Dwell and Timing Explained

What You Need to Know for Points Engines

spark plug wires
Spark plug firing order is crucial to engine performance. Getty

Although many ignition systems on new cars today are computer controlled, your classic or late model car likely has a points ignition system. And if you enjoy working on the car, there are a few things you need to understand before setting its timing, including the importance of setting the dwell.

Into the Gap

Ignition points are a set of electrical contacts that switch the coil on and off at the proper time.

The points are opened and closed by the mechanical action of the distributor shaft lobes pushing on them. Getting the best gap between the points is essential for proper engine performance and reliability. Set the points too wide and the spark plugs don't get enough juice. Set them too close and the engine stops working after a few miles.

At ordinary engine operating speeds, the points open and close a couple of hundred times per second, the exact number depending on the number of cylinders and the engine RPM. The points need to be closed for an appreciable amount of time in order to build up the maximum magnetic flux in the ignition coil core. It might sound like something from "Back to the Future" (in fact, there was time when this process was considered almost magical), but today it's basic automotive knowledge.

Dwell on It

The period of points closure is specified by the ignition system designer and is typically expressed as degrees of distributor rotation.

In a four-cylinder engine, the angle between each ignition cam lobe is 90° and the period of points closure or "DWELL" is usually a bit over 45° of distributor rotation. In a six-cylinder engine, the lobes are 60° apart and the dwell time is 30° to 35°.

The dwell is adjusted by setting the points gap to a specified distance at maximum opening.

A narrower gap gives more dwell and a wider gap gives less. Taking it to extremes, excessive dwell means that the points close too soon after opening, cutting off the magnetic field collapse before it delivers all its energy. Too little dwell gives the magnetic flux insufficient time to build up to the maximum.

Set Your Timing Last

Both conditions give a weak spark which gets even weaker as the engine RPM rises and produces misfiring at normal operating speeds. The dwell, as well as spark plug gap, do have an effect on ignition timing. The later the points open, the later the spark comes and retards the timing. The earlier the points open the sooner the spark comes and advances the timing. That is why timing is the last thing to be set in a tune-up.

How to Set the Dwell

You read above that the ignition timing is the last thing to set when tuning the engine. Your dwell, and thus your points gap, needs to be set before you get out the timing light. To set the dwell, remove the distributor cap and rotor, ground the coil wire and remove all the spark plugs from the engine. Set up your dwell meter and hook up a remote starter. If you don't have a remote starting loop, you can always ask a friend to be your key operator for this procedure.

Turn the key ON and crank the engine. Using a feeler gauge to get close, adjust the points to the desired setting according to dwell readings and tighten the points. Crank it again to be sure the dwell angle is still correct.

You can now go on to set your timing.