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Ignition Basics

Inductive Vs. CD Ignitions

Jeff Smith Aug 1, 2001

Step By Step

The world of high-performance ignitions is full of claims of superior performance. There are basically two types of systems: inductive-discharge systems (left) and capacitive-discharge systems (right). They both light the fire, but there’s more to ignitions than just sparks.

The original inductive-discharge ignition system used points to both charge the coil and then trigger the spark. Unfortunately, with only 8 or 9 volts on the primary side, spark energy is limited. The GM HEI improved spark energy, and now aftermarket units like ACCEL, Performance Distributors, Pertronix, and Crane’s XR700 inductive unit offer ignitions that can support over 400 hp and 7,000-rpm engine speeds.

The classic CD system has to be the MSD-6A (left). MSD has since upgraded this system with a digital version (right) that offers a built-in rev limiter using a rotary switch instead of plug-in pills.

Crane’s version of the entry-level CD system is the HI-6 and comes in several versions. When matched with Crane’s E-core coil, this system can put some serious power to the spark plugs.

Mallory also has a direct competitor for the MSD-6A and Crane HI-6—the HyFire VI. The upscale Mallory digitally controlled system also uses excellent Weatherpak connectors that make wiring as easy as plug and play.

ACCEL’s street CD system is the 300+ and can be purchased to plug into several existing ignition systems. For example, ACCEL offers a 300+ that will convert a typical HEI into a CD ignition with multi-strike capability.

This shows the classic inductive-spark scope pattern using points. The voltage spike (A) is the high initial voltage required to ionize a current path across the spark-plug gap. The voltage required to maintain this current flow is comparatively low (B) but you can see that the spark duration is relatively long. After the spark breaks down, the voltage trails off back to zero (C). There are dozens of different patterns based on the type of ignition system. Even an inductive HEI circuit will differ from this pattern slightly.

A performance ignition system allows you to open up the spark-plug gap. This demands more voltage to jump the gap, but it usually creates a bit more complete combustion at low engine speeds. Stock plugs gaps are usually 0.035, inch and you can experiment by increasing the gap to 0.045 inch. Higher-powered CD ignitions tend to increase plug wear with the greater voltage and amperage delivery.

The safe bet is to never mix ’n’ match ignition components. There are entirely too many ways to screw up. For example, a coil designed for an inductive ignition will not work very well with a killer CD system.

Conventional oil-filled coils (left) worked OK with point-type ignition systems, but the new E-core coils (right) are more efficient. There are several variations on turn ratios, internal resistance, and other parameters that make coil choice crucial to the success of the ignition system.

The illustration shows how the E-core coils have only minimal magnetic flux loss (the dotted lines), which makes the E-core coils more efficient.

Jacobs offers an excellent digital Pro Street CD system with rev control. The Jacobs Pro Street system uses an innovative microprocessor-controlled system that increases current output based on the rate of change of rpm.

The ignition is easily the most misunderstood system in any car. Most enthusiasts treat ignitions like some sort of weird, black magic/voodoo curse best left alone until it fails. Either that or they spend tons of money on parts they may not need. Given the large body of existing misinformation, we decided to zero in on the two basic types of performance ignitions. We’ll explain how they work and the advantages of each. While we’ll keep it simple, there is some basic physics here that you’re just gonna have to wade through.

Triggering a spark across a spark-plug gap requires both voltage and amperage. Voltage is the electrical pressure that “pushes” the current (amperage) through a conductor. In order for a spark to jump the gap, a large amount of voltage is required. For a 9:1 compression street engine, this can be between 8,000 and 12,000 volts. But voltage is not what initiates combustion. The voltage ionizes the air in the plug gap, creating an electrical path across this gap. Once this occurs, the voltage requirement is reduced and current flows across the gap, creating that blue arc that typifies a strong spark. This current flow of energy across the gap is what begins the combustion process.

The amount of current flow combined with the voltage plays a big part in an efficient combustion process. The more current pushed across the plug gap, the better the chance for more complete combustion. As an analogy, it’s tough to light a big log in your fireplace with a match. It’s much easier with an oxyacetylene torch. In the combustion chamber, the match represents a low-amperage ignition, while the torch represents a high-amperage system.

Now that we have a grasp on basic combustion, let’s move upstream to the ignition system. There are two basic types of ignitions. The most common is called inductive discharge, while more performance-oriented ignitions are designed as capacitive-discharge (CD) systems. Let’s take a closer look at both and compare performance characteristics.

Inductive Discharge

Inductive-discharge ignitions date back to the earliest points-type ignition systems. In its simplest form, this system uses a set of points to trigger a coil. All coils operate on the same basic principal with a small number of primary windings outside a large number of secondary windings wrapped around a metal core. The positive side of the coil is connected to battery power while the negative side is connected to the distributor points and eventually to ground. The high-tension tower lead directs the output of the coil to the distributor and eventually to the spark plugs. Since points cannot handle a full 12-14 volts, a ballast resistor is used to reduce operating voltage to 6-8 volts and to limit the amount of current passing through the points.

There are generally about 100 times the number of secondary windings to the primary windings in the coil called the turns ratio. When the points are closed, current flows through the coil, charging the primary windings. The length of time that the points are closed is called dwell time. When the points open, the current flow is interrupted and the magnetic field created by the current in the primary windings collapses across the secondary windings, creating a much higher voltage, but at a reduced amperage. Generally, a stock points-type coil can create as much as 25,000 to 30,000 volts. Even though these coils are capable of this, high-voltage street engines generally only require about 8,000 to 12,000 volts to ignite the mixture.

The advantage of an inductive ignition system is its simplicity, requiring only a few easy-to-build components. Inductive-discharge ignitions also deliver a relatively long-duration spark, which is especially good for lighting lean mixtures. The limitation of a points-switched inductive system is that the primary side of the system (the points) suffers from low current and voltage. The ballast resistor is used to lower the current to a level that the points can handle without excessive arcing. When emissions requirements in the ’70s dictated lean air/fuel mixtures that were more difficult to ignite, GM came up with the High Energy Ignition (HEI). The HEI is still an inductive-discharge ignition, but it replaces the points with a magnetic pickup and uses a module to regulate the amount of current on the primary side of the ignition—without the need for a ballast resistor. Think of an HEI as an electronic version of a points distributor with the coil built into the top of the distributor cap.

Time is what handcuffs inductive- discharge ignitions. This system requires dwell time to “charge” the coil up to its maximum capacity. Think of the inductive system as having to slowly pour water into a glass until it’s full, then dumping the water out when the spark is required. As rpm increases, there is less time to saturate the coil (fill the glass), which reduces the voltage and current output (the same as reducing the amount of water in the glass). GM created the HEI as a low-engine-speed emissions-style distributor, so early HEIs were not designed to generate high voltage and current at high rpm. Therefore, the HEI gained a reputation for “laying down” at over 5,000 rpm. High-perf HEI modules offer dramatic amperage increases over earlier systems and can now deliver a hot spark through 7,000 rpm.

Capacitive Discharge (CD)

A CD system is designed to overcome the high-rpm limitations of an inductive-discharge system. In a CD system, alternator voltage feeds a high-voltage power supply (varying by manufacturer from 450 to 550 volts) connected to a discharge capacitor. When signaled by the switching device in the distributor, the capacitor applies the 450 to 550 volts to the positive (primary) side of the coil. The coil transforms this voltage into as much as 30,000 volts, which is applied to the spark plug. Think of the CD ignition as having the capacity to instantly fill the glass with water before it is dumped out.

A CD spark is much different from an inductive discharge spark. The arc generated at the plug by a CD system is extremely short in duration, limiting the amount of energy that the spark can deliver. But because a CD system can recharge the capacitor very quickly, it has the capability to deliver multiple sparks at low engine speeds. A multiple-spark CD system like an MSD, Crane, Jacobs, or Mallory can fire a spark plug as many as 8 to 12 times per combustion cycle at idle. Unfortunately, as engine speed increases there is less time for these multiple strikes. By 3,000 rpm, all multiple-strike systems revert back to single-spark ignitions.

Pros And Cons

The advantage a CD has over an inductive-discharge ignition is that the CD system can produce strong spark energy all the way up to 10,000 rpm. Given the design limitations of an inductive system, this is difficult for an inductive ignition to perform. However, at conservative street-driven engine speeds, inductive systems deliver a very long duration spark that helps driveability. CD systems compensate for a short-duration spark with multiple ignition strikes to effectively extend the spark duration.

One way to improve combustion efficiency with any engine is to increase the spark-plug gap. This requires a higher voltage spark, which can do a better job of igniting the mixture, especially at idle. This comes at the cost of greater demands on the ignition system. Performance inductive and CD systems can handle larger plug gaps of up to 0.045 to 0.055 inch, but this demands excellent performance from the entire secondary side of the ignition system (coil wire, cap, rotor, and plug wires) because of the higher voltage required to jump the wider gap. Typically, this also increases the amperage demands on the ignition system as well.

CD ignitions are virtually a necessity when building supercharged, turbocharged, or nitrous-oxide–injected engines. These engines create tremendous cylinder pressures that increase the resistance that the ignition faces when lighting the spark. Generally, higher cylinder pressures require more voltage to initiate the spark. Violent misfires under maximum, high-rpm load in these situations are often caused by an inadequate ignition system that cannot fire the spark plug because of the resistance caused by the increased cylinder pressure.

The disadvantages to CD systems are that they are more complex, require more components, and can be more expensive. CD ignitions also take up more space. These are the main reasons why OEM manufacturers continue to use inductive ignitions. One solution to the problem of energizing the coil for an inductive system is to create a distributorless, coil-on-plug ignition system such as the one used on the current-generation LS1 engine. There has been a significant growth in the number of both inductive and CD aftermarket ignition systems from which to choose. The most popular street CD ignition is the MSD-6A. A few years ago, Crane jumped into the ignition market with a powerful, multi-strike CD ignition system called the HI-6. Other companies offering CD systems include the 300+ CD from ACCEL, Holley’s Annihilator lineup, Mallory’s several versions of the Hy-Fire VI, Jacobs new FC-1000 box, and others.

Of course, there are plenty of high-performance inductive ignitions as well. The stock HEI distributor can be modified into an excellent street ignition system with the addition of a matched combination of a high-performance module and coil. Companies like ACCEL, Performance Distributors, Pertronix, Moroso, and others offer HEI ignitions that work very well. ACCEL, MSD, Crane, Jacobs, and others also offer add-on boxes that will enhance an inductive-discharge ignition. The best way to improve an inductive system is to get rid of those ancient points and move up to an electronic distributor. Recently, a couple of companies have added to the CD stockpile by replacing analog switching devices with digital technology. These include the new MSD Digital-6 Plus and the Mallory digital multi-strike HyFire VI. Of course, as with any digital device, this demands the use of spiral-wound spark-plug wires since solid-core wires will create enough electromagnetic interference (EMI) to shut down even the best digital devices.

We’ve tried to keep this ignition primer simple to give you an idea of the differences between the inductive-discharge and the capacitive-discharge systems. Both ignitions offer certain advantages. Understanding the differences makes it much easier to select the right ignition system for your needs.


Crane Cams
Daytona Beach, FL 32117
Performance Distributors
Memphis, TN 38132
Holley Performance Products
Bowling Green, KY 42101
Jacobs Electronics Inc.
Midland, TX 79701

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