In effect, Dwell is the same thing as point gap.
It's geometry.
The point cam profile determines the time the points spend open and closed, when using the specified gap. Dwell is the amount of crank rational degrees where the points are closed. This time is used to charge up the coils. Because of the cam profile, if there is a correct point opening gap, when the points closed time will also be correct.
The cam is actually supposed to close the points through 195 degrees of crank rotation on the SOHC4. The way dwell meters measure this is by comparing a time constant internal capacitor/resistor charge/discharge times with the time power is supplied when the points are open. They don't actually measure crank angles, they measure time constants, and compare that to an internal reference set by the meter designer. Standard ignitions for autos "of the day" used a single coil to fire individual spark plugs. Each revolution of a 4 cycle engine crankshaft fired 4 cylinders of an eight cylinder engine, 3 for a 6 cyl and 2 for 4 cyl), and the point cam was shaped for a duration of coil charge times sufficient to provide spark for the next cylinder. This is why you see dwell meters either provide different scale markings to reference or additional switches (changing internal time constants) to make the scale read correctly for the engine under test.
None of the scales used for 4, 6, or 8 cylinder 4 cycle engine will read correctly for our 4 cylinder, two coil, wasted spark systems. Our coils discharge and charge once per crankshaft revolution as a 2 cycle engine would experience, and we have two points systems operating off a single crankshaft mounted cam.
They are adjusted independently. In essence, our ignition system dwell meter should be designed for a 1 cylinder 2 cycle engine to give an accurate dwell indication.
Does this mean we can't use an automotive dwell meter? No, we can. We just have to calibrate the scale to be meaningful to us.
To calibrate, we use a brand new set of points with known flat contacts. Set the gap at the optimum opening, and note where the needle points on the dwell meter with the engine running. (If possible, use a switch setting the puts the needle mid scale.) This is now your reference point for all future measurement on the SOHC4. It won't matter if the point contacts are worn irregularly, you can set the proper gap and dwell from your calibrated dwell meter without using a feeler gauge. Just make the meter needle point to your calibrated tick mark while adjusting the gap.
The danger of all this is when the points mounting plate moves about later during the timing operation of a running engine, as lateral movement of the plate effects both timing AND the point gap/dwell. The desire is to set dwell and then the timing, but, if the timing operation changes the dwell, you've created a recursive (perhaps endless) loop of adjustments. With the plate loosened for timing changes, the cam position and point springs can move the plate about and negate or randomize your precision dwell adjustment in direct relation to the amount the plate can move within its mounting. With a gap setting of .014" and a lateral plate movement of .007", your possible point gap can vary from .021" to .007", and that will make a significant difference in your dwell reading. The tester then puts on a face like...
I didn't proofread the above for spelling etc.. ...out of time. Maybe do that later...
