Modified Mustangs & Fords
Ignition System's Basic Concept - Ford Ignition Basics
Without Spark There's No Fire
Ignition System Basics
No Ford engine project is complete without a properly functioning ignition system. This system has the important job of delivering the spark to the required cylinder at precisely the right time. Any deviation from the micro-second accuracy required and the engine won't be running correctly. If you can't get the ignition dialed in exactly then all the high-performance hardware in the world won't add anything to your engine's performance. The classic Ford cars we love were delivered with breaker-point distributors as original equipment. After a few thousand miles of wear, you'd begin to notice subtle changes in the way the engine runs as the points wear and the dwell, or point gap, changes. While many of us now circumvent these problems by installing a more modern system, the requirements for a correctly running engine are the same, so it's important to understand the basic principles regarding how you car's ignition system functions.
Whether you've got a Pertronix ignition or not you still have primary and secondary sides to the ignition system and you'll still have to have the correct engine timing. A breaker-less ignition setup, like the Pertronix, eliminates the mechanical variable of wear on the point set but the voltage to the ignition coil must still be delivered in the same manner. New cars are freed of these types of concerns because a computer controls the ignition system and there is no mechanical dimension to their function. However, on our 289-powered '67 Fairlane the distributor and its functions are still very relevant to our world. Let's examine some of the more basic concepts related to your vehicle's ignition system.
Primary And Secondary Ignition
Your classic Ford ignition is divided into primary and secondary ignition systems. The juncture between the two systems is the ignition coil. The primary system carries the 12-volt DC current from the battery through the ignition switch directly to the positive side of the ignition coil. This charge goes to the coil directly during start up, and through a resistor (which lowers the voltage) when running. The resistor prolongs the service life of the point set.
The ignition coil has both primary and secondary wiring in the form of winding circuits. The coil primary winding consists of more than 100 turns of insulated heavy copper wire. The primary circuit wire goes into the coil through the positive terminal and loops around the primary windings and then exits through the negative terminal.
The coil secondary winding circuit contains perhaps 20,000 coils of fine copper wire. The secondary windings sit inside the loops of the primary windings. As the 12-volt current flows through the primary side of the coil it passes through the outer windings and out to the breaker points. When the points are closed, the circuit is complete. In this condition a strong magnetic field is produced inside of the coil, with the outer windings becoming a powerful electromagnet. When the points open, the circuit is interrupted and the magnetic field inside the coil collapses. When this happens the inner, or secondary, windings in the coil react by producing a powerful electrical charge in the 20,000-volt range. This transition represents the juncture between the primary and secondary systems.
Once the 20,000-volt spark is produced, it's managed by the secondary ignition system. The big electrical charge travels out the top of the coil and through the coil wire to the center terminal of the distributor cap. The current then arrives at the rotor, which will deliver the charge to the correct spark plug at exactly the right moment.
Once the required spark is produced there are several variables that must be managed in order to assure that the spark is delivered to the spark plug at the correct time. Two of the most important factors are engine timing and breaker-point dwell. Both involve the distributor.
In the Beginning: Points and Condenser-Based Ignitions
With the distributor cap off you will see the points and condenser. The condenser is an electrical capacitor that can store a small amount of current. When the points begin to open, current, flowing through them, seeks an alternative path to ground. It will try to jump across the gap of the points as they begin to open and this would eventually cause damage to them. To prevent this from happening the condenser provides an alternative path to ground. It's not really a ground but functions as one for a short time. By the time the condenser is saturated with this residual electricity the points are far enough apart so that the small amount of remaining voltage won't be able to jump across.
Ignition Upgrades:Electronic Ignition
There are many modern alternatives out there that eliminate the troublesome variables inherent in a mechanical system. As mentioned earlier, the Pertronix Ignitor is just one of several available that will eliminate dwell and gap concerns completely. This photo shows a complete Pertronix replacement distributor for a 351W engine.
Another factor that affects service life of the points is mechanical wear. There is a rubbing block on the points that is in contact with the distributor cam and this block wears down over time. Hence, the points require periodic adjustments to compensate for this. There are two ways that the points can be measured to see if they need an adjustment. One way is by measuring the gap between the open points when the rubbing block is on the high point of the distributor cam. The other way is by measuring the dwell electrically. The dwell is the length of time measured in degrees of distributor cam rotation that the points stay closed. On our classic Ford cars there is no dwell window in the distributor cap to allow for an adjustment while the engine is running. The points are adjusted with the engine off and the distributor cap removed. A feeler gauge is used to measure the point gap at the open position. The points are loosened and moved slightly to achieve the desired gap adjustment and then retightened in the correct position. Once the distributor is reassembled, the engine is run with the dwell meter attached. Any further adjustment will require a repeat of the procedure. Measuring dwell is much more accurate than setting the points with a feeler gauge alone.
On a complete tune-up for a classic Ford car, the points, condenser, and spark plugs would be replaced. With the new equipment in place the point gap would be set manually, the dwell checked, and the engine timing set to specifications. The timing is set by loosening the distributor hold-down clamp and rotating the body of the distributor. A timing light connected to the No. 1 cylinder is used to determine the timing adjustment. The light flashes each time the cylinder fires and the light is directed at the engine timing pointer, which has the harmonic balancer rotating beneath it. Marked on the balancer is a scale showing degrees of advance or retard of the engine timing in deviation from top dead center. A timing adjustment set "before top dead center" or BTDC, is an advanced setting. An adjustment with the engine running after top dead center, or ATDC, would be a retarded setting.
With the engine on TDC the No. 1 piston is at the top of its compression stroke. Rotating the distributor body changes the position where the points contact the distributor cam. Because the cam is geared to the engine rotation, this adjustment changes when the sparks occur with respect to the rotation cycle. Engine base timing on our classic Ford cars is usually set advanced (BTDC), between 4 and 16 degrees. While setting the initial or base timing is important for an engine to run properly, the timing needs to change depending on the speed of the engine and the load that it's under.
By using either mechanical centrifugal advance or a vacuum-actuated mechanism, we can change the engine timing adjustment and advance the timing as the engine runs to suit the requirements according to rpm or load. The reason for the variation in timing requirements is because although the engine runs at different speeds and under different loads, the air/fuel mixture burns at only one speed. The spark merely ignites the mixture and it will only burn at that one rate regardless of how fast the engine is turning. Thus if the piston is going to return sooner, as happens at higher rpm, the air/fuel mixture must be fired off sooner in order to allow optimum combustion time for the rapidly returning piston. Hence, as engine speed increases, the engine timing must automatically advance to occur sooner in the cycle. In other words, the faster the engine is spinning, the earlier we have to fire the plug to produce optimum power. Once computer controls were able to directly control the engine's timing, vacuum and centrifugal advance mechanisms were no longer necessary and were eliminated.
If you'd like to enjoy the advantages of an oversized distributor cap but you don't want to replace the whole distributor you don't have to. You can have the larger diameter using production Ford equipment pirated from later-model Mustangs and other Fords of the late '70s.
Ignition Upgrades:Capacitive Discharge
A stock ignition system uses an inductive discharge. In this arrangement the coil must store and then step up the voltage between each firing. At normal rpm ranges this system works fine. However, as engine rpms climb the process begins to happen too quickly for the coil to have time to store up a complete charge between each event. The result is that the firing occurs faster than the coil's ability to store up a complete charge, meaning that the spark is fired at less than peak efficiency. The use of an MSD capacitive discharge ignition box, such as the 6AL, will eliminate this problem. The 6AL unit also incorporates a built-in rev limiter that can be set to the desired rpm with a simple adjustment.
Distributor Gear Gotchas!
One thing to be aware of when changing distributors in your engine is the material that the distributor drive gear is made out of. It must be made of a material compatible with your camshaft. If your car has the original flat tappet cam, be sure that your new replacement distributor is also equipped with an iron drive gear. If your car has a more modern roller cam, then the camshaft will be made of steel. In this case the new distributor must have a steel or bronze distributor drive gear. Any mismatch between the material the cam is made from and the distributor drive gear will result in premature wear for both gears. Besides destruction of these gears, metal shavings will end up in the engine oil and permanent damage to the engine could result.