Digital dwell meters and point-type ignition systems don't play well together. The digital readout reacts too fast to changes in voltage and you get an incorrect reading. The old fashioned sweep-needle (analog) meters work much better.
Here's a couple of definitions for you:
"Dwell"- - - -degrees of distributor rotation the points are closed, or "dwell together" The most efficient dwell setting for most distributors is a "points closed" duration of 66%. On a 60-degree (6 cylinder) cam that would be 40 degrees of dwell. Some early stovebolts actually had a dwell specification of 39 degrees. Later, it was reduced to 32 degrees, I believe. Most V8's have a dwell specification of 30, which is 66% of the 45 degree distributor cam they have.
"Dwell" is actually an average of the voltage seen at the downstream connector of the coil primary terminal when the points are open, then closed. Follow closely here- - - -this gets a bit confusing. With the points closed, the voltage at the distributor side of the coil will be zero, since one side of the points is grounded, and a few inches of wire won't make a difference between ground and the downstream end of the coil primary. You can check this by measuring the voltage with the engine stopped, ignition on, and the points closed.
With the points open, you'll see full battery voltage at the coil (-) terminal, since no current can flow through an open set of points. Once again- - -don't take my word for it- - - -it's easy to confirm.
Now- - - -with the engine running and a good analog voltmeter at the coil (-) terminal, you will see an average between battery voltage and zero, depending on how long the points stay closed. Since the points are supposed to stay closed 66% of the time, with a 14 volt alternator reading you will see about 1/3 of that voltage- - - -about 4.6 volts. If the point gap is widened and the dwell decreases, the voltage will go up. Don't try this with a digital voltmeter- - - -all you'll get is random gibberish on the readout!
Jerry
