MG Restoration

The Electronic Ignition for MG Sports Cars


Les Bengtson


Much has been discussed about the advantages of converting over to an “electronic ignition” system for older automobiles. Many claims have been made as to their effectiveness and their greater ease of operation. “Never have to replace your points again” was the main claim when these systems first came out back in the mid 70s. Today, they also claim additional horsepower.  Therefore, it might be time to examine exactly what the distributor does, how it does it and what are the advantages of converting over to an electronic ignition system.


The distributor is a two system piece. You have the low tension (LT) system which consists of the points, condenser and the low tension circuit of the coil. This system operates at 6 or 12 volts depending on car model. The high tension (HT) circuit consists of the high tension circuit of the coil, the coil lead (king lead in Brit), the distributor cap, rotor, spark plug leads and the spark plugs. So, how do they work?


The low tension circuit is used to operate the coil which is a form of step up transformer, converting the basic 6 or 12 volt input into an output of up to 40,000 volts into the high tension circuit. Since cars operate off direct current (DC), a transformer will not work without help. A transformer works by inducing an electrical charge into a series of windings which are surrounded by another series of windings. This creates a magnetic field which, as it forms and then collapses, generates a higher or lower electrical charge in the secondary windings. Since direct current does not create and collapse the field, a method of doing this must be devised. In the distributor, the points do this, creating an electrical field when they are closed and allowing the field to collapse when they open. Because the opening and closing of the points causes a small spark, they wear and begin to loose effectiveness over time. To a certain extent, this wear is slowed down by the use of a condenser which serves to store the excess energy when the points open and reduce the spark to almost nothing. Thus the points are kept working efficiently for many thousands of miles. (There are also points in the SU fuel pumps and the older, mechanical voltage regulators, not all of which have condensers to lengthen the life of their points).  The points are opened and closed by a cam on the distributor shaft which is shaped like a square with rounded ends. How long (in degrees of a circle) the points are left closed will effect how fully charged the coil will become. This figure is known as “dwell” and on the Lucas 25D4 is about 60 degrees. On an eight cylinder engine, the figure is about 28 degrees which is why dual point distributors were a worthwhile modification to the older V-8 muscle cars—the two points sets gave a dwell of about 34 degrees, a great improvement at high rpms. A dual point distributor offers no real advantage on a four cylinder car as the dwell is more than adequate to build up a full charge even at the higher rpms. What happens when the points close then open? They trigger the high tension circuit.


The high tension circuit of the coil is triggered by the opening of the points and the collapse of the electro-magnetic field it has created in the LT circuit of the coil. This puts out a pulse of  high voltage current through the coil lead to the center of the distributor cap. The standard coils (6 or 12V) put out 17-20,000 volts, while the Lucas Sports Coil and other aftermarket “performance coils” put out up to 40,000 volts. Very impressive, but it really does not mean much. Even the “standard” coil has more than sufficient output to cause the plugs to spark under normal driving conditions. This takes about 10,000 volts and the charge builds up until the spark jumps between the electrodes of the spark plug. At that point, there is a release of energy and no more build up. Thus, if you are running standard spark plug gaps (.025” for older cars and .035” for later cars), you will never exceed the capacity of the standard coil to fire the plugs. The Sports Coil is great if you are running larger than standard spark plug gaps or operating consistently at high rpms. In fact, you are wasting money on a performance coil unless you open up the spark plug gap to take advantage of the increased power available.  


So the coil has just released its many thousands of volts to flow to the distributor cap. The current travels down the center electrode of the cap and hits the flat bar on top of the rotor. The rotor is swinging around at one half engine speed with its outer edge almost making contact with the four outer terminals on the distributor cap. As the charge is induced and the rotor swings by the cap, the current flows from the king lead, through the rotor to the spark plug wire terminal, up the wire and through the spark plug, causing the plug to fire and ignite the explosive mixture in the cylinder which will drive the piston downwards.  This is all that happens, but it needs to happen at the right time and in the right order. This is known as timing.


Timing refers to the firing of the spark plug when it is most efficient to cause a controlled burning of the air-fuel mixture in the cylinders. Because it is a controlled burn and not an explosion, everything happens fast, but not instantaneously. Thus, you would normally ignite the mixture slightly before the piston reaches top dead center (TDC) so the burn will be most efficient when the piston is at TDC and beginning to go downwards. This is why timing figures are expressed in crankshaft degrees before top dead center (BTCD).  Basic timing is established by rotating the distributor until it is firing at a specified number of degrees BTDC.  Differing operating conditions mean that this is not always the best timing, especially as the engine is turning faster or is put under load as when going up hill or accelerating. To change (advance) this timing to a more efficient setting, two mechanisms are used—the centrifugal advance and the vacuum advance.


The centrifugal advance is built into the distributor and allows the points cam to rotate in relationship to the position of the points, thus causing the firing to take place earlier. They consist of a set of weights, springs to control and retract the weights and a movable cam. They are dependant only on the engine rpms. The vacuum advance, on the other hand, is not rpm dependant.


The vacuum advance operates when vacuum is applied. This vacuum operates a spring with a loop on the end which is attached, via a stud, to the base plate on which the points are fastened. As vacuum is applied, it causes the plate to move in relation to the points cam and the timing to advance. As vacuum is reduced or removed, the points plate or base plate returns to its original position. There are two types of vacuum advance systems. Those that operate on “ported” vacuum (a small hole by the throttle butterfly on one car) and “manifold” vacuum (those attached to the intake manifold). They operate differently and do different things. Ported vacuum advances the timing as the throttle is opened to provide increased performance. Manifold vacuum actually drops slightly as the throttle position changes and, thus, the vacuum “advance” actually retards the timing slightly for better emissions performance. Both the vacuum and centrifugal advance systems must be checked periodically to ensure they are working properly or their advantages may be lost. On an intake vacuum connection, a bad vacuum advance canister will cause an intake leak which will result in rough running.


Now that we understand the basic function of the distributor, how does this relate to the “electronic ignition”.  First of all, the electronic ignition isn’t. It is not an electronic ignition system, it is an electronic points replacement.  All it does is replace the points and the condenser in the low tension circuit, using either a beam of light or a magnetic effect to trigger the LT circuit to induce a charge in the HT circuit. Thus, you do away with the problem of having to replace the points and condenser and, perhaps, you will sustain a slightly higher level of performance. Tests conducted using a new set of points and an electronic ignition show that the level of performance is virtually identical. But, point wear and reduce the level of performance while the electronic ignition maintains the same consistent level as long as the system is working.  REGARDLESS OF THE TRIGGERING SYSTEM USED, THE REST OF THE DISTRIBUTOR SYSTEM MUST BE MAINTAINED ON A REGULAR BASIS IF THE SYSTEM IS TO WORK EFFECTIVELY.  This means that the distributor should be disassembled, cleaned, lubricated and reassembled on a regular basis (annually on a daily driver, biannually on others).  You still have to replace spark plugs, rotor, distributor cap and check the HT wires regardless of the triggering system used.  With this in mind, is it worth replacing the points with an electronic ignition?  Maybe.


If your distributor is in less than perfect condition (as many of them are) and the distributor shaft bushing is a little worn, you will get some wobble to it which can vary the amount the points open, thus affecting dwell and timing.  The electronic ignition is not affected as much by this as the points are and would provide a better, more consistent ignition under these circumstances. So would a new or properly rebuilt distributor, but most of the electronic ignitions cost less than a new distributor, thus delaying the requirement to install a new distributor right now. Thus, the advantages of the electronic ignition system are small, but real. While I would not install one on my car (unless given one, of course), it might be a reasonable investment for many.  However, the only time it is a “must have” is to replace the old electronic system in the Lucas “Opus” system, few of which still survive.


This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.


Copyright © 2001 by Les Bengtson

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