Large RPM and Vacuum drop when shifting from park to gear
Timing and fueling work together.
Pages 5,6,7 in this Chrysler booklet cover the concept. Ignition System Analysis (Session 259) from the Master Technician's Service Conference
Mechanical or Centrifical Advance
Compensates for the reduced amount of time (miliseconds) available for the burn as rpm increases. Its not directly proportional because combustion efficiency increases with rpms. This means decreased burn time as rpms climb. A 'low performance' 318 usually has good lower rpm efficiency, but does not gain as much efficiency with increasing rpm. A 'high performance engine' is usually the opposite, poor - slow combustion at low rpms, but more efficient and quicker pressure development at higher rpms.
Vacuum Advance:
Timing and fueling work together.
Pages 5,6,7 in this Chrysler booklet cover the concept. Ignition System Analysis (Session 259) from the Master Technician's Service Conference
Mechanical or Centrifical Advance
Compensates for the reduced amount of time (miliseconds) available for the burn as rpm increases. Its not directly proportional because combustion efficiency increases with rpms. This means decreased burn time as rpms climb. A 'low performance' 318 usually has good lower rpm efficiency, but does not gain as much efficiency with increasing rpm. A 'high performance engine' is usually the opposite, poor - slow combustion at low rpms, but more efficient and quicker pressure development at higher rpms.
Vacuum Advance: