The distributor ignition system is a relatively simple system in theory, but begins to get a little complex when it comes to the operation. This is because it has to react to differing conditions in order for the engine to recieve correct ignition timing so the spark is always delivered at the correct time for the amount of fuel and air entering the cylinder, and the amount of time available for the combustion process to occur.
Basically there are three types of ignition timing:
1st- There is the BASE setting, this is the setting where there is no vacuum advance or mechanical advance acting on the distributor. The engine would run on this setting as long as it was at a steady RPM or one didn't want to rapidly advance the throttle.
2nd- The VACUUM ADVANCE, this is powered by a vacuum line that usually gets it's vacuum source from the carb or throttle body above or on the outside of the throttle plate. The reason for the source needing to be above or outside the throttle plate is because the vacuum needs to increase momentarily as the throttle is opened. If it was below or inside the throttle plate, the vacuum would decrease when the throttle was rapidly opened.
3rd- CENTRIFUGAL advance. This is usually induced with offset spring loaded weights mounted on a cross head located on the distrbutor shaft. As the RPM of the distributor shaft increases the weights begin to "fly out" away from the center, because of the way they are pivoting, and the way they are shaped, they act against the plate on which the rotor is secured, and this mount is allowed to rotate independently a few degrees from the distributor shaft and advance the rotor relative to the driven part of the distibutor shaft, thus advancing the ignition timing beyond the base setting.
Now for the reason for the two advance mechanisms.
The VACUUM advance advances the ignition timing from the base timing during rapid throttle opening in order to allow more complete combustion of the sudden increase in fuel and air being introduced into the combuston chamber from the opening of the throttle during acceleration. If the ignition didn't advance rapidly, the driver would notice a hesitation until the centrifugal advance has time to react from the increase in RPM to induce mechanical ignition advance. The centrifugal advance cannot react rapidly enough because it must increase in RPM before it can actually advance the ignition.
The CENTRIFUGAL advance advances the ignition timing beyond what the base timing is as the engine RPM increases. This is needed because as RPM increases, the combustion process has less time for flame propogation to occur, therefore if the ignition is advanced at higher RPM, the combustion process has more time to occur and that leads to a more thorough combustion, and greater combustion chamber pressures, therefore greater horsepower and efficiency of the engine can be created for the given conditions.
The second part of your question deals with the actual vacuum advance diaphram and housing. There is almost no difference in operation among differing brand names. However, the matrials of which the diaphram inside the vacuum advance are made of may differ in quality. Seeing as how there isn't much that can go wrong with the vacuum advance system components other than the diaphram becoming porous, you can probably use either as long as they both mount to the distributor housing correctly, and the vacuum advance rod is aligned and doesn't bind.
I hope this helps, and answered your question.
vettepilot
My $.02
