what is causing the advance if not for vacuum ?
I am confused......J.
There are three sources of spark advance:
Initial advance: Set by the position of the distributor housing. Aligning the ball and pointer and adjusting the distributor housing so the points just break contact sets the initial timing. People who think they're smarter than the engineers who designed the engine have all sorts of voodoo rituals to set initial timing.
Centrifugal advance:
A set of weights and springs below the breaker plate moves the distributor cam in the direction of rotation as engine speed increases, allowing the spark to fire sooner at speed so the flame front across the combustion chamber and its resultant pressure wave meets the piston exactly at the top of the stroke. Flame travel is almost a constant speed, with a pretty big variation according to air/fuel mixture. Lean mixtures burn extremely fast; rich ones burn slowly. The centrifuugal advance is set up with different weights, springs, and slots to limit the movement of the weights so the spark advance corresponds to the needs of the engine. Generally speaking, heavily-loaded trucks that spend a lot of time lugged down and pulling hard need less spark advance than lightweight vehicles to prevent detonation while pulling heavy loads. The rate of advance can be altered by changing springs and/or weights. The amount of total centrifugal advance is altered by changing the length of the slot where the weight travels.
Vacuum advance:
At light-throttle cruise, another 8-10 degrees of spark advance can be added for good fuel economy, but only under very light-load conditions. The advance port in the carburetor is designed to provide vacuum to the advance unit only when the throttle plates are partway open and intake manifold vacuum is high. As soon as the throttle is opened for acceleration or load-pulling, the vacuum signal disappears and the distributor goes back to its initial/centrifugal advance system. As the load decreases and cruise conditions return, the vacuum advance comes back into operation for gas mileage.
Very few people actually understand ignition timing. It's very closely related to fuel mixture, load, camshaft profile, and RPM. The research engineers who blew up untold numbers of engines on the dyno figured all this out many years ago. People who weren't even a gleam in grandpa's eye back then think they can throw away all the rules of physics and do their own thing! NOT!
Jerry
