Ignition advance via distributor

Can anyone explain how ignition timing is advanced in our cars?

I understand that the hall effect and shutter wheel mechanism is used to generate a signal input to the computer and therefore records ignition timing.

I have also read here that the computer's only ignition timing related output is grounding the coil:

https://www.autozone.com/repairguide...00c152802514de

Supposedly, based on how the coil is grounded on and off, the ignition timing can be changed. However, this doesn't quite make sense to me.

If ignition timing were solely dictated by the time at which the coil was grounded, wouldn't the position of the rotor only need to be "somewhat close" to the plug wire its igniting? If this is the case, why does physically rotating the distributor affect timing at all?

I do not realize how the computer's outputs are capable of adjusting this timing. If anyone could elaborate, it would be much appreciated.

Almost everyone now uses a computer controlled advance system. The Reference HEP is the one used for ignition signal to the engine computer, whether the two part (Logic Module and Power Module) SMEC or SBEC. The curves are in the program chip. The computer picks up the pulse from the HEP, but most likely uses the previous pulse and delays it to where the desired advance is. On our engines, the initial or static timing is set by unplugging the ECT (engine coolant temperature sensor) the two lead one on the water box. If you have one of the carbureted cars with the spark computer, then the idle contact is grounded and vacuum disconnected and plugged.

Either of these put the system in a "limp" mode locking the timing at the base setting (12° BTDC on the turbos), on my F350, there is a plug near the ignition module that disables the advance, GM used a jumper at the diagnostic connector to do the same thing. As for the timing in relation to the rotor tip, yes, the tip needs to be in an area in relation to the plug wire end that it can jump to without a problem. This can occasionally, with a lot of advance (and it happened before computer systems) cause a misfire or even cross fire where the wrong cylinder fires or two fire at the same time. This is the reason Ford and GM went to larger diameter distributor caps on most engines. Our 4 cyl caps. even though small do not seem to have a problem since the plug wires are 90° apart.

The ever increasing "cruise" advance that the high output ignition systems are capable of is part of the reason for first DIS systems and finally COP systems where the ECM can adjust the spark on each cylinder.

Ah Bill beat me to the punch...

When you are adjusting timing via the dizzy, you are moving the reference timing of the engine.
Here is a useful document
http://www.turbo-mopar.com/forums/kn...BEC-Tuning-FAQ

To make sure I understand you guys correctly, I'm pretty sure I understand all of the following:

The timing is advanced solely by the time at which the coil is grounded.

Relation of the rotor to the plug terminal, does not have an effect on timing (assuming that there are no jumps to an incorrect plug and assuming that the rotor is close enough)

Removing the ect will force the computer to run at default parameters where timing is set at 12° btdc

Now here is what I'm still unsure of:

The timing remains at 12° at idle only? With the ect plugged in, could timing vary to 11° or 13° or more?

When the distributor is rotated, the hep is rotated and this would cause the computer to see a signal sooner or later. The computer adds 12° of timing under the assumption that the hep signals right at 0°?

It sounds like the 12 degrees of timing is going to be added no matter what by the computer. But real timing can change based on how soon or late the hep generates a signal.

Don't overthink it, just accept the fact that it works. The 12° BTDC is the static timing and the computer advances it from there. It will not remain at 12° once you reconnect the coolant sensor, it will move far enough to vanish inside the transaxle housing. If you have one of the adjustable advance timing lights, you can see where it sits, but be warned, it will not be a steady value, it will move as the computer adjusts it.

My F350 is converted to MAF/SEFI and when I unplug the SPOUT connector (Ford's method of locking the timing) and set it at 10° BTDC, then reconnect the SPOUT, it will jump to 20-30° BTDC, and if I increase the RPM, go off the scale (scale is from 10° ATDC to 30° BTDC).