09-09-2022, 07:39 AM
Dickie65 that's nearly right -
The "worst case" for ignition setting is low engine speed under high load; in a car with fixed timing this is the condition which determines the maximum advance which can be tolerated, and everything else is a compromise (unless the car only runs at 6000 rpm as on a race track).
Bob weights advance the ignition as engine speed increases, and retard it as engine speed decreases.
For sporting use the revs are up and the ignition is advanced; when engine speed drops off e.g. slogging up a hill the ignition is backed off to prevent pre-ignition (which is a Bad Thing). A better compromise than fixed timing is arrived at, where ignition timing is adjusted with engine speed.
Vacuum advance is an added layer of sophistication, in that it senses whether the engine is under load or not.
Whereas a slow-running engine under load has to be retarded considerably to avoid pinking, the same engine running at the same speed on the flat with a mere whiff of throttle applied can tolerate - indeed would welcome - considerably more advance. Vacuum is essentially a function of throttle opening and a vacuum-controlled distributor - working alongside the bob weights - backs off the advance when the throttle is open and increases it when the throttle is closed.
Obviously the two opposing mechanisms are tuned to give the best compromise - but still a compromise - which enables ignition advance with engine speed but moderates it according to the engine load as indicated by throttle position.
Pondering the above suggests to me that vacuum advance is of little interest to the go-faster crowd, but might improve running efficiency for those who spend a lot of time pottering along at low engine speed and barely-open throttle - a number of my continental holidays have been like that.
The "worst case" for ignition setting is low engine speed under high load; in a car with fixed timing this is the condition which determines the maximum advance which can be tolerated, and everything else is a compromise (unless the car only runs at 6000 rpm as on a race track).
Bob weights advance the ignition as engine speed increases, and retard it as engine speed decreases.
For sporting use the revs are up and the ignition is advanced; when engine speed drops off e.g. slogging up a hill the ignition is backed off to prevent pre-ignition (which is a Bad Thing). A better compromise than fixed timing is arrived at, where ignition timing is adjusted with engine speed.
Vacuum advance is an added layer of sophistication, in that it senses whether the engine is under load or not.
Whereas a slow-running engine under load has to be retarded considerably to avoid pinking, the same engine running at the same speed on the flat with a mere whiff of throttle applied can tolerate - indeed would welcome - considerably more advance. Vacuum is essentially a function of throttle opening and a vacuum-controlled distributor - working alongside the bob weights - backs off the advance when the throttle is open and increases it when the throttle is closed.
Obviously the two opposing mechanisms are tuned to give the best compromise - but still a compromise - which enables ignition advance with engine speed but moderates it according to the engine load as indicated by throttle position.
Pondering the above suggests to me that vacuum advance is of little interest to the go-faster crowd, but might improve running efficiency for those who spend a lot of time pottering along at low engine speed and barely-open throttle - a number of my continental holidays have been like that.