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.