On Wed, 10 Apr 2019 13:22:23 -0700 (PDT), TimR
wrote:

On Wednesday, April 10, 2019 at 1:52:47 PM UTC-4, Rod Speed wrote:

But in some situations the problem is that the car that wont
start will turn the engine over fine but the voltage sags enough
so that the ignition cant supply a good enough spark to get
the engine to fire and all the car that is helping to start it
needs to do is supply enough voltage to get the engine
to actually fire and that doesn’t require much current.

I'm throwing the bull**** flag on this. It has always taken more power to turn an engine over mechanically than to get a spark plug to fire.



Yes, that is true - but you can run the flag down the pole. If the
battery "sags" from the cranking current there is not enough VOLTAGE
left to make a good spark. That's why on Kettering ignition (typical
points and condenser type of years gone by) there was often a ballast
residtor that ewas shunted out of the circuit for starting. The coil
was designed to run on 7 or 8 volts - about the voltage available from
the battery when cranking a cold engine. Once running the resistor in
the circuit limited the coil current to what it would be at 8 volts.

This was done because a 12 volt coil running on 8 volts coukld not
fire the plugs reliably.