Hi Dan,
In his book MG Tech Talk, Norman Nock writes:
"The Negative Spark--Coils are normally wound to give a positive earth
spard--that is, the spark plug insulated electrode is negative with
respect to the engine block or earth. We usually refer to this as a
negative spark.
"Several distinct advantages are obtained. We have the same sparking
efficiency at a 10% reduction in HT voltage required to break down the
gap. By lowering the voltage, the strain on the insulation throughout the
high-tension circuit is considerably reduced --i.e cable insulation,
distributor cap and all mouldings and plugs.
"We will point ou that if the external connections to the SW and CB
terminals of the coal are reversed, current will flow in the opposite
direction though the coil, reversing the HT spark polarity. In addition,
the auto-transformer action will be lost.
"A further advantage gained with this negative spark is little or no
wear of the rotor arm. WIth the negative spark, the metal transference is
from the contacts to the rotor and wear is divided equally among the four
fixed electrodes.
Question #1---is this theory accepted today?
Question #2---he speaks of the winding of the coil to give a positive
earth spark. If one changes a + earth car to a - earth and reverses the
leads to the coil, is this beneficial negative spark effect still
present?
Bob
On Mon, 10 Aug 1998 14:19:59 EDT DANMAS@aol.com writes:
>In a message dated 98-08-09 08:36:13 EDT, hjillson@argolink.net
>writes:
>
>> The only thing anyone has left out that I can think
>> of is that the coil, being an auto-transformer is indead polarity
>sensitive,
>> seeing as how the coils of the transformer are wound in oposite
>directions
>> for positive and negative ground.
>
>Harlan,
>
>Good point! This creates a bit of a conundrum, though. They are
>polarity
>sensitive in that the output polarity depends on the input polarity,
>and they
>are polarity sensitive in that a positive ground coil is constructed
>differently than a negative ground coil. They are not polarity
>sensitive in
>that when connected to a spark gap on the workbench, either type of
>coil will
>produce a spark, regardless of input polarity. In one case, the spark
>will
>jump from point A to point B, and in the other, will jump from point B
>to
>point A, but the same spark will be produced either way.
>
>This is not true when the coil is operating in a real engine - the
>spark
>produced is weaker if the polarity is not correct, but that is not a
>function
>of the coil, per se, but a function of the spark plug characteristics.
>
>Normally, when I think of something being polarity sensitive, I think
>of
>something that will work only if it is connected the right way, and
>will not
>work, or will be damaged, if connected the wrong way. Of course, this
>begs the
>definition of what the term "will work" means. 80% effective? 90%?
>99.99%?
>
>Hmmm. Perhaps we need to not use the term "polarity sensitive" at all,
>but
>what should we replace it with?
>
>> About the only thing I can think of on the plugs is that the
>polarity
>(hence
>> direction of current flow) will affect which element of the plug is
>going
>to
>> pit , the electrode or the tang.
>
>Another good point!
>
>Dan Masters,
>Alcoa, TN
>
>'71 TR6---------3000mile/year driver, fully restored
>'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion
>- see:
> http://www.sky.net/~boballen/mg/Masters/
>'74 MGBGT---3000mile/year driver, original condition - slated for a V8
>soon
>'68 MGBGT---organ donor for the '74
>
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