Classic Inlines
603 W Pecos Ave
Mesa, AZ 85210

HEI Tech

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 sheet metal, but those monster capacitive-discharge (CD) systems aren’t necessary for a typical street engine. We thought we’d investigate the advantages of a high-performance inductive system, like the High Energy Ignition (HEI) system.

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 HEI's 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 Petronix 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 Petronix 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.

HEI modules, even most all of the so called "lower quality" ones, are not problem prone as a rule, they are fairly bulletproof, actually. They do adversely react to other components that have issues, though, HEI and other modules such as MSD, Crane, etc.

Peterson Publishing (Hot Rod or Car Craft, don't remember just which one) did a test of three modules a few years ago, a high end "racing module " from a major manufacturer, a standard replacement module from a local NAPA store, and one of them white "junkers" from offshore. The difference was less than one quarter of a percent from the "junker" to the custom module, which cost four times as much.

The single most cause of HEI module failure is coil related. Number 1 is the in-cap coil "layer shorting" When this issue arrises, the epoxy coil has overheated (common for those coils due to the epoxy being almost the worst heat transfer material known to man), and has the primary windings insulation compromised and/or just plain burned away from some or more of the windings. This allows the windings to come together, short circuiting the length they have to be to operate correctly. Resistance is changed for the adverse, and the coil just plain overworks the module to failure.

It isn't enough any more to simply resistance test an in-cap, or any other coil. Real world, powered up tests are the new way to get it done correctly. Stores such as Auto-Zone have special testers that run the coil and test it off the vehicle, and give accurate feedback as to the coil's real condition/health. Do not just rely on "ohm'ing" a coil, get it run/load tested.

Next, two different scenarios, both as deadly as the other.

A) The carbon brush is compromised, and/or installed incorrectly. The correct sequence is: cap, carbon brush with spring positioned upwards, then silicone heat transfer grease on both sides of the insulator, installed onto the spring, then, coil. If this method isn't done correctly, the distance from the end of the carbon brush to the rotor connector bar becomes excessive, resistance on the secondary side and primary side of the coil rises, heat becomes too high, and the module fails.

B) The ground strap that grounds the coil from one of the coil holddown screws to the center terminal in the cap either is dirty and/or corroded. This makes for added resistance, and that is...module heat. Failure will occur. Contrary to what some will say, if this bar isn't in place with an in-cap coil, the coil WILL NOT MAKE SPARK. This bar HAS to be in place for a coil in cap coil for it to make spark, Doesn't have to be there for a remote, off cap coil changeover.

B-2) The small black wire that comes out of the coil on the in-cap coils is the second part of the coil ground, and has to be connected to the coil yoke to work. This wire grounds through the coil yoke, to the ground strap, three prong connector on the cap and ground wire, to the body at the module mounting area. If this wire isn't connected, no spark.

Spark plug wires. An HEI requires a wire set that has these features, magnetic suppression, spiral, or "magna" core, and be of sufficient diameter to allow the spiral core to effectively shield the voltage running through the wire. That said, steel and copper wire aren't shielded as a rule, and shouldn't be used with any good HEI or better electronic ignition system. The reason we need to run this specialty wire type is that wires will leak if given the chance, and when that happens, the leakage, refered to as 'RF leakage" (radio frequency) and can cause wires to magnetically feed voltage to more than one spark plug at once. Feeding more than one plug at any one time overworks the module and coil, and failure can result, along with misfire, early/late fire and other tuining/performace issues/problems.

Contrary to what some will also say, low voltage, as in still using the resistor or a resistor wire, will not kill an HEI module,. It will make the overall spark weaker exponentially as volts are reduced. After a certain level of volt input lowering, the module will simply stop working from lack of volts, and the system will simply not idle. Once again, low volts will not harm an HEI module, it'll just stop malking spark when it gets to the point it can no longer run on what isn't there. Add volts, it will come back alive again.

HEI modules need two things to be happy, a good ground and insulating grease to stay as cool as possible.

Grounding is accomplished with the screws on some modules, others ground from their bottom pads. Some modules will have a steel ring at one of the holddown screw bosses on the module top, THIS is the ground for those type modules. Modules that don't have the ring ground through their metal back plates.

Excessive heat can also destroy modules. The grease used on the underside of an HEI module is special silicone based grease designed to transfer heat from one surace to another. IT IS NOT DI-ELECTRIC GREASE. Di-electric grease is used for other things electrical, and shouldn't be used on module to body interfaces. Also, the silicone greases that we need for modules are HEAT TRANSFER TYPES, NOT HEAT BARRIER TYPES. Obvioulsy, barrier greases would retan heat at the module source, not help to cool them.

Electrical connections at the module terminals need to be kept clean, and tight. Corrosion isn't wanted there, nor are loose connects. That kind of issue causes resistance, which is heat, which is death to the modules. These terminal connects ARE the correct places for di-electric greases to be used.

Caps/rotors can also make for excessive resistance, and can also lead to module failure. We all have heard of HEI rotors with burn thorugh under the contact springs. It happens, and when it does, resistance and heat will kill the modules, and coils as well, if a person has a great deal of luck. And, as ozone buildup accumulates on the rotor tip and wire terminals, resistance is raised, usually, this isn't a serius issue, but a copper terminal cap is mandatory for good cap health. Avoid aluminum terminal caps.

Rotor phasing isn't usually an issue with common HEI's. One "performance" TV show a couple of years ago, showed an "expert" checking the phasing of the rotor to the cap terminals with a cap that had a hole drilled into it so a timing light could be used to view the rotor to terminal orientation. Only issue was, the vacuum advance wasn't in operation.

Here's how that all works.

Mechanical advance will keep the rotor in the phase it was left at in relation to the pickup. The rotor end will not change position in relation to a cap terminal on mechanical curve operation. The magnetic pickup isn't moved, so the phasing status remians where it was left.

If the rotor firing tip was left centered on the wire terminals inside the cap, it will remain centered with mechanical advance operation, period.

HOWEVER, VACUUM ADVANCE does move the magnetic pickup through a number of distributor degrees of rotation. When the pickup is moved, so is the firing end tip of the rotor. This movement is split between no advance and fully advanced locked out of the vacuum cannister. Therefore, a rotor in a vacuum advance situation is "parked" on both ends of the vacuum advance curve the same distance from centered on both ends of the vacuum advance curve. In this scenario, the rotor tip is only centered when the vacuum advance is at HALF ITS TRAVEL in distributor degrees, so, it is ONLY phased correctly when the vacuum advance is at its centered position.

It is no wonder why the "expert" on the TV show found the rotor off. He was checkng a vacuum advance distributor with the vacuum advance disconnected, and the pickup parked on the low stop of its travel. So...the rotor would appear misphased. "Fixing it" (re-phasing) really wouldn't get it done if the vacuum advance were to be connected and used again. And, rotor tips/cap terminals today are wider than they used to be, especially in large, coil in cap HEI's, so...misphasing really isn't an issue. Ternminals/rotors in those nightmares are just not prone to having phasing issues, unless someone does a real Bozo booboo, like Pontiac/Olds left rotation parts in a Buick/Cadillac/Chevy right rotation HEI. Caps are made to fit both, as are rotors, but other parts between different rotation aren't.

home ]
Copyright 2012 - Classic Inlines - All Rights Reserved