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Old January 22nd 05, 02:59 PM
Matt Whiting
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aarcuda69062 wrote:

> In article >,
> Matt Whiting > wrote:
>
>
>>>The rotor air gap increases firing voltage.

>>
>>How so?

>
>
> An ignition coil does not normally output maximum voltage, but
> will if the resistance in the secondary circuit is increased,
> creating a gap that can be jumped essentially is an increase in
> the circuit resistance, since there is a gap between the rotor
> and the cap contacts, the firing voltage realized will always be
> higher than if there is no gap such as with DIS.


I don't see how. The voltage output of the coil is a direct function of
the strength of the field around the coil that is collapsing. The
strength of the field is a function of the current through the primary
of the coil when the field is being established. The field doesn't have
any "knowledge" of the resistance in the secondary circuit. Greater
resistance in the circuit will result in less current (simple Ohm's law
application given the EMF induced by the collapsing field) and thus a
weaker spark, not a stronger one. I searched around last night and
could find not reference whatsoever to either this theory.


> I may also be wrong on this, but my observations on my ignition
> scope reveal that firing voltages on conventional ignition
> systems tend to be a few KV higher than firing voltages on DI
> systems.


That could well be. I haven't measurement them personally. However,
that isn't what is usually claimed. In any event, the strength of the
spark is not a huge factor in starting the car in cold weather. I
proper fuel/air mixture will ignite with only a small amount of
provocation. The really strong spark is needed in cases where the ratio
isn't idea. That is why I can believe that the ignition system on a
non-FI car might need to be more robust as it is much harder to ignite a
mixture that isn't ideal.



>>>Comparing spark patterns on an ignition scope, the DIS voltages
>>>(all else being equal) are lower, lower ionization voltage and
>>>lower voltage across the plug gap compared to a distributor type
>>>ignition system.
>>>An old tow truck drivers trick when trying to start a stubborn
>>>engine in the winter is to pull the coil wire slightly loose from
>>>the distributor cap, this increases the voltage output from the
>>>coil secondary (greatest gap theory).

>>
>>I'm not familiar with this theory, but my EE degree is nearly 20 years
>>old so maybe I just forgot. Can you explain or provide a reference?

>
>
> The greatest gap theory was authored by Mac VandenBrink in 1965,
> Mac worked as an EE for many years for Allen Test Products.
> The only references I know of WRT Mac's theory are on i-ATN,
> easily found if you're a member. Unfortunately I can not copy
> them and post them here because all i-ATN posts are copyrighted
> and doing so would jeopardize my membership standing.
> I became familiar with Mac's teachings way back in the early 80s
> when I was a GM dealership mechanic during training on the Allen
> Smart Scope.


Sorry, but this sounds like cold fusion to me. If this were true, then
somebody else somewhere would have researched or commented on it. Thus
far I've found zilch.


Matt
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