an inductor in the circuit. This tends to make the current much more constant during each spark.

        With electronic ignition, it is not uncommon to have both magnetic suppression wire AND resistor plugs. They act in concert to limit peak current, to smooth out the average current, and to reduce radio (and computer) interference.

        If the coils and ignition control module you have selected were designed to use magnetic suppression wires and resistor plugs, you would probably be smart in continuing to use these type plugs and wires. The higher peak current from non-resistor plugs can also damage the coil or ICM if they are not designed for that use.

        Just my eight cents. :^)
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