Dwell can be checked at cranking speed, no problem. So far as a "formula", it's just a straight percentage calculation. Let's try a V-8, which fires every 90 degrees of crankshaft rotation. Since the distributor runs at half crankshaft speed, each distributor cam lobe has a maximum "points closed" rotation of 45 degrees. "Dwell" indicates the time, in distributor degrees, that the points "dwell together". If the points are closed all the time, that would be 45 degrees of dwell. Most engines run best with the point dwell set at about 66% of the available rotation, or 30 degrees for a V-8. If the voltmeter reads 1/3 of the voltage applied to the coil while running, that means the points are closed 2/3 of the time, or 66%. Check the voltage applied to the coil from the ignition switch, then adjust the point gap so the voltage at the distributor side is 1/3 of applied voltage while cranking or running. Bingo- - - -the dwell is set!
Since a 6-cylinder engine has 60 degrees of available dwell, a 66% reading would be 40 degrees. The pre-1955 distributor specs list a point gap of .016", which is 38-45 degrees. In 55, they increased the gap to .019", which translates to 26-33 degrees of dwell, so set the "dwell" voltage a little higher than the theoretical 66% dwell when working on a 55-62 235/261 or a point-type 230/250/292 engine. Cornfuzed yet?
Jerry
