MODERN TRANSISTOR IGINITION FOR THE MODEL "A"

Increased reliability and Performance From Your "A's" Ignition System

The breaker point-condenser-cam-coil ignition system was developed by Napier and Renault in Europe, and Winton in America about 1904. It was used on cars until the early 1970's. By the mid-70's, all domestic manufacturers changed over to solid-state transistorized systems, obsoleting the point system.

Point type and transistorized systems differ primarily in the method the ignition coil primary current is turned "on", "off", and "on" again.

Point Type Ignition
Refer to Figure 1. When the ignition switch and points are closed, current flows through the primary winding of the ignition coil. This produces a magnetic field around the primary and secondary windings.

When the points are opened by the distributor cam, the current falls to zero. The magnetic field collapses, inducing a momentary high voltage in the coil secondary windings.

This high voltage is connected to the distributor rotor to sequentially "fire" the appropriate spark plugs. The condenser acts to provide a rapid collapse of the primary current and reduce arcing at the points.

For the four cylinder engine, the primary current is on and off 40 times per second at 1200 RPM, 80 times per second at 2400 RPM. At maximum RPM of 3000, this occurs 100 times per second.

The spark energy and voltage generated by the coil are highly dependent upon the primary current, speed of contact point opening, and leakage paths. Fouled and dirty plugs, dirty coil tops, dirty spark plug insulators, and carbon leaks inside the distributor reduce the energy and voltage available to fire the plugs.

Point current is normally limited to about 4 amperes maximum for life of thousands of miles. Point resistance and wear increases with time and reduces the coil primary current.

Cam and point assembly rubbing block wear further reduces the current, changes the current with RPM, and changes the spark timing (approximately 1°/0.001 change in point gap).

Point bounce and cam non-uniformity also change successive spark energy/ voltage. This causes missed sparks and erratic spark timing with accompanying engine malperformance.

The foregoing performance and maintenance deficiencies are the major reasons for the point-contact system change to semiconductor transistorized ones.

Transistorized Ignition
As shown in Figure 1A and 1B, the point contact and transistorized systems are similar. They differ in the way the coil primary current is switched: "on", "off", and "on."

As the distributor shaft rotates, a toothed iron wheel (reluctor) changes the magnetic path between the magnet pole piece and a wheel tooth. This path change generates an alternating voltage in the pickup coil surrounding the magnet pole. Figure 2 shows a detailed view of the reluctor and magnetic circuit

The induced voltage (after suitable processing) "triggers" (turns "on", and "off", and "on") a power transistor connected to the ignition coil primary winding. The coil primary current is switched "on", and "off", and "on" to generate the desired secondary voltage and "spark" for the plugs. This is now similar to the action of the points as a switch, but the switching action is more uniformly and accurately controlled.

The system consequently improves ignition and engine performance as follows:

• no maintenance-no rubbing components, sealed and potted components
• uniform spark energy each "firing"-constant primary coil circuit current, pulse to pulse
• uniform spark timing at each trigger point - reluctor tooth spacing, width, and transistor trigger controlled to a fraction of a degree of shaft rotation
• increased life-no components prone to wear
• immunity to oil, water, and engine heat.

The above result in smoother and more powerful engine performance.

Ignition System Comparison
The output voltage of the ignition coil rises until the spark jumps the spark plug gap. It next falls to a short time sustaining value and finally to zero as the coil magnetic field energy is dissipated.

The typical plug gap requires about 12,000 volts to jump the 0.035 inch spacing. The rotor to distributor contact gap requires about another 3,000 volts. So, the coil must develop at least 15,000 volts to produce an ignition spark.

To provide for worn dirty plugs, changes in point wear, coil current, cylinder pressure with throttle opening, etc., the ignition system must be capable of producing output voltages that are higher than the nominal 15,000 volts. The usual breaker point-coil system was designed to produce approximately 22,000 volts. Transistor-coil systems usually produce approximately 30,000 volts.

The transistor-coil system thus has greater reserve voltage compared to the conventional point configuration. It consequently causes ignition under more severe conditions of plug gap wear, fouling, and engine load operating conditions.

Output Voltage With Engine Speed
Ignition system output voltage change with engine speed is shown in Figure 3 for typical point and transistor coil systems.

The point type is at about 22,000 volts at idle and low speeds. It then falls off rapidly at higher speeds. The change is due to the reduction of coil primary current at higher engine RPM and the effects of point bounce and float. The further effect is to cause misfires and loss of engine power.

The transistor-coil system higher voltage output is due mostly to greater primary coil current. The output voltage is essentially constant with RPM. The effect is to maintain engine performance over the entire speed range of the engine from idle to maximum RPM.

The Ignition System meets the preceding conditions. The complete kit consists of the following items:

• Precision machined/ spaced Reluctor (replaces the cam).
• Potted Magnetic Pickup Coil Assembly mounted on a new upper plate stamped from original tooling (replaces existing upper and lower plates. A dummy condenser may be retained for external appearances).
• New Armored Cable Conduit from distributor to new ignition switch (identical to original cable).
• New Ignition Switch (simple on-off switch replaces the costly pop-out switch and cable).
• Sealed Transistor Control Module with all color coded and keyed connectors mounts up under the dash.
• Complete instructions for installation, timing, and use.

There is no change from the original "A" external appearance. The visible coil wires, junction box to distributor armored cable, and ignition switch look original in colors, make, and appearance. The Transistor Control Module is located under the dash and is not readily visible.

The kit is compatible with both 6 Volt Positive Ground and 12 Volt Negative Ground battery, generator, or alternator configurations. The furnished wiring accommodates both grounding systems.

The original/ existing coil is used for 6 Volt application. A new 12 Volt coil (no external resistor required) is recommended for the 12 Volt system. A series ballast resistor (1.5 ohms) may be used if one retains the 6 Volt coil for a 12 Volt system.

The system has been extensively tested both in the laboratory and on several "A's" over the past two years. The installation instructions are complete and detailed. They cover both 6 and 12 Volt applications, tests, and timing methods.

The kit was designed for and is being produced by Nu-Rex, Box 9332, Akron, OH 44305. It is available from both Bratton's Antique Auto Parts, Inc., 9410 Watkins Road, Gaithersburg, MD 20882 and Snyder's Antique Auto Parts Inc., 12925 Woodworth Rd., New Springfield, OH 44443.

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