Originally posted by smlblcks10
so the only way the idle could affect the timing is if it is too high and at an rpm where the mechanical advance is starting to kick in? does it affect how fast the mechanical comes in later in the rpm range or the initial/total advance at all? thanks
Mechanical advance is controlled by three things (all related to each other). The RPM of the distributor shaft causes the weights to move outward by centrifugal force. The heavier the weights the faster (lower RPM) they will move outward. The return springs attached to the weights are also to be considered as their strength (or lack of strength) control how much resistance the weights have to overcome in order for the weights to move outward. The springs will also control how quickly the weights return to there base position. Full mechanical advance can be regulated with a combination of springs and weights.
EXAMPLE: heavy weights and light springs will enable the weights to move fully outward at a lower shaft RPM than the same weights with heavier springs.
A good combination would enable a smooth increase in the advance and reach the maximum movement around 2500-2800 engine RPM.
Initial timing is set by moving the distributor base to whatever degrees BTDC you wish to use. It is set at idle (the lowest idle you can get with your combo). If you have a cam that requires the idle to be set higher, the engine RPM (and distributor shaft RPM) may cause the advance weights to begin to move outward. This is when stronger springs and/or lighter weight may be needed. Idle speed does not affect how fast the mechanical advance begins to move. Again, this is determined by the weights and the springs. Idle speed can affect the initial timing inasmuch that if the idle speed is higher you may need more initial to keep the engine running.
Basically; most street-able engines (does not apply to blowers or when using NOS) should be able to idle (in drive if an automatic) around 650-750 RPM. The initial timing (always set with the vacuum advance disconnected and plugged) for most performance applications will be somewhere in the 10-12 degree range and even as high as 18-20 degrees BTDC. The Total mechanical timing (initial plus mechanical) all in around 2500-2800 RPM will be in the 32-36 degree range and even as high as 38 degrees (again with the vacuum advance disconnected and plugged). It is not advisable to exceed 40 degrees Total mechanical for most street-able engines. None of the timing degrees I have mentioned include any vacuum advance adjustments. If you were to hook up the vacuum advance and check the timing again you will see a figure well above 40 degrees. You will not have that condition when the engine is under load (driving) due to the loss of vacuum at WOT or hard acceleration.
Since I made a reference to NOS I wish to state that the timing MUST
be able to be retarded when using NOS or serious engine damage will occur. That's another story!