Date: Sat, 8 Feb 1997 12:39:19 EST From: Russ Maak Subject: SPARK ADVANCE 2 Heres the rest of the article THE SPARK ADVANCE By Petersons Basic Ignition and Electrical Systems For many years the only distributor used on Ford passenger cars was the Loadomatic. It was made by Holly Carburetor Co. the same people who made the famous 4-barrel carburetors. The third type of distributor uses both centrifugal and vacuum advances. This " dual advance " type is found on the vast majority of both domestic and foreign cars. The vacuum advance unit receives it's vacuum supply from one of two sources - from either below the throttle valves ( intake manifold vacuum ) or from a location above the carburetor throttle valves ( spark-ported vacuum ). Manifold vacuum is the highest at idle and decreases as the throttle valves open but spark-ported vacuum functions differently for it is lowest at idle but increases with throttle opening.During idle and part throttle running, manifold vacuum advances the spark for more economical operation but, when the throttle is opened, vacuum advance is not provided, thereby reducing the chance of engine " ping " or damaging detonation. Since 1966 most manufacturers have switched over to spark-ported vacuum for emission reasons. Spark-ported vacuum is more tailored to the performance and economy demands of the engine. It is not available at idle or any other time the throttle closes, so the spark is not advanced. As the throttle opens, vacuum increases and timing advances to increase performance and economy when you need it most - that is, as the throttle opens. The centrifugal advance ensures the best settings for maximum acceleration rates and top speed. The two advance methods work independently of each other, according to the demands of the engine at the time. All vacuum advance units on duel-advance distributors are controlled by either intake manifold vacuum or spark-ported vacuum. Regardless of vacuum source, all single-diaphragm and single-action diaphragm units operate in the same manner. A tube running from a special vacuum port above or below the throttle valves is connected to a vacuum chamber on the distributor. Inside the vacuum chamber is a thin diaphragm that separates the chamber into two halves. One side is exposed to vacuum and the other to the ambient pressure of the outside air. When there is vacuum in the chamber, air pressure deflects the diaphragm against its spring loading so that a rod attached to it can advance the timing of the spark by rotating the breaker plate. This changes the relative position of the breaker point rubbing block and the distributor cam. When vacuum drops, the diaphragm in the vacuum chamber is not so greatly deflected, retarding the spark timing slightly. The only significant departure from this system is the distributor on older Chevrolets 6-cylinder engines. On these cars the vacuum unit moved the entire distributor body, not just the breaker plate. The centrifugal advance works by changing the position of the cam in relation to the distributor shaft. This is accomplished by means of two governor weights held close to the distributor shaft by small springs. As the shaft's speed of rotation builds up the weights tend to fly apart, stretching the springs. Pins on the weights act against a plate fitted to the base of the distributor cam. The further the springs allow the weights to fly out, the further the cams position-and the spark timing - is advanced. The precisely calibrated tension of the springs is therefore the prime controlling factor in the operation of a centrifugal advance. Auto makers provide data on advance curves, etc.in the shop manual for your make and model car. Applying this information, however, requires special testing equipment that is not generally available to the home mechanic. The important thing to remember is that any defects that thorough machine testing might uncover are usually the direct result of dirt, lack of lubrication, wear or faulty parts in the distributor. Most repairs are just a matter of cleaning, lubing or replacing. There you have it in a nut shell, or three nut shells if you may. centrifugal advance only. vacuum advance only centrifugal and vacuum advance ( duel advance ) Hope this helps answer a lot of questions about advances. RUSS 56 oval 58 ragtop BAJA daily driver 63 convert. ( Snubs ) 65 notch and more SPARK ADVANCE 2