Dwell and Timing Explained Even More

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

There are all sorts of questions coming to me here at Auto Repair Central, and believe it or not, most of them are very good questions. I especially like to hear from people who are asking about common curiosities or topics that are important to understand in order to comprehend your engine as a whole. This question comes in from Jack who is very interested in understanding dwell and ignition timing.

He writes:

I can tell by reading your web page that you know what you are talking about. I know as the points gap decreases the dwell increases. I'm wondering when the dwell increase how will it affect the timing degree. Will it increase or decrease?

 

I was wondering too if a person removes the high voltage coil wire from the distributor cap and grounds it, and also leave the distributor cap off the distributor and remove the rotor would it be safe to turn the engine over with the starter and set the dwell angle that way? You mentioned removing the spark plug wires. Would appreciate hearing from you if you have to time to answer.

Thank you,
Jack

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.

Sounds like something from Back to the Future, right? It may be commonplace in engines today (or actually outdated at this point!) but there was a time when this type of technology actually was considered almost magical. I digress, but it always amazes me to think about what was happening around the time that something like an ignition coil was invented.

 

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.

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. The way I set the dwell is this: 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.

With GM points it's a lot easier. They have an adjuster built into the points and a little door in the distributor cap.

This allows you to adjust the points while the engine is actually running.

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Your Citation
Wright, Matthew. "Dwell and Timing Explained Even More." ThoughtCo, Jul. 7, 2017, thoughtco.com/dwell-and-timing-explained-even-more-4059400. Wright, Matthew. (2017, July 7). Dwell and Timing Explained Even More. Retrieved from https://www.thoughtco.com/dwell-and-timing-explained-even-more-4059400 Wright, Matthew. "Dwell and Timing Explained Even More." ThoughtCo. https://www.thoughtco.com/dwell-and-timing-explained-even-more-4059400 (accessed October 23, 2017).