Re: Motorcraft Duraspark question...

[Tom Jennings <tomj@xxxxxxx>]

On Mon, 13 Jun 2005 farna@xxxxxxx wrote:

> Sounds like an awful lot of juice is being pulled through that resistor! I didn't think the TFI ignition needed/used a ballast resistor. Better check some Ford wiring charts or applications -- may not be a resistnace wire or ballast resistor requirement for the TFI. In that case it might be pulling more amps since it can't get teh volatage reqquirement through the resistor, which would cause the resistor to overheat.

I'll got RTFM again, but I'm fairly sure I did the research and a
ballast is required. Without a ballast resistor, the coil would
see MORE current, regardless, simple Ohm's Law (when engine's not
running).

And here's an interesting observation:

With the ignition on but engine not running, after 30 minutes or
so the coil is hot. With the engine running, the coil does not get
hot. Same ballast resistor, etc.

The reason for the lower temperature when running is that the
energy stored in the coil (in the magnetic field) is dumped into
the spark plugs when the current is switched off (by the Duraspark
box); the coil heats up only with the enrgy losses in the copper
and excessive turn-on time (at low RPMs).

When on, but not running, the 4.8amps/7V in the coil simply
becomes heat, since it's not making spark. Ohm's Law at work.

Same issue as a cylinder full of burnt fuel; if you expand it you
take the energy out at the crank and the temperature of the charge
goes down. If you dont it all goes out the exhaust. I remember
Smokey Yunick was running some crazy high-temperature motor where
the exhaust gas temperature was 300C! Now that's efficient!

It's simply Conservation of Energy at work.

A properly sized and tuned ignition coil should actually run
coolest up near it's maximum design RPM, and hottest at idle!