Terms Related to Timing
Initial or base timing: Timing at an rpm with no advance added.
Normally on a stock engine timing at idle is the baseline. Idle is usually 500 - 650 rpm (pre-smog), or 600 - 700 rpm smog era.
Hot rodded and race engines requires some trial and error tuning to get the idle speeds down anywhere close to 700 rpm.
When you are measuring timing at idle it is critical to note the rpm. Timing at 800 or 900 rpm will likely be base timing plus some advance unless the spring tension is increased. Don't worry about it. Just write down the rpm.
Mechanical Advance: Degrees added by the distributor's weights moving outward with rpm. Usually just called advance.
Total Timing: Initial timing plus the maximum mechanical advance. With hot rods and race engines, sometimes the idle timing is very unstable, especially early in the tuning process. For that reason, a high rpm timing measurement is a better reference. For example, mopar performance instructions suggest measuring at 2800 rpm.
Main Purpose of Spark Timing
View attachment 1716259195View attachment 1715447028
Mechanical or Centrifical Advance:
Compensates for the reduced amount of time (miliseconds) available for the burn as rpm increases. Because engine efficiency decreases burn time as rpms climb the rate of advance is not always propertional to rpm. 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 opossite.
Vacuum Advance:
View attachment 1716259196
Vacuum advance is in addition to the initial timing and the mechanical advance.
Removing and plugging the vacuum advance hose (golf tee works well) is the standard procedure when measuring timing.
_____________________________________________________________________________________________________________________________
With vacuum advance connected 57* at 3000 rpm is on the high side but in the ballpark . Factory would be closer to 50*
Your '69 4 speed was originally setup to reduce emissions at idle and while coasting. It came with a distributor setup with a long and fast primary advance curve to compensate for the idle emissions. IF your car is setup with that distributor, or a distributor with similar advance, then initial has to be set lower.
More about both of this the next post