[FC] Why Vacuum Advance?

[corvanatics@corvair.org]

In a message dated 4/8/2004 2:16:56 AM Eastern Standard Time, 
galaxyflyer1@yahoo.com writes:
But what is the purpose for getting top advance when accelerating only, 
therefore getting a retard when de-accelerating or while the engine is just idling. 
Why not remove the vacuum line (plug it) and set the timing for peak advance 
before ping (road test) and therefore have optimum advance all the time? 
>>>>>>>>>>>>>>>
    You got it all wrong in what you are thinking above.
    The vacuum advance is a ported vacuum source. After about 3/4 throttle it 
no longer works and goes to it's retarded position. But, you don't want any 
advance during acceleration or at idle.
    So the vacuum advance will advance at part throttle openings and as you 
open the throttle the advance will retard so you don't get spark knock and when 
you hold a steady throttle it will advance again. This process goes on all 
while you are driving and with every move of the throttle. As you decelerate, 
foot off the throttle, the vacuum advance should go to the retard position 
because the port should be covered and this helps prevent backfiring in the muffler.
    What the vacuum advance does is give you gas mileage, plain and simple. 
The distributor can be made to work very well without a vacuum advance but I'll 
bet your mileage would be cut in half.
    Now if you want to get into the mechanical advance and combine what the 
two do together, well it could take several pages to adequately explain.
    Ken Hand