Join Date: Sep 2000
Location: Washington, Michigan USA
In basic terms, spark advance refers to when the spark plug fires relative to the position of the piston as it comes up on the compression stroke. The air-fuel mixture in the cylinder takes a little time to burn once it's ignited, so the spark needs to fire before the piston reaches top dead center in order for the burning mixture to develop maximum cylinder pressure by the time the piston starts down on the power stroke.
The higher the engine rpm, the less time there is during each compression stroke, so the spark has to fire sooner at higher rpm to "light the fire" sooner than it does at low rpm in order to develop maximum cylinder pressure at the same point of piston travel as the piston starts down on the power stroke; that's why the spark has to be advanced as rpm increases to maintain efficiency.
Also, lean mixtures (like at steady cruising speed or while at low speed around town) burn slower than rich mixtures, so the engine needs more spark advance under these conditions for best efficiency and fuel economy - that's what the vacuum advance does - adds advance (over and above whatever the distributor centrifugal advance mechanism provides at that rpm) under high vacuum (low engine load) conditions to burn lean mixtures efficiently. Then when you accelerate, the mixture richens (and can burn faster), engine load increases, manifold vacuum drops, and vacuum advance decreases, as the extra advance it provides isn't required any more. When you ease off the gas, the mixture leans out to cruise levels again, vacuum increases, and the vacuum advance comes back into play to fire the leaner mixture earlier again for fuel efficiency.
Spark intensity remains essentially constant at all times and isn't related to advance, which only determines WHEN it fires. Hope this helps you understand the concept.
'69 Z28 Fathom Green