CC Tech: HEI – GM’s Superior Sparks, At Home and Elsewhere

There’s a sturdy argument to be made that GM’s HEI (High-Energy Ignition) is so damn good because GM were so damn fixated on being so damn cheap. Wait…what? How does that reckon? Simple: it was less costly than anything else they could’ve done.

US automakers made some respectable efforts to harvest the lowest-hanging fruit when serious emissions regulations started taking effect in the late 1960s and early ’70s. Chrysler, having been the first(?) automaker to experiment commercially with electronic ignition—they offered a Motorola transistor ignition system as a fleet-only option on certain 1966 models—went on to be first to market with a –pointless– breakerless electronic ignition system, a homegrown one optional on ’71 V8s and standard on all engines from ’73 on. The main goal was to eliminate breaker points, which begin deteriorating the moment the engine is first cranked. Even with perfect tune, brand-new points and everything, at the tailpipe you’ll hear misfires as chuffs and splutters mixed in with the exhaust note, every one of which is a slug of unburned gasoline—more and more of them as the points wear and burn and the condenser degrades. So a system with less and slower deterioration was an effective emissions reduction strategy. Marketable, too! In fact, Chrysler’s electronic ignition was a little zappier, but not much; coils and caps and rotors and distributors stayed cost-savingly unchanged, and spark plug gaps remained small at 0.035″ (0.89 mm). The main advantage was a more durable state of tune.

In the bigger picture, the American auto industry put forth what might diplomatically be called largely a cynical response to progressively stricter emissions regulation. They resented lawmakers telling them what to do, and behaved as though emission control were a silly passing fad not worth putting much serious thought or effort into. They assured each other in selfgratulatory technical papers that the 1970-model cars’ exhaust was squeaky- (and sufficiently-) clean and no further cleanup effort was warranted. And they applied cheap, nasty, hang-on-and-pray strategies to squeak the cars past their Federal emissions certification tests so they’d be legal to offer for sale. Driveability, performance, fuel economy? Pfft! Those weren’t on the test; they went right down the crapper. Besides: Oh, gee, Mr. and Mrs. Carbuyer, oh gosh, your new car runs poorly and guzzles gasoline? Tsk, what an awful pity. Too bad, so sad, not our fault; you’d best run get busy writing to your congressman about it!

So why, then, did GM devise such an excellent ignition system, better than all the others, convincingly the best of its type in the world? Because it was less expensive than upgrading primitive fuel systems and stone-age combustion chambers. Honda demonstrated the latter by making a set of CVCC cylinder heads for a Chev 350; tables III and V tell just about the whole story. In response, GM carried on scornfully yukking about ‘Japanese beetles’. Meanwhile, the improved combustion resulting from a strong ignition system compensated, at least enough to get the cars past the certification tests, for extensive slack and sin elsewhere. Besides, GM were sort of boxed into a corner: they had to work and spend hard to have a terrific ignition system because their catalytic converter development program was cursed, or maybe it was just riddled with incompetence.

Or perhaps, given the connection between GM and Ethyl Corporation who made most of the tetraethyl lead used in gasoline, the GM catalyst program was meant (nudge, nudge) to serve as an exemplar (wink, wink) of the foolish, costly waste of the very idea of cleaning up auto exhaust emissions in general, and particularly with catalytic exhaust treatment. Along with Ford and Chrysler, GM spent a whole lot of money, effort, and time waging legal war on every aspect of the regulations, from their technical prescriptions to their timetable to EPA’s authority to issue and enforce them. One wonders what they might’ve accomplished if they’d put that effort, money, and time into engineering rather than lobbying and lawyering. Eventually they lost those battles; the 1975 U.S. emission standards were going to be the 1975 emission standards no matter how big a temper tantrum they threw about it nor how hard they dragged their feet.

So GM launched catalytic converters for the 1975 model year, trumpeting that a ’75 GM car running on unleaded cost less to run than a comparable ’74 running on leaded, in improved fuel mileage—because the engine didn’t have to be strangulation-tuned so much to meet the tailpipe standard, with the catalyst cleaning up the trash in the exhaust—and in reduced maintenance, because spark plugs and engine oil last longer with no-lead fuel—so do exhaust systems, but they didn’t talk about that. Click this ’75 Chevelle ad for a readably-large version in a new browser tab:

Now, too much raw fuel hitting the catalyst makes it heat up and melt down. And even under optimal conditions there was going to be a lot of raw fuel in the exhaust; the cars had dirty-burning engines equipped with carburetors capable of only very crude, sloppy control over the fuel/air ratio on their best day—and prone to many different kinds of bad days. So the last line of defence against converter meltdown was a super-strong ignition system that maximised the chance of lighting each and every cylinder, each and every time. That system, after a couple of predecessors, was HEI.

Like the Mopar system and similar aftermarket setups—the Hays Magna-Pulse was a popular one—and other automakers’ electronic systems, HEI eliminated breaker points. It went a big step further, though, in that it materially increased the available spark energy and duration: more volts (with a wider spark plug gap) and more amps over a longer time. That did a better job of lighting even difficult (lean or stratified) cylinder charges.

What makes the difference? The Mopar system and others like it use the positive temperature coefficient of a ballast resistor or resistance wire (resistance increases with temperature) to regulate primary current to the coil with changes in engine speed, just like in a points-condenser system: at low engine speed the power pulses through the primary circuit are relatively long, so the resistor heats up, reducing power to the coil so parts don’t burn. Not much spark is needed to run an engine at low speed and load. At higher speeds, the pulses are short; the resistor runs relatively cooler, increasing power to the coil to better meet the engine’s greater needs. This feedback loop works slowly, though, so the coil is incompletely charged during sharp increases in engine load, such as acceleration.

A GM HEI module does not rely on slow thermal coefficients. It is typically based on an integrated circuit which regulates coil current using an internal variable voltage reference, which responds much faster to changes in duty cycle. This gives a much hotter spark during sudden-onset high-demand conditions (acceleration), and less coil heating during low-rpm operation. The IC’s program includes a 1-millisecond “off” time to ensure complete discharge of the coil field during a spark event.

So what’s all that mean, practically? See for yourself in this comparison of ignition performance, shown on a sillyscope, of Mopar electronic ignition versus GM HEI in a Slant-6 engine. The HEI spark is of longer duration, and of sufficiently high power to support a larger spark plug gap (0.045″/1.1 mm is the gap GM eventually settled on for engines originally equipped with HEI) for higher secondary voltage and better combustion in the cylinders:

There are those who come over all Klingon, bellowing and snarling indignantly about how they’d rather die than pollute their [brand] car with [other brand] parts, implying [brand] always only ever has the finest, most bestest engineering and [other brand]’s stuff is always only ever super-poopy. With all due respect to them, that’s kind of nutty; no automaker manufactures all their own parts, and most suppliers sell to most automakers. More to the point, a thoughtfully-done HEI retrofit is a worthy, cost-effective, not-very-difficult upgrade for engines equipped a compatible distributor—and that’s a whole lot of engines. Chrysler, Ford, and AMC used compatible reluctance triggers in their electronic distributors. So did Bosch—ditch the Bosch box and put HEI in a Volvo or Mercedes or Bimmer or VW, open the plug gaps from 0.028″/0.71 mm to 0.045″/1.1 mm and go for a drive!—as well as others. People have even figured how to trigger an HEI module with breaker points—you still have rubbing block wear, but you get the other big benefits.

The quality of the outcome depends on the amount of sweat applied to the details. Most cars not originally equipped with a powerful ignition system won’t dependably feed one properly through the stock ignition circuit, for example, and the resultant voltage drop can make starting, driveability, and dependability problems; a power relay is an easy, cheap, and wise way to address this. Parts selection is important, too; the market is flooded with poor-quality trinkets and junk, from whole distributors down to individual components. Module mount location matters—it has to be well grounded, well heatsunk, out of any splash zone (that under-distributor mount plate is just right for V8s, but not for Slant-6s!), and reasonably close to the distributor.

And it’s fine and necessary to have a distributor with a compatible trigger, but that’s not the whole scope of compatibility. HEI distributors are of very large diameter compared to others. That was done deliberately; a high-power spark is more likely to jump where it’s not supposed to, and a large-diameter cap means greater distance between where the spark is properly destined and where it’s not. One reason why Chrysler—despite their strong expertise and capability in electronics at the time—focused on durability of tune rather than high ignition power: that way they didn’t have to retool for larger distributors, which wouldn’t have fit some of their engines, anyhow. An HEI retrofit on a non-GM car means living with the constraints of a small-diameter cap not really designed or intended for high-power sparks. And today’s best caps and rotors, as bought off the shelf, are markedly poorer than what used to be available; there’s scant money to be made in obsolete kinds of parts any more. Nevertheless, a top-notch cap, rotor, and plug wires are all very important for durable good performance and dependability in an HEI retrofit. Some do-it-yourself work will pay great dividends, as described at great (and tiresome, unless you care) length in threads like this one, which over the course of six pages identifies the issues, quantifies them, identifies theoretical fixes, and develops them to practicability. Never let it be said that auto hobbyists are lazy, unmotivated, or unimaginative!

TBM3FAN’s account the other day of installing HEI on his ’73 Polara inspired this present article. He mentioned having read an HEI swap how-to article on another site; I wonder if it might’ve been mine—maybe or not; there are many. I’ve kept largely silent about OE systems other than Chrysler’s and GM’s, but everyone had one; Ford foisted eleventy-seven “no, no, wait, we promise this one will work!” versions of their Dura-Spark on hapless buyers, AMC bought bits and parts from here and there and called it BID (Breakerless Ignition Distributor), and so on and on. And aftermarket electronic ignition system retrofits are outside the scope of this article, except to say that an HEI retrofit is practically better than most of them, as well as more dependable and less expensive.

Further reading:

• CC Automotive History: Losing the Points Part I · Part II · Part III

GM’s SAE technical paper on the development and deployment of HEI