In the Beginning: Points and Condenser-Based Ignitions
This photo shows the internal...
This photo shows the internal layout of a stock Ford distributor installed in a 351W engine and about to receive maintenance. The condenser is shown on the left while the breaker points are in the process of being removed. The octagonal shaped distributor cam is seen on the distributor shaft directly below the top notch in the shaft, which orients the rotor in the correct position.
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.
This close-up of a points...
This close-up of a points and condenser set (dual point application shown) shows the details of the point set construction. From this photo it's easy to see the rubbing blocks and to visualize how their wearing down will change the adjustment of the point gap or dwell. The condenser is a simple reservoir for residual electricity looking for a place to go once the point set opens. Although very simple in construction and function, without the condenser the points would be fried in a short time.
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.
This photo shows both the...
This photo shows both the mechanical and vacuum provisions to advance timing while the engine is running. As rpm increases, the weights, which are restrained by calibrated springs, are drawn outward by centrifugal force, advancing the engine's timing. The metal fixture on the left is a vacuum advance canister, which is connected to a vacuum source at the carburetor. As manifold vacuum increases, the diaphragm inside the canister is drawn toward the vacuum and it moves the distributor baseplate and points, also contributing to engine timing adjustment.
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.
The Pertronix cylindrical...
The Pertronix cylindrical coil, shown here, is of the same type and construction as discussed earlier. It has an iron core to enhance the magnetic field and is oil filled to aid in cooling. The heavy-duty 8mm Flame Thrower spark plug wires shown are superior to OE sizes because, in addition to having a heavier center wire, they also have thicker insulation to prevent accidental grounding in the form of arcing to exhaust manifolds or cross fire. Such robust plug wires are also more resistant to extreme heat, making then a must for use with headers.
Another cool way to go for...
Another cool way to go for an ignition upgrade is this replacement distributor from Davis Unified Ignition. It offers two other advantages over the stock ignition in addition to eliminating breaker points. First is that the distributor also contains the ignition coil in the cap, thus eliminating the separate coil from the engine bay and the remote coil-to-cap wire connection. This DUI distributor also features a larger diameter distributor cap. As the voltage figures grow because of a more powerful spark, the extra distance between the contacts helps reduce the chance of the spark crossing over to the wrong terminal. Like the Pertronix distributor, this DUI unit employs a GM-style cap and rotor.
This 351 Windsor engine in...
This 351 Windsor engine in a '69 Mustang has the original distributor but it has been topped off with a later-model 5.0 Mustang distributor cap and rotor. You get the advantage of much greater terminal separation under the cap for just a few bucks. This cap also employs the later-model terminal ends requiring the later-style plug wires.