Fire in the hole

Jan. 1, 2020
Viewing the secondary ignition pattern is a great example of figuring out what's different between two photos.

Remember when you were a kid, and your parents took you for breakfast (or lunch, or dinner) at the local diner-style restaurant? Kids got a special menu, with the food choices on one side and lots of cool games on the other. One that I still remember to this day was the one that had two pictures, nearly identical, and the object of the game was to study the one and then find the differences in the second.

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Troubleshooting an automobile is very similar. The more you know about how a system operates, the easier it becomes to identify what’s wrong when it shows up in your bay broken.

Viewing the secondary ignition pattern is a great example of that. In learning what the picture is supposed to look like, you gain an appreciation for just how tough a job the ignition system has. And when looking at the broken picture, you can apply what you learned to determine what’s making its job harder.

In the Beginning Let’s start looking at the picture just as the piston nears the bottom of its intake stroke. The injector added fuel behind the intake valve nearly 180° earlier, before the intake valve even opened. Now it has begun to close and the piston begins its ascension to Top Dead Center (TDC) of its compression stroke. Cylinder pressure rises slowly at first and then accelerates as the piston passes the halfway point.

Crankcase pressures are now almost at their peak. Increasing the cylinder pressure has also increased the temperature of the air (and the fuel it contains) to a point that should make it a little easier to ignite. Just before the cylinder reaches TDC, the Engine Control Module (ECM) issues the command to fire off the spark plug.

But wait a minute. There’s a huge gap between the plug’s center electrode (the one connected to the coil side of the circuit) and its ground electrode (the one we need to get to in order to have a complete electrical circuit). If I put that kind of gap into any other electrical circuit on the car, the circuit won’t work. Isn’t that an open?
CDI Ignition

High performance engines using distributor ignition systems with a single coil often need a lot more to keep the fires burning than the original systems can supply. Output for these older designs was already low in comparison to modern systems, and the addition of higher rpms limits the amount of time the coil primary can be saturated, further reducing its potential output. The answer? Capacitive discharge ignition, like the popular systems sold by MSD Performance.

“The biggest advantage to a CD ignition, is that the spark is always at full output and ready. On an inductive ignition, the coil must step up the voltage from battery supply to thousands of volts. Plus, it must store that voltage for when it is triggered. Sometimes, at higher rpm, there simply isn’t enough time to charge up to full output capacity and at that point, the output suffers which could result in a misfire,” says MSD’s Todd Ryden.A CD ignition system takes voltage straight from the battery and steps it up (using a transformer) to upwards of 500 volts. This energy is stored in a capacitor contained in the ignition control unit that dumps it all into the coil primary when triggered by the distributor. This produces a secondary voltage in the range of 50,000 volts and will do so regardless of the rpm of the engine because coil saturation is nearly instantaneous. The problem with C
D ignition is the spark has a very short duration. To allow for this, modern CD ignition systems provide for multiple sparks (as many as six) through about 3000 rpm for every combustion event. This actually provides a benefit by aiding in initial start-up and improving idle quality.

CD ignitions are used on applications ranging from the weekend warrior’s bracket racer to NASCAR Nationwide series cars. For more information on CD ignitions, check out the technical resources available at

We’ve all seen a spark plug fire outside of the combustion chamber. Did you know that it takes 80 volts of electromotive force to get a spark to cross a gap of just 0.001 inch (at atmospheric pressure)? For that little spark plug gap of 0.040 inch, we needed more than 3,000 volts from the coil. This is the voltage needed to prepare the plug gap for electron flow. Before the electrons can pass from one electrode to the other, the path in between has to be prepared, a process called ionization. Ions are atoms that have too few (or too many) electrons and this ion trail provides the electrical path we need to form a complete circuit.

Do you think that checking for good spark by grounding a plug to the engine is a valid test of the ignition system?

Of course not. There are other factors at play when the plug is installed. The gap is under a lot more pressure, requiring even more initial voltage to ionize. On the other hand, the addition of fuel to the air in the gap increases its ability to conduct and reduces the voltage demand.

Do you think you could use firing kV as a relative compression testing tool? Sure, lower cylinder pressures will cause lower demand. However, if you’ve ever watched a firing kV line on a scope, you’ll see that it varies as the engine runs and there are other relative compression testing methods that are more stable and easier to interpret so it wouldn’t be my first choice.

With that said, though, monitoring firing kV can provide clues on weak running cylinders. Look for the consistently low as a clue to a cylinder that isn’t producing the pressure it should but keep in mind it doesn’t have to be a sealing issue. What impact do you think there would be on firing kV if a cam lobe was worn?

What about as an indication of the air/fuel mixture in the cylinder? Lean mixtures would require more firing kV and rich mixtures less. Firing kV can be used as an initial clue to a mixture issue.

It Makes Sense

The burn line, and what occurs just before and just after, can tell us a lot about what is happening in the combustion chamber when we look at the pattern on our scope. On some models, the ECM is looking at the same information and making decisions based on it.

Ion sensing ignition systems have been in use for years by some manufacturers for knock and misfire detection. It is more sensitive than conventional knock sensors and eliminates the need for the additional sensor(s). Ion sensing misfire detection is more stable and more accurate than the more common method of monitoring crankshaft speed fluctuations.

And there are more possibilities considering the advances in technology and computing power.  According to Lars Eriksson (who wrote “Spark-Advance Control by Ion-Sensing and Interpretation”), “The spark plug can, using signal interpretation, function as a sensor for several parameters.” 

Along with knock and misfire detection, this method could allow the spark plug to provide individual cylinder information to the ECM on fuel trim and peak cylinder pressure position allowing the ECM to constantly adjust fuel and ignition timing on a per cylinder basis.  This would improve overall engine efficiency and even give techs more diagnostic detail when dealing with engine drivability complaints.

Do you think a high resistance plug or plug wire would affect firing kV? Not so much. High resistance wires are certainly more easily overcome than a 0.040-inch hole, so any impact on the firing kV demand would be negligible.

Gap Is Ready, Now What?
The next element to look at in the ignition pattern is the point at which the spark is actually initiated. This is spark kV, and is the amount of voltage needed to overcome the resistance in the electrical path the spark is going to follow. This is a measure of resistance, so think about what factors will impact resistance. What comes to mind?

Will cylinder pressure impact resistance?  No, so spark kV is of no additional help when considering factors affecting cylinder pressure while running.

How about fuel mixture? Absolutely! And by looking at both firing kV and spark kV, you can gain clues at the mixture present in an individual cylinder. As mentioned, lean mixtures increase resistance (and increase spark kV) while rich mixtures will decrease resistance (and lower spark kV).

How about resistance in the plug wire? Yes, high resistance in the plug wires can be seen here. So can high resistance in the spark plug itself. You did know that there is an internal resistor in the spark plug that can fail, right? If not, now you do and your known good picture just got a little better.

Now we move on to the burn line. Remember the spark you saw fly across the gap? That’s what you’re looking at here. This event is taking less than two milliseconds to complete but what happens to that line in that time can tell you a lot about what is happening inside the engine. A burn line that slopes gently upward is an indication of a lean condition, while one that slopes down can be indicative of a rich condition. Think of it as a continuing representation of the resistance of whatever is in the gap.
Sometimes you’ll see the burn line act erratically, moving up and down. This is not uncommon when the engine speed is higher and turbulence swirls around inside the combustion chamber. At idle, however, it can clue you in to a cylinder that isn’t sealing as it should, typically caused by valves with excessive carbon build-up, sticking valves, or valves that are slightly bent.

Putting It All Together
Studying static scope pictures is not the best way to view ignition patterns. Watch for anomalies that stand out while you watch a running pattern. You can further distinguish between mixture/pressure issues by watching for changes at 2,500 rpm versus idle, and the reactions in the pattern when you snap the throttle to wide open.

If you aren’t sure if the problem is related to something in the cylinder (mixture/pressure) or outside of the cylinder (bad wire/plug), try moving the wire to a spark tester. If the same abnormal ticks are visible, focus on the outside elements. If not, you know it’s inside.

Often, it’s easier to backprobe the ignition coil’s groundside control to get an ignition pattern. Spark kV and burn time will be mirrored in the primary pattern but firing kV will not. Be sure you use a required attenuation to prevent damage to your scope by exceeding the input voltage limits.

Last, don’t memorize any scope pattern. Strive to consider what is happening, what the picture is supposed to look like versus the picture you’re viewing on the scope’s screen. A side benefit? When you take your kids to the local diner-style restaurant, you’ll kick their butts at the find what’s wrong in this picture game.

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