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Ask Anything - April 2014

Inductive Ignition

Richie Green; via Great article on the HEI conversion (Junkyard Builder, Nov. '13). I thought those modules were no good above 5,500 rpm because they couldn't keep up. Is this true?

Jeff Smith: I think this may be a new record for the shortest tech question to date! As usual, in the best tradition of Ask Anything, your short question requires a more detailed answer. I will attempt to be brief. You are mostly correct that the HEI has trouble above 5,500 or 6,000 rpm, but not because of the module. The answer has less to do with the module and more to do with single-coil inductive ignition systems, in general. Let's start, as always, with some electrifying basics. The HEI (and all factory ignition systems) are designed as inductive ignitions where system voltage (14 volts) is fed to the primary side of the coil. An inductive ignition coil uses a primary side with a given number of windings wrapped around a metal core. When the distributor module completes the circuit (or the points close, if you remember ancient automotive history), this feeds voltage and current to the primary windings. When the module disconnects (opens) the circuit, the magnetic field energy created around the primary windings collapses across the secondary (high tension) side of the coil that is wrapped with 100 times the number of windings. These windings are connected to the high-tension lead for the coil wire. As the primary field collapses, the greater number of secondary windings increases, or steps up the voltage. This is how a basic step-up transformer works. This increases the voltage from 14 volts to between 20,000- and 40,000-plus volts. This has also been referred to as a Kettering ignition system, developed by Charles Kettering and first used in Cadillacs in the very early 1900s.

This system works the best when there is sufficient time to allow the primary coil windings to fully saturate, which occurs with no problem at 3,000 rpm. But at 6,000 rpm, there is only half the time and primary coil winding saturation degrades. This consequently reduces both the voltage and spark energy available at the spark plug. In the '60s, the idea behind dual points was to increase the dwell time (the amount of time the points were closed) to increase coil saturation. But that was a Band-Aid applied to the real problem. One solution was the capacitive discharge (CD) ignition. A CD uses a charged capacitor to hit the primary side of the coil with around 500 volts, which radically increases the secondary voltage—and it does all of this very quickly. This is why MSD is able to fire the spark plug three times (below 3,000 rpm) because the capacitor can recharge the coil so quickly. It was this feature that created the brand name Multiple Spark Discharge, MSD. The disadvantage to a CD ignition is extremely short spark duration. But it is also capable of delivering high voltage and good spark energy at extremely high engine speeds of 8,000 to 10,000 rpm. The advantage for an inductive ignition and why it is used in all production engines is that it creates a very long duration spark, which helps ensure the fire is lit in the chamber at low speeds.

So this leads us back to the HEI and any single-coil inductive ignition system's weak point, the single coil. Unless we hit the coil with higher voltage, it requires lots of time to recharge the primary windings, even with a full 14 volts. The old points systems were even worse because they could only deliver barely 6 volts to the coil. Any more would quickly burn the points to dust. The HEI delivers better spark energy than points, but it still suffers from the same coil saturation dilemma, which results in reduced ignition power above 6,000 to efficiently help make more power. When GM created the HEI, they weren't concerned with high-rpm power. Remember that the HEI was created during the '70s when emissions requirements demanded high voltage to ignite lean air/fuel ratios. When we say power, we don't just mean voltage. Lots of coils are capable to 40,000 volts, but if the spark plug only requires 25,000 volts to ionize the spark-plug gap and push the electrons across that gap, then that's all the voltage the coil will deliver. Many feel that the spark energy delivered across the spark-plug gap is more important to helping complete combustion. Here is where the inductive ignition system has a slight advantage with its longer spark duration compared to a CD. The OEMs have solved the coil saturation problem by using a dedicated coil for each cylinder. On a V8 engine, each coil now has eight-fold more time to saturate before it fires again versus a single-coil ignition system. I suspect that the main reason that the new car companies went with distributor-less ignition system (DIS) is because the coil-near-plug design reduces emissions because of higher spark-plug energy and improved timing accuracy. This improved ignition performance results in fewer trace misfires, so power improves and emissions—especially unburned hydrocarbons (HC)—are reduced. So now we have an inductive ignition system that is capable of running as high as 8,000 rpm or perhaps higher with very little appreciative loss in spark energy.

Other problems with the HEI can be traced to misfires that often occur due to faulty modules and grounding spark through the rotor. We've outlined several simple HEI improvements in previous Car Craft stories that can be found online, so we won't go into them here, but starting with a high-quality cap and rotor are essential steps. Always choose a cap with real brass contacts instead of the cheaper aluminum spark plug wire contacts. Aluminum is a poor conductor of electricity because it is subject to oxidation, which increases resistance. Copper has a 55-percent-higher electrical conductivity than aluminum, making it a far better choice for connections on the high-tension side of the ignition system. Then invest in a really high-quality set of spark-plug wires like MSD or Moroso, and you're most of the way to optimizing an HEI distributor.

Converting Hot to Cold Lash Clearance
(Chart courtesy of Crane Cams)

Iron block Iron heads Add 0.002 inch
Iron block Aluminum heads Subtract 0.006 inch
Aluminum block Aluminum heads Subtract 0.012 inch

More Info

Autotronic Controls Corp. (MSD); 915/857-5200;
Moroso Performance Products; 800/544-8894;

CC Quickies

We saw these 6.4L Hemi heads (aka Apache) at Superior Performance in Placentia, CA. Superior owner Joe Jill claimed that they will flow more than 400 cfm at 0.650 lift with the right valve job.

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