You are right - within the distributor of an EST ignition system there is no provision to physically vary the relative position of the rotor to the posts. Once the base timing is set and the shaft is clamped down, that's it - the shaft/rotor are rigidly coupled to the cam. The rotor to post position will always be what it is then.
But everything above also applies to a standard HEI ignition system. Once the distributor is clamped down, the relative position of rotor to post is fixed.
I think the misunderstanding here is what parts are actually moved to vary ignition timing. It isn't the distributor shaft or rotor that are indexed by the vacuum advance actuator. If you look closely, the vacuum advance arm is connected to the base ring.:
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Mounted on the the base ring is the reluctor ring. Timing is controlled by changing the position of the reluctor ring/pickup coil to the reluctor.
Here is how the vacuum advance operates. When the rod that comes out of the “can” and up through the pickup point indicated by the yellow arrow is pulled toward the can under vacuum conditions, it rotates the base (white arrow). This advances the timing.
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The reluctor is fixed to the distributor shaft and the pickup coil is mounted on the base ring described above. The base ring is free to rotate and is mechanically coupled to arm of the vacuum advance. As the reluctor spins, the relative motion across the stationary nodes of the reluctor ring coil produces an induced AC voltage in the pickup coil. Rotating the base/reluctor ring around the reluctor varies the timing of when peak voltage is attained. That is how the ignition timing of a non-EST style HEI distributor is controlled.
The problem is that when voltage is produced this way, it gradually rises and falls in the form of a sine wave. But, the main ignition coil requires a pulsed trigger signal - one that is either on or off - to trigger the collapse of the magnetic field in its windings.
Thats where the ignition module comes in. The module takes the sloppy sine wave voltage supplied by the pickup coil and cleans it up (so it snaps on and off). It then uses that sharply pulsed signal to make/break the ground leg of the ignition coil's windings. Each time it snaps the ground leg open, the magnetic field in the windings collapses and the coil fires - first down into the rotor, then into the plug wire and finally to the plug.
So, as described above - probably in greater detail than you needed - control of ignition timing in an HEI system
without EST, is all done by the distributor itself - using it's internal electronics and vacuum advance. All it requires externally is 12 VDC at the B+ terminal of the main coil.
Now for your question about how this is accomplished with EST:
The acronym "EST" stands for Electronic Spark Timing. With these systems, the trigger that the ignition module uses to break the coil's ground leg (which collapses the magnetic field which produces the spark that fires the plug) is externally supplied. It comes from the ECM and is already in the form required by the ignition module - a pulsed, electronic 5V signal.
So - under normal operating conditions - instead of being responsible for control over its own ignition timing, the EST distributor is externally controlled. It opens the coil's ground leg only when the ECM tells it to.
The reluctor and pickup coil are still installed and part of the system, but their only purpose is to control timing at initial start up. As soon as engine RPM exceeds 400, the ECM switches control over to itself. The trigger signal from the reluctor and pickup coil continues to be generated and sent to the control module, but it is kept isolated from the system and not used again until the next startup. (how this is done is explained below)
So, the ECM takes control of ignition timing at 400 RPM and keeps it until the engine is shut down.
EST is considered to be a vast improvement because the ECM has access to real-time data which is supplied by a number of engine management sensors. Using those inputs, it is able to calculate the optimal degree of ignition timing for any given operating condition.
On the other hand, the standard HEI distributor - while very dependable and a huge improvement over the breaker point type - is still essentially flying blind. Other than an indication that low manifold pressure exists - via the vacuum advance actuator - a non EST distributor has no other inputs from external sources that would help it adjust timing for best engine performance.
Ignition Control Module pins:
NOTE: the pin designations in this example are for discussion only - they stand for E = EST signal, B = By-pass signal, R = reference signal. The pins on an actual module will be A, B, C & D
The ignition control module has an internal by-pass switch that is controlled by a signal from the ECM (on pin B). Under 400 RPM, the by-pass switch connects the output of the signal converter to the transistor. This is the default position. The input signal for the converter originates from the pickup coil (on pins P & N). The transistor then uses the signal from the by-pass switch to cycle the coil's ground leg open/closed.
At 400 RPM the ECM sends a 5V signal (on pin B) to the module commanding the by-pass switch to disconnect from the pickup coil and connect to the EST pin at the ECM. From that point on, the trigger signal to the transistor is provided by the ECM to the module (at pin E).
The remaining two pins - "R" & the unlabeled one - provide a reference signal to the ECM that it uses to confirm proper ignition system operation. If the ECM doesn't see a pulsed signal, for more than 2 seconds, it will command the fuel pump relays open and cut off the fuel supply.
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