distributor
This may help too.
TDC is when the #1 piston is at the very top of the compression stroke. Top Dead Center
Timing is the number of crankshaft degree either before or after TDC the spark will be fired.
Initial or base timing: Timing 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 - 850 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. On many distributors timing at 800 or 900 rpm will likely be base timing plus some mechanical advance. Don't worry about it, just be aware of it and always write down the rpm of the timing measurement.
Mechanical Advance: Degrees added by the distributor's weights moving outward with rpm. Sometimes called centrifical advance, often 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, in those situations a high rpm timing measurement is a better reference. For example, Mopar Performance instructions suggest measuring at 2800 rpm. It is critical to know the RPM of every timing measurement.
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 slows down as rpms increase. 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:
This may help too.
TDC is when the #1 piston is at the very top of the compression stroke. Top Dead Center
Timing is the number of crankshaft degree either before or after TDC the spark will be fired.
Initial or base timing: Timing 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 - 850 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. On many distributors timing at 800 or 900 rpm will likely be base timing plus some mechanical advance. Don't worry about it, just be aware of it and always write down the rpm of the timing measurement.
Mechanical Advance: Degrees added by the distributor's weights moving outward with rpm. Sometimes called centrifical advance, often 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, in those situations a high rpm timing measurement is a better reference. For example, Mopar Performance instructions suggest measuring at 2800 rpm. It is critical to know the RPM of every timing measurement.
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Purpose of Spark Timing: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 slows down as rpms increase. 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: