Spark Talk

High performance is all about horsepower. But even the ultimate in heads, compression, cam, and the rest won’t make power if you can’t light the fire. Ignition systems don’t make horsepower, but a weak ignition can cost you power if it can’t get the combustion process started. There are plenty of killer ignition systems that could probably weld sheetmetal, but those monster capacitive-discharge (CD) systems aren’t necessary for a typical street engine. Last month we looked into the differences between inductive-discharge and CD ignition systems. This month, we thought we’d investigate the advantages of high-performance inductive systems like the High Energy Ignition (HEI).

General Motors engineers designed the original HEI system to replace the points ignition that had been around for decades. In the mid-’70s, leaner mixtures for emission reasons demanded greater voltage and more spark energy to initiate combustion. An HEI distributor is still an inductive-discharge ignition, but it exchanges the points for a solid-state electronic switching device called a module. Since this system produces more voltage and amperage output, it demanded a larger-diameter cap to prevent voltage crossfire inside the distributor cap. This larger cap also offered space to position the coil, making the HEI distributor self-contained. All you have to do to run this system is to apply a straight 12 volts to the distributor and you’re ready to run.

Early HEIs gained a reputation for giving up above 5,000 rpm, so many enthusiasts avoided them. This was true of the early-model distributors, but it didn’t take GM long to modify the modules and coils to offer greater spark energy at higher engine speeds. Today, there are several aftermarket high- performance modules and coils that can be added to a stock GM HEI distributor to give it excellent spark energy and rpm potential up to 7,000 rpm.

Module Power

The combination of the right module and coil are the key elements to a high-performance HEI distributor. Since charging-system voltage is applied directly to the distributor, the module acts as a current limiter on the primary side of the ignition system. One of the misunderstood aspects of points ignition systems is that the ballast resistor (or resistor wire as used in most GM points-type ignitions) just limits voltage. While voltage is reduced, the resistor’s main function is to reduce current, since points cannot handle more than about 2 amps of primary current without drastically reducing operating life. With an electronic module, an HEI can safely operate at much higher current loads. More power applied across the primary circuit of the coil means the coil can apply greater spark energy to the spark plugs. This additional power ensures more complete combustion at both idle and at wide open throttle.

Increased horsepower and torque are a direct result of higher cylinder pressures. However, these higher pressures require more spark voltage and current in order to ensure complete combustion. The beauty of an HEI is that even stock, it applies much more spark energy than a points-type ignition. Generally, stock HEI modules are current-limited to about 4 amps. Better GM performance modules increase this current to around 5.5 amps. Some of the more high-performance aftermarket modules can draw as much as 7.5 to 8 amps of power. The problem with higher amperage is heat. The heat can eventually build up inside the distributor, melt the internal circuits, and destroy the module. This is especially true at idle since the energy remains in the module (since little is needed at idle), which adds to the heat problem.

The key to reducing heat buildup in the module is to use the special white grease supplied with new modules on the mounting side of the module. This grease creates a better heat transfer path between the module and the distributor body so that the module tends to run cooler. According to ACCEL, you should not use the clear dielectric grease on these modules (like MSD’s Spark Guard) since this is actually an insulator type of grease that will act as a heat transfer barrier, which could lead to trouble.

Another function the module performs is electronic control of the dwell circuit. The key adjustment for points was to measure the dwell time, which was the length of time, in degrees, that the points were closed. Generally, this was set at 30 to 32 degrees. Dwell time is the amount of time the primary circuit is complete (points closed). Long dwell times are used with inductive ignitions to fully charge, or saturate, the coil. This is especially important at higher engine speeds because there is less time to charge the coil. This was why dual-point distributors were popular since they extended the effective dwell time by closing a second set of points. The HEI performs this dwell time electronically and can increase the dwell time at high engine speeds to effectively improve a coil’s performance by ensuring adequate coil saturation.

Coil Power

While a good performance module will make a big difference in a stock HEI, matching it with a performance coil allows the system to work as efficiently as possible. In fact, all the aftermarket ignition companies sell matched coil and module sets to ensure optimal ignition operation. Coil design is very much a dark art and would require a textbook-sized story to detail all the different variations on the theme. Suffice it to say, by decreasing internal coil resistance, it is possible to decrease the amount of time necessary to charge the coil so it can fire the next spark plug with maximum energy.

This is an important factor since all the ignition companies we spoke to stressed the idea of properly matching the coil and module. The best way to do this is to use the coil specified by the manufacturer for its specific module. For example, ACCEL offers a stock replacement and two different performance modules for the typical four-pin HEI. Each requires its own coil to create optimal ignition power. What this means is that you should not mix and match coils and modules. In one particular situation, we combined a stock replacement module with a Pertronix coil and the engine just seemed to run flat—as if the ignition timing were retarded, even though it wasn’t. As soon as we replaced the stock module with the matched Pertronix module, the engine instantly responded and was again crisp and fun to drive.

Later, we tried to duplicate this coil and module mismatch in a different vehicle, but we did not see the same results. We also tried several mismatches of coils and modules with no apparent differences in driveability, idle emissions, or throttle response. However, it’s clear that the best plan for optimal ignition performance would be to use the factory-matched module and coil.

What we did find when testing several aftermarket ignitions was that the greater spark energy at idle allowed us to open up the spark plug gap from 0.035-inch to 0.045-inch. While this demands more voltage from the ignition, it also creates a fatter spark in the combustion chamber. Combining this with a performance HEI ignition, we were able to lean out the idle mixture settings without suffering from lean misfire. Using a Sun emissions test machine measuring hydrocarbons (HC) and carbon monoxide (CO) we were able to reduce the emissions using a high-performance HEI module and coil compared to a stock combination. Overall, we were able to reduce HC by 20 percent and CO by almost 30 percent with a stronger ignition system.

While high horsepower supercharged and nitrous-equipped engines can always benefit from those mega-power CD ignitions, most street engines can employ a quality, high-performance HEI ignition without fear of losing power to a weak ignition. Combined with a well-designed spark curve, quality spiral-wrapped plug wires, and the proper heat-range spark plugs, it’s possible to ignite up to 500 reliable horsepower with little more than a hopped-up HEI distributor. That leaves you money left over to spend on those cylinder heads you’ve had your eye on.