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 rates 335 hp--when the ignition fires correctly. This high
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.
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