On vacuum advance-only Loadomatic-type distributors, the carburetor has special passageways to provide vacuum from both the ported and venturi areas. As ported vacuum falls off, the carburetor's speed-related venturi vacuum takes over. Venturi vacuum, unlike manifold vacuum, does not drop dramatically under engine load.

Many high-performance ignition systems have only a mechanical (centrifugal) advance. Performance engines use a racy camshaft profile and higher compression. Racy camshafts provide a limited amount of manifold vacuum at lower engine speeds, and high compression does not tolerate substantial spark advance at low speeds. Such engines "come on" at higher speeds and benefit from a speed-regulated ignition--without vacuum advance.

For the typical performance engine, launch or throttle tip-in performance comes from more degrees of base or initial spark advance. This couples with a somewhat quicker response of the mechanical (centrifugal) advance. Base timing plus centrifugal advance degrees target a total number of advance degrees, achieved at a prescribed rpm. The purpose for limiting this total is to prevent detonation as engine speed continues to increase.

Primary And Secondary Ignition Systems
The secondary ignition system is easy to recognize. It consists of a distributor cap, a rotor, the coil lead, spark plug wires, and the spark plugs. The distributor cam drives the rotor, and the distributor driveshaft drives the cam.

As current flows through the coil, the rotor "distributes" spark to each spark plug contact in the distributor cap. At the correct time for each cylinder, spark crosses the gap between the rotor's tip and the spark wire contact in the cap, sending spark to each spark plug in the firing order. The intensity (voltage) of the spark and spark plug gap determine the spark's duration.

The ignition signal at the coil comes from the breaker points or an electronic ignition module. Since breaker points are common to vintage trucks, and the aims of a breakerless ignition are similar, let's use breaker points to illustrate how a primary ignition circuit works.

Think of the breaker points as an on/off switch. When they open, a crisp spark (generated in part by the condenser) jumps across the point gap. This triggers the coil to unload its built voltage, sending current through the high-tension spark wire lead to the distributor's rotor. Here, the rotor transmits that current to the available spark lead contact. The lead carries the current to its spark plug.

Depending upon the number of cylinders, this process repeats itself until each of the cylinders has fired. Then the rotor repeats the process, once again distributing spark to each cylinder in the firing order. As a point of interest, the distributor rotates at half the speed of the crankshaft. On an eight-cylinder engine, coil discharge takes place 12,000 times per minute at 3,000 crankshaft rpm--eight times for each two rotations of the crankshaft!

The Coil Winding And Signals
How does the coil build up the voltage necessary to fire a spark plug under engine compression? The coil, essentially a transformer, consists of primary and secondary windings. An ignition's breaker points are part of the "primary" coil circuit. That circuit operates on battery voltage--or less. Many 12-volt systems resist or drop the primary voltage to 9 or less volts to extend point life and keep the coil running cooler.

The circling of primary current around the fine secondary wire within the coil creates a magnetic line of force. (That force moves away from the coil's iron core.) The field builds until it is interrupted by the opening of the breaker points. Essentially, the circuit interruption redirects the magnetic field's line of force. This interruption causes a surge of voltage from the high number of fine secondary wire winds. The surge moves inward to the soft iron core of the coil.