Understanding Ignition Dwell

by Kelli Murrow

There are a number of different components required to produce the spark that occurs at the tip of the spark plug including the distributor, ignition coil, plug wires and of course the spark plugs. To ‘dwell’ on those components a little closer, there is also a trigger device such as breaker points or a magnetic pickup, two sets of coil windings, an advance assembly, cap, rotor, distributor gear, electrode and more.

Each of these components plays a key role in the function of the ignition system but we’re going to take the time to understand the term ignition dwell and how it affects the performance of an ignition system. Ignition dwell, or dwell time, has to do with process of building up high voltage in the ignition coil for each firing of the spark plug. This charging of the coil takes place in a fraction of a second every time a cylinder fires and is very important to any vehicle running breaker points. The concept of dwell time will help you understand how the advanced dwell control circuitry of a Pertronix Ignitor Electronic Conversion Module can help with performance.

Coil Operation

To understand ignition dwell, we first need to review the operation of the ignition coil. It may seem like magic to have 12 volts from the battery go into the coil only to have it output a spark with at least 15,000 volts, but it is all physics and principles of electricity. Inside every coil there are two sets of windings surrounding an iron core. The primary windings are generally made up of several hundred turns of heavy wire while the secondary set are made up of a much smaller gauge wire consisting of thousands of turns. The number of turns between the two windings is referred to as coil ratio or winding ratio such as 100:1 or 80:1. (Engineers can actually manipulate the voltage and amperage that the coil creates with these turns ratios, but that is for another tech blog.)

As voltage runs through the primary windings to the negative coil terminal and to ground, a magnetic field builds strength around the iron core of the coil. When the trigger device of the distributor opens, such as the breaker points in our example, the ground path is suddenly opened causing the magnetic field to collapse to the thousands of windings of the secondary side. During this event, the voltage is increased creating a much higher voltage that is directed to the center terminal of the distributor, through the rotor and plug wire finally jump across the spark plug gap in the cylinder.

What is Ignition Dwell

What does this have to do with ignition dwell? Dwell time is measured in the number of degrees of the distributor’s rotation through which the contacts of the points are closed. This is also known as saturation time, or the time when the coil is producing a strong magnetic field in the primary side of the coil. If the dwell time is reduced, the voltage output of the coil is weak. If the dwell time is too long (the gap barely opens), an arc can travel across the points reducing the output of the coil and even damaging the coil or points. It’s a fine balance and is why the breaker points must be adjusted correctly, which is also called setting the dwell.

On the distributor shaft in a V-8 engine, there is a cam with eight flat areas and eight lobes. The switching arm of the breaker point assembly is spring loaded and has a rubbing block that rides against the cam. Each time a lobe comes up, it pushes the rubbing block which opens the points contact causing the primary magnetic field to collapse to the secondary side releasing the spark. A rule of thumb is to have about 28-32 degrees of ignition dwell time which can be measured with a good old dwell meter and adjusted with the Allen-head screw on the points assembly.

Breaker Points

Breaker points were used to trigger the coil for decades and there is no denying their ability to get the job done on a stock engine for daily drivers, however they do have their shortcomings. First, they’re mechanically operated which means there is going to be wear over time and require adjustment. Secondly, points were designed to handle a small amount of current which limits their potential output, a hinderance for performance applications. Going back to the mechanical side of things, at higher rpm, the mechanical arm of the points assembly tends to bounce (sometimes called points float) resulting in ignition misses and burning of the points themselves.

Electronic Dwell Control

For years, racers had to contend with the shortcomings of breaker points but eventually electronic ignition controls and trigger devices came to their aid. Pertronix developed their Ignitor Electronic Conversion Module which fits in place of the breaker points assembly of most any distributor. The Ignitor is triggered electronically resulting in improved accuracy and consistency. Thanks to this technology, the Ignitor is capable of delivering twice the voltage output to the plugs through improved control over the dwell time of the coil. In fact, the Ignitor II and III Conversion Kits provide an adaptive dwell control technology and high rpm correction to help maintain peak energy through minute adjustments of the dwell time.

With the use of modern electronics, there is no reason to be concerned with setting the dwell of an ignition system, however the concept of dwell time still takes place in the coil. It’s just not something you need to worry about thanks to Pertronix Performance Brands. With the largest selection and most knowledgeable staff, PerTronix Ignition is your one stop shop.