This Tech Tip is From the Full Book “SMALL-BLOCK CHEVY PERFORMANCE: 1955-1996“. For a comprehensive guide on this entire subject you can visit this link:
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The ignition system may seem relatively uncomplicated. All you need is a power source and a means of distributing a high voltage (spark) to each cylinder at the proper time. In reality, the problem is far more complex. The wide range of possible operating conditions makes optimum ignition timing and spark delivery vital for overall engine efficiency, especially when emissions and economy are part of the picture.
Selecting an ignition system for your small-block Chevy requires the consideration of several important factors. Since timing is the basis of all engine functions, it is critical that, above all else, the ignition system maintain rock-solid integrity. This was easy enough in the early days when the primary mission of the ignition system was providing smooth engine operation, but now that it has been called upon to help control emissions and to compensate for other less-than-ideal conditions, the standard ignition system has become quite complex.
Chevrolet has offered a variety of Delco-Remy-built ignition components over the years, but two basic types have been used on the vast majority of production vehicles. Each of these systems is suitable for normal high-performance use once they have been correctly tuned. With some minor modifications most of these Delco ignition systems are also suitable for semiprofessional racing.
Prior to 1975, all small-block Chevys used a conventional Delco breaker-points distributor with mechanical- and vacuum-advance systems. This setup used a single set of breaker points as a switching device to control the flow of electrical power to the primary windings of the ignition coil. When the points opened, electrical flow was disrupted and the electromagnetic field created around the primary windings collapsed. The collapse was assisted by a condenser (a storage device for electrical energy, like a small battery), which reduced arcing across the points and ensured the rapid collapse of the magnetic field in the coil. The field collapse generates a burst of high voltage in the secondary windings of the coil. This voltage is routed to the appropriate cylinder through the distributor and spark plug wires.
You can visualize voltage like electrical pressure. The more voltage you have, the more power you can push through a circuit. If your engine is operating under adverse conditions or the ignition system components are not in top shape, voltage requirements for effective ignition can easily double. On the other hand, a welltuned engine running with an optimum air/fuel ratio may be able to begin ignition with only average secondary energy. In these situations, any additional voltage is unused. The high voltage potential of a highenergy ignition system can be considered just that: potential. It is available to fire the plugs under adverse conditions such as lean mixtures, high speeds, and cold-start conditions.
The conventional breaker-point system can be reliable, but it requires maintenance. The common downfall of a breaker-point system is rubbing-block wear and dwellangle variation. The points are opened and closed by a cam on the distributor shaft. It works against a rubbing block located on the breaker points. When this rubbing block eventually wears, ignition characteristics change because the point gap is altered, changing the amount of time the points are open or closed (called dwell time).
In the 1970s, electronic ignition systems and ignition controls were developed to eliminate breaker-point problems altogether and to provide a much hotter spark to work with lean, emissions-calibrated mixtures. To this end, the GM High Energy Ignition (HEI) has been used in all General Motors cars since 1975, and it has proven highly reliable under a wide range of operating conditions. HEI is the logical extension of earlier electronic systems and features a number of improvements and a drawback or two. The major difference in the HEI is the relocation of the coil and control electronics to within the main distributor housing. HEI is designed as a self-contained system, optimized to fire lean fuel mixtures at slow and moderate engine speeds. It’s just the ticket for late-model emissions-calibrated engines, as it requires virtually no maintenance, provides accurate ignition timing, and greatly extends spark plug life. Many knowledgeable ignition people feel that a properly set-up HEI is all the ignition needed for 95 percent of all high-performance applications.
HEI upgrades are available from specialists such as Performance Distributors, and MSD. MSD, in particular, offers a wide variety of specialized components, including a billet HEI distributor (also available from Chevrolet) that provides the best features of an HEI along with the performance and reliability of MSD racing technology.
There are so many types and brands of specialty ignitions available it becomes nearly impossible to sort the good from the also-rans. Almost as soon as there were ignition systems, somebody began figuring out ways to improve them or to make money by claiming new and improved designs. Every ignition system advertisement sings the praises of high energy, and they all claim to have more than the next guy. Unfortunately, there is a lot of junk floating around and much of it will do you little good.
For early model cars, a stock breaker-point ignition provides good performance when it is well maintained. If you decide to add a hot coil and a multispark ignition module, like the potent boxes from MSD, you’re in great shape for a street machine. Still, many people are attracted by the low maintenance requirements of an all-electronic ignition. And now that these systems are available with a broad range of performance capabilities for reasonable cost, electronic ignitions have become a virtual necessity.
This inexorable drive for electronic ignition power and reliability began in the mid 1960s, when the capacitor-discharge system, or CD, came into vogue. They were originally designed to augment a breakerpoint system, but eventually they were combined with breakerless distributors. CDs featured a control box that boosted battery voltage to about 300 volts. This higher voltage was applied to the primary side of the coil when the points opened and that really shocked the coil into producing 30,000 or more volts on the secondary side. This high-energy spark, along with a fast rise time, made these units suitable for racing, provided the distributor and breaker points were able to operate at the desired RPM limit. They had a lot of spark energy, but the spark duration was very short. Normal and rich mixtures were easily lit with a CD, but lean mixtures were another story. There was a chance that no combustable mixture would be near the plug at the precise moment the spark was delivered, and the almost instantaneous nature of the spark limited the possibility for reliable ignition.
The first truly breakerless ignition developed was a magnetic-impulse system. It used a constant magnetic field passing through a pickup coil. A toothed wheel, called the reluctor, moved past the pickup coil and the changes in the magnetic field sent a small electrical impulse through the pickup coil to the switching electronics that fired the spark plugs. Many of these early system designs worked well, but some were rate sensitive. In other words, the output to the plugs was directly tied to the speed of the reluctor. So at low engine speed, spark energy was quite weak; in some cases it was too weak to start the engine. However, high-RPM spark energy was very good, making them desirable for racing where engine speed was almost always high. They had no point bounce and a good immunity to dirt and grease. They were unaffected by wear and required virtually no maintenance.
Variations of magnetic systems included various triggering devices, three of which remain available. The least common uses a distributor with the old breaker-point cam lobes to trigger a proximity sensor mounted near the cam. An electronic control module fires the spark plugs when the lobe tips come close to the sensor. Interestingly, in this system the original breaker points may be left in place so that the system can be converted back to a stock ignition by flipping a switch in the event of an electronics failure.
The second, and perhaps the most common, design uses a separate trigger wheel either mounted over or in place of the stock distributor cam. The sensor detects the teeth of the trigger wheel as they rotate past, signaling the control box to fire the coil. These units work effectively, but worn bushings can degrade their accuracy. Both types of these breakerless systems are currently used in many specialty and factory ignitions.
The third type is an LED (light emitting diode) system that makes use of light-sensing circuitry. The LED is mounted on the breaker plate and positioned so that a shutter wheel, mounted to the distributor shaft, can alternately cover and uncover the light beam. This alternates power to the primary circuit and fires the spark plugs.
Optical systems are quite reliable and provide stable timing. They are insensitive to most ignition problems, but some systems can be affected by dust and grease. A variation of these three systems combines the construction of the optical and magnetic systems. It is called the Hall-Effect ignition, and it uses a magnetic field and a chopper wheel similar to those used in an optical system. Each arm of the chopper wheel is a small magnet.
The sensor, called a Hall Cell, is located on the contact plate. This electronic chip senses the magnetic field generated by each arm of the chopper wheel. The associated electronics reacts to the Hall Cell signals and switches the primary current to the coil. Like optical systems, the Hall- Effect ignition is relatively insensitive to environmental conditions and does not require close operating tolerances.
In recent years, the development of both multi-firing and extended-spark (sometimes called extended-burn) ignitions have added unique capabilities to both high-performance and conventional ignition systems. Instead of producing a single short spark for ignition, these systems either produce multiple high-voltage sparks or one long-duration spark. In multispark systems, the number of sparks per ignition cycle at idle can be as high as six, when time between power strokes is the greatest. As engine speed increases, the number of sparks decreases to about two at high RPM. In extended-spark ignitions, a single, long-duration spark jumps the plug gap during the time a multiplespark system would generate several sparks at the plug.
Potential horsepower increases from multiple-spark or long-duration systems depend on the flame propagation characteristics of the combustion chambers. Cylinder heads with larger chamber volumes can benefit most from these ignitions, and although it’s impossible to predict the benefits on any single engine, gains may vary from negligible to as much as five percent. Small-blocks with small-volume chambers usually show little or no improvement from multifiring ignitions. However, multiplespark or long-duration ignitions almost universally help smooth out a rough idle and minimize plug fouling that can hurt engine performance during the first critical seconds after leaving the starting line. For the street, these high-tech ignitions can make stubborn starting a thing of the past, and improvements in gas mileage are not unusual.
There are other types of systems found mainly on racing engines. These includemagnetos, crank-trigger systems, and more complex forms of MSD systems, including high-power multiple coil ignitions with electronic advance curves, high-speed retards, and other exotica. Magnetos have been popular racing pieces for many years. The faster they spin, the hotter the spark they produce. They are really a simple generator that operates without outside power. The instant a magneto begins to turn, it starts generating electrical power. If you have ever been the victim of the popular racer’s prank of spinning a magneto while holding the lead you know how much kick they can produce. At low RPM, they barely have the energy to fire the plugs, so they are usually reserved for high-RPM, race-only applications.
Crank-trigger systems were developed for drag racing to combat ignition problems due to camshaft twist, timing chain flex, and distributor wear. They are basically magneticimpulse systems, except that the timing wheel (reluctor) is much larger and mounted next to the harmonic balancer at the front of the engine. There are four magnets embedded in the wheel, and a pickup/sensor positioned near the wheel senses the passing magnets, sending ignition pulses to the control box. The distributor is still used to channel the secondary voltage to each cylinder, but primary timing is handled entirely by the magnetic pickup (where it is unaffected by camshaft twist and other problems).
The parts of your ignition system between the output on your coil and your combustion chambers are often referred to as the secondary side of the system. These parts carry the high voltage from the coil to the spark plugs in the combustion chambers. Spark plugs in your small-block should be the proper heat range, and they must be gapped to match the ignition system’s requirements. Greater secondary voltage often permits the use of a slightly colder plug. Dyno tests sometimes indicate that plugs one or two heat ranges colder than stock can produce an increase in power, although this may not occur in every case. In addition, a higher secondary voltage has more energy to jump across an air gap, so increasing spark plug gap by 0.010 to 0.020 inch may provide a fatter spark that will more reliably ignite the air/fuel charge. This can be of particular help with lean mixtures or high compression ratios (but don’t go over 0.050- inch gap, since secondary voltages may go high enough to damage ignition components, and electrical emissions also dramatically increase). More reliable ignition can, in turn, increase the speed of flame-front propagation, and this may require slightly less total ignition advance to reestablish optimum power. If the ignition timing is not optimized when the flame propagation times are altered, the result can be a reduction in power. Usually, just a small change is all that’s needed, assuming the ignition advance was right-on prior to modifications.
Platinum-tipped plugs offer measurable advantages in both long life and reliable ignition under adverse conditions. Platinum-tip designs are widely available in auto-part outlets and are very reasonably priced. They are certainly worth your consideration. Other new spark plug designs seem to offer even more power-producing potential. Split-Fire plugs are one of these unusual variants. They have shown reasonably reliable power increases in some dyno tests. The patented plugs have a unique construction that helps expose the air/fuel charge to the electrical discharge between the center electrode and the ground strap. Other experimental plugs of which we are aware may produce even more power in some applications. If manufacturing and legal issues are settled, they may be available within the next year. Stay tuned! Spark plug wires used withmodern electronic ignition systems must withstand higher voltage levels and prevent spillover into vehicle electronics, including engine-control computers. High-quality, high-temperature silicone- jacketed wires are available with solid cores for racing (where interference- causing emissions are not a consideration) or with a carbon-fiber or helical-wound cores for street or racing applications where interference suppression is important. MSD, Mallory, Moroso, and others make excellent 8- mm heat-resistant cable that does an excellent job of electrical emissions containment. These wire sets are substantially superior to the carbonimpregnated, string-core stuff that is supplied as standard equipment by many automanufacturers. You can add additional protectionto your secondary wires by jacketing themin tubingmade of glass cloth that is highly resistant to heat, the most common cause of premature wire failure. MSD offers glasscloth tubing and a self-vulcanizing silicone rubber tape that can be wrapped around the wires to secure the cloth tubing or to add additional heat protection at critical points, especially around header tubing. High engine compartment temperatures will bond the tape permanently to the ignition wires, improving their insulation resistance to both heat and high voltage.
Many speed shops sell a variety of great looking colored plug wires. Some of these wires are much better suited for performance use than others. Carefully examine the core and the insulation before you buy. If they have a carbon-string core, put them down or make your mind up to replace them every year or so. If you have headers, make sure the plug wires are insulated with temperatureresistant silicone rubber. Keep in mind that wire manufacturers can claim they use silicone insulation as long as the jacket material is composed of only some silicone rubber. These cheaper wire sets will not withstand the heat radiated from headers. If you want the best, stick with top-ofthe- line wires from ACCEL, MSD, Mallory, Moroso, or others designed for serious racing applications.
Setting Ignition Timing
There are numerous backyard methods to test and adjust ignition timing on a breaker-point system. Some of these can be accomplished without any fancy equipment. However, all of these methods are questionable in accuracy, and since most modern ignitions use magnetic or other breakerless pickups that make it virtually impossible to visually determine the distributor position that will trigger the coil, it’s best to forget oldfashioned techniques and only set timing with a strobe-type timing light. If you don’t have a top-quality timing light, it’s worth investing in one. Watch out for cheap units that have low light output; these strobes make it impossible to read the timing mark unless you’re in complete darkness (and that’s a great way to get your fingers chopped up in the fan blades). The best timing lights use an inductive pickup that quickly clamps on the outside of the ignition wire. Top-quality lights will provide a stable indication of timing from idle to over 8,000 rpm.
Checking initial timing with a strobe light is a relatively straightforward process. Connect the timing light to the number 1 spark plug terminal or wire according to the instructions provided with the light. Disconnect the manifold vacuum hose leading to the vacuum advance canister. Leave the hose connected to the manifold and plug the open end to prevent a vacuum leak. Start the engine and make certain the idle is below the point at which the centrifugal advance starts to activate. Onmost engines centrifugal advance may begin as low as 800 to 1,000 rpm. Point the timing light at the timing plate on the front of the engine. The flashing light will illuminate a timing mark on the spinning crankshaft damper and the stationary marks on the timing plate. If you have trouble seeing the marks, try enhancing them with a narrow stripe of white paint. The relative position of these marks will indicate the amount of initial ignition advance. In all but the cranktrigger systems, timing is adjusted by rotating the distributor housing (the pickup must be adjusted on the crank trigger system to vary timing). Loosen the clamp securing the distributor and advance or retard (rotate) the distributor slightly until the timing light indicates the correct number of degrees (move the distributor counterclockwise to advance timing on the smallblock). Once this is set, the distributor clamp should be firmly tightened.
This procedure is fine if you already know what the initial ignition timing should be for optimum performance. On a modified engine, the stock timing figure may no longer be applicable. Information presented in this chapterwill help you find the best initial advance for your engine, but if you’re just looking for a place to start, try setting initially at about 5 to 10 degrees advanced.
A timing light can also help troubleshoot the vacuum and centrifugal ignitionadvancemechanisms.By reattaching the vacuum hose to the advance canister while viewing the timing marks (make sure the vacuum hose is connected tomanifold vacuum not a ported source on the carburetor), it’s easy to confirm vacuum advance function. The timing mark on the vibration damper should move substantially ahead of the stationary TDC mark. In a similar fashion, the mechanical advance can be tested by simply increasing engine speed above idle (make sure the vacuumadvance is disconnected for this test).
Finally, a quality timing light can help you determine how accurately the ignition system functions at high engine speed. Slowly increasing engine speed to near peak RPM while observing variations in the timing mark can reveal spark scatter, high-speed retard, or other mechanical or electronic abnormalities. For safety reasons, never stand in line with the fan or fan belts or remove the belts when checking high-speed ignition timing.
A performance ignition system should generate a rock-solid timing mark at all engine speeds; there should be no visible signs of widening, spreading, or jumping. If any of these problems are indicated, they can usually be traced to several mechanical and/or electronic sources, but the most common causes are a loose timing chain, worn distributor bushings, or a sticking mechanical advance. In addition, since the oil pump is driven off of the bottom of the distributor, spark scatter can often be traced to pressure pulses generated by the oil pump, especially when high oil pressure is used; refer to the oiling chapter for possible cures.
Written by John Baechtel and Posted with Permission of CarTechBooks