Vacuum advance information and myths

Pressure, Load, and Temperature in the chamber all effect the combustion development.
Changing rpm changes the time available for the pressure to develop before its largely wasted (in the form of heat into the cylinder walls and out the exhaust).
lead time is needed for maximum pressure during the maximum crank leverage.
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from Chrysler Master Technicians Conference

Density of the fuel air in the combustion chamber is another factor.
Important enough to have a method to automaticly compensate for the effect on the burn rate.​
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from Chrysler Master Technicians Conference 259