I've only heard this mentioned in passing, but never explained. I've searched here and all over the web, and never found any answers. If anyone would know, I figured it'd be somebody here.

Here's what I've gathered:

The "yellow-grommet" Duraspark box comes with a third plug. It's a box used for high-altitude applications, and it varies the spark advance based on the input to that third plug. Basically, it varies the advance with the amount of resistance between the two terminals on that third plug. The Motorcraft part #'s for this box from the M-block page are DY-237, DY-237A and DY-297.

Here's what I'd like to find out:

* What is the range of resistance that the factory sender (I'm guessing it's an atmospheric pressure sensor) would introduce between the two leads on that third plug?

* What was the range of ignition advance that this box could alter?

I thought that adding a rheostat in-line between the leads on the third plug would give a degree of in-cab ignition advance adjustment, but wanted to get some idea of what kind of adjustment I could get, and what specs I would need for the parts in the dial control, before I dive into the project.

On a related note, the white wire for the blue-grommet box is supposed to retard the ignition timing when the ignition switch is in the "start" position. Anyone know how much a signal to this wire retards the ignition? Could 12V(+) be fed to this wire at other times to back down the ignition in other situations (i.e. flipping a switch if it's pinging under load)? Anyone tried this, with good or bad results?