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 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.


> > 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.