Whether you service your own ignition or need more troubleshooting savvy for road rallies and cruising, this month's lesson focuses on spark and ignition system basics. Understanding ignition fundamentals will improve your tuning skills.
Internal combustion gasoline engines require metered fuel and ignition spark at the right time. The traditional ignition system consists of a gear- or slot/key-driven conventional distributor, a distributor cap, and live wires leading to each spark plug.
Spark must be delivered at the right time. Timing is critical to complete fuel combustion and maximize the energy yield in the fuel. When the spark ignites fuel too early, the rising piston receives a counterforce. The rattling sounds of detonation, or what we call "ping," signals the ominous piston shake that results from this counterforce. Be aware: Ping also occurs when overly volatile, low-octane fuel burns across the piston's crown too quickly.
For best performance, spark begins at a precise number of degrees before each piston reaches top dead center (TDC) on its compression stroke. The time interval or duration of the spark across a spark plug's gap is relatively constant. Correct spark timing begins a specified number of degrees before the pistons reach TDC. Since spark duration is constant as engine speed increases, the ignition point must begin earlier.
Ford's Cobra Jet 428 V-8 engine...
Ford's Cobra Jet 428 V-8 engine rates 335 hp--when the ignition fires correctly. This high-performance classic used a breaker point ignition. The stock appearing distributor has a Pertronix breakerless (electronic) conversion, which eliminates breaker point service. A Pertronix kit works with stock or performance coils.
When air/fuel ignites, the expanding gases push the piston downward. Ignition and the expansion of gases must start sooner when piston speed increases. The goal of spark timing advance is to light off the air/fuel mix in a way that allows a peak energy release--ideally, a strong downward push just as the piston crests TDC. Too early, and the piston is still rising. Too late, and the utilization of energy will be diminished.
Fuel octane plays a role as well. Lower-octane fuels are more volatile and burn quicker across the piston's crown. The energy release takes place sooner, and if timing is not retarded, the result is a counterforce, which is why ping occurs when octane is too low. One means for compensating is to retard spark timing. This may cure ping, but it also reduces the horsepower output.
Spark Advance Mechanisms
Most conventional distributors use two methods to accomplish this variation in spark timing: a centrifugal advance mechanism and a vacuum advance mechanism. Some vintage distributors, like Ford's Loadomatic type, use solely vacuum advance (in conjunction with a particular carburetor design). The late '50s and muscle car era high-performance dual-point distributors typically have a mechanical advance with no vacuum advance.
Centrifugal/mechanical advance mechanisms consist of flyweights and calibrated balance springs. The weight and spring method responds directly to engine speed. Vacuum advance, by contrast, receives signals from an engine vacuum source and advances the timing. The factory recommends a base (idle or static) timing for each engine, described in tuning literature under the vehicle's make, model, and engine type.
Ported vacuum is a high-vacuum signal available just as the throttle opens. While engine load might drop manifold vacuum substantially at part throttle, ported vacuum does the opposite. As the throttle begins to open, vacuum increases just above the throttle valve(s) of the carburetor. This is where ported vacuum receives its signal.
Of the two advance sources, the one that varies most is vacuum advance. As the throttle opens further, vacuum to the ported source drops. If timing calibration is correct, the increasing centrifugal advance takes over the spark timing. Centrifugal timing advance is now sufficient to meet the engine's needs without the support of vacuum advance.
Classic GM distributor is...
Classic GM distributor is a "window cap" design. An engineering breakthrough, the window allows adjustment of the breaker points without removing the distributor cap. You can set the preliminary gap with a feeler gauge, install the cap, and finish the adjustment after starting the engine. (Be careful inserting the Allen tool. If you miss the screw, a shock is possible!) This oil-filled coil works with a block-type ceramic ballast resistor.
Spark at the plugs is crucial....
Spark at the plugs is crucial. These plugs show carbon buildup from excessive idling and city traffic at the Hot August Nights (Reno, Nevada) event. Distributor, coil, wires, and ballast resistor must be in top condition. Spark plug heat range and electrode gap is important for proper firing.
The 12-volt breaker point...
The 12-volt breaker point ignitions often use a ballast resistor like the white one shown here. (Note: Crimp connectors are inappropriate for primary circuits.) This is the ballast for GM trucks built in the '50s and '60s. Ford and others have used a resistance wire to the primary side of the coil. When wiring, do not eliminate resistance circuits. Know the voltage requirements of the coil, and check available voltage. Some aftermarket coils require a full 12 volts.
This is also a problem: The...
This is also a problem: The wire from the coil to the ballast resistor has frayed to two fine strands. Wire on vintage trucks is prone to wear and fatigue over time. Six-volt systems are often cloth-wrapped wire. Twelve-volt systems have plastic coatings that get brittle over time or short internally. When in doubt, rewire the system, using an appropriate resistor on the primary circuit if required.
A primary wire must be intact...
A primary wire must be intact and have no load resistance. This late-'50s Chevrolet wire shows high resistance, which would overheat the coil and this wire. Ignition misfire, poor starting, and cutting out under load would be likely symptoms for this primary wire to the coil. The best testing device is a digital volt-ohmmeter. Check circuits for continuity and ohms resistance.
This ignition distributor...
This ignition distributor has lost its original ground to the breaker plate. The red wire is homemade using crimp connectors. The best connections are solder or Solid Seal connectors that have a solder ring incorporated. Crimp connectors can create resistance, as contact is seldom complete to all wire strands. One modern approach is solder splices with plenty of heat shrink insulation.