Ask Away with Jeff Smith: Capacitive Discharge vs. Inductive Ignition

My buddies and I got into a discussion the other day about why all the new car companies went with individual coils for each cylinder as opposed to sticking with a simpler distributor. And if a CD (capacitive discharge) ignition is so great, why isn’t that on new cars?

D. B.

Jeff Smith: Let’s start with the last one, which will lead us directly into a discussion of CD vs inductive ignition. Let’s first state that while I’m not an electrical engineer, it appears that an inductive ignition offers many more advantages for a typical production car that operates for a majority of its life at low engine speeds–let’s say below 5,000 rpm. An inductive ignition employs a coil and a trigger device which a long time ago used to be a points distributor. When the points were closed, that connected the charging system to the primary side of the coil and charged the coil. When the points opened, this interrupted the flow of current into the primary windings in the coil, which then collapsed across the much higher number of windings in the secondary side. This is how a step-up transformer works. This pumps the voltage up from a points system with six or seven volts into the primary side to more like 30,000 volts.

This ignition system has been around since the 1910 when first developed by engineer Charles Kettering. Engineers have made some significant changes since then with changes to electronic control within a distributor and then again with complete computer control over this system. But the inductive system still works. The advantage of an inductive ignition is it is brutally simple and it provides significant electrical power to light the spark plugs. Its limitation was that a single coil had to fire all eight spark plugs on a V8 engine. At engine speeds above 6,000 rpm, there is very little time to recharge the coil and as a result, ignition power drops off dramatically. This was based on a given amount of dwell time–the amount of time when the points were closed to charge the coil. Attempts to increase the dwell time created new products like dual point distributors and later GM’s HEI ignition that used electronics to vary the dwell time.

The ultimate advantage with full electronic control of the ignition system was to eliminate the distributor altogether and use a crank and cam sensor to tell the computer when Number One cylinder was nearing TDC during the compression stroke so the computer would know when to fire the spark plug. Then the OEs decided that adding an individual coil to each cylinder would allow them to completely saturate the coil to fire it with a full voltage charge every time the spark plug needed to fire even at higher engine speeds of 6,500 to 7,000 rpm. This means that the coil has much more time to recharge at these higher engine speeds because it only fires once every 720 degrees of engine rotation where with older single coil systems, the coil had to fire eight times.

A CD ignition system works a little differently by employing a capacitor to store something like 440 volts of electrical energy. This high voltage energizes the primary side of the coil. As you can imagine, it doesn’t take very long for the primary side of the coil to completely energize. With the primary side energized, the connection is broken and the voltage dissipates across the coil’s secondary windings and a much higher potential voltage is created. The advantage to a CD ignition is this higher secondary potential voltage can jump across the open gap of a spark plug under high pressure–as with high boost supercharged or turbocharged applications. The disadvantage of a CD is that the spark duration is much shorter. That was the marketing genius of MSD’s ignition program when that company started back in the 1970’s. MSD stands for multiple spark discharge. Company founder Jack Priegel said that his ignition could fire three times at speeds below 3,000 rpm to help completely burn the fuel in the cylinder. The reason for this was the CD’s ability to recharge the primary side of the coil very quickly. The disadvantage of the CD ignition is that the spark duration is not nearly as long as an inductive. There are plenty of arguments on both sides of the table as to the advantages and disadvantages of each ignition system.

As an interesting side note to this argument, PerTronix created an ignition box call the Second Strike that combines an inductive ignition initial spark with a second capacitive discharge strike that can be adjusted with what PerTronix calls its crank angle offset (CAO) feature that is available throughout the entire rpm range. I’ve never tested this system, but it does seem that it might allow an astute engine builder the ability to tune either an EFI or even a carbureted engine to run at much leaner air-fuel ratios at part throttle while minimizing misfire with the second strike potentially recovering what might be an otherwise complete misfire.

I think the reason that the OEs chose to retain the inductive ignition is because of its long spark duration. By using individual coils, this eliminates the problem of a weak high-speed spark and now with computer control, spark timing accuracy has to be far better than all that distributor monkey motion. I think there might be some power to be found by converting an engine like the small-block Chevy over to an individual coil type ignition system. A company called EFI Connection sells all the parts to do this conversion. It’s expensive, but it does create a more accurate spark timing by eliminating the distributor. You will still need a basic distributor body to drive the oil pump, but that’s all it would need to do.