Dwell Angle
Most people know that if the car has a mechanical contact beaker then the point's Gap has to be set. The gap itself is not the important thing, it is the Dwell Angle that is effectively being adjusted.

The Dwell Angle is the angle of rotation of the distributor cam during which the points "Dwell" together, and they will go through that angle 4 times per rotation of a 4 cylinder engine. Again, for a 4 cylinder engine, each cam lobe opens and closes the points for a total angle of 90° of distributor rotation per cylinder. On most cars, manufacturers specify that the points should dwell together for about 2/3 of the total 90°, that is for 60°. Changing the contact breaker Gap, changes the Dwell Angle. If the Gap is very small, the contacts will Dwell together for a long time, then open (and close again) very quickly. Conversely, if the Gap is very wide, the points may not close at all, or only for a small proportion of the time, and hence the Dwell Angle is low.

During the Dwell of 60°, the coil builds up magnetic energy which it then dissipates during the remaining 30°. Dwell really only matters at high engine speeds when, should the dwell be too short, insufficient energy is stored in the coil to produce a strong enough spark to reliably fire the engine and there may be insufficient time to dissipate all the stored energy.

At 1,000 rpm of the crankshaft, the distributor rotates at 500 rpm or 8.3 rotations per second. Each rotation takes 0.12 seconds (1/8.3) or 120 mS. (mS means milli second or thousandths of a second). Each cylinder (of a 4 cylinder engine) is addressed for 25% of that time or 30 mS. With a Dwell Angle of 60° the coil gets
60/90 X 30mS or 20mS to charge and the remaining 10mS to discharge. Obviously at 5,000 rpm crankshaft speed, the Dwell time is 1/5 of that, or only 2mS. See also Math .

Measuring the Gap is a crude method of measuring Dwell. Unless the contact faces are new, unpitted and absolutely flat, the result will be approximate at best. Unless you have several cars, borrow a Dwell meter, the frequency at which you will use one does not warrant the outlay. You may also wish to consider electronic points, that never require adjustment.

If you are experience misfiring at high engine speeds and suspect poor spark energy (it could be due to other causes like fuel starvation - check pump, filter and float level) then it is worth checking the Gap or Dwell, before changing the coil. However, Dwell usually increases (Gap decreases) because of wear on the cam follower. The real problem is its effect on the Ignition Timing, which will be retarded.

The main effect of Dwell is, as stated, on timing. Whether it is set to 50°, 60° or 70° is not really important (unless you run the car at high engine speeds for racing, in which case invest in electronic ignition). What is important is that it doesn't change, or if it does, it is set back to the same reading again. That's where a Dwell meter scores over feeler gauges.