I don't get it. In the olden days, the rotor would be advanced by spring weights and vacuum diaphragms. The points would deliver the spark at the appropriate time. Nowadays, the rotor is fixed in position, bolted to the distributor shaft, and the computer acts as the points. How does the distributor itself advance? If the computer advances the spark, but the rotor doesn't advance, how does the spark line up with the spark terminal?

 

Think about it: The rotor & cap terminals didn't exactly line up in a mechanical/vacuum advance system, either.

The computer reads the signal coming from the trigger wheel in the distributor - when it sees a pulse, and the time between each pulse. Using other system inputs such as throttle position, manifold absolute pressure, etc., it decides how much sooner to trigger the spark for the next cylinder coming up (shortens or lengthens the time between coil firings). It is doing this without actually knowing the crank position, because you set the "base" timing by taking the advance capability out of the loop during the timing adjustment activity.

It's pretty slick, actually. They really had to "think outside the box" to come up with this.

 

...So rotating the distributor housing set the base timing works because you line up the trigger wheel, right? Lining up the terminals is not as important to timing? That's enlightening! Thanks!

 

Cap/rotor terminals, horseshoes, and government work all have something in common - "close enough".

 

five7kid hit it about right. Yes, the tip of the rotor doesn't line up perfectly with the posts on the distributor cap when the ECM is putting and taking advance. maybe a little to one side at low advance, little to the other and high advance.

Old mechanical distributors didn't have this problem QUITE as bad since the centrifugal advance moved the rotor and the the little points cam wheel or mag pickup wheel in unison on the top of the distributor shaft. So the tip fo the rotor stayed basically stationary relative to the cap terminals regardless of how much centrifugal advance kicked in. However, the vacuum advance portion would cause it to go out of phase since it only moved the outer pickup ring or points plate and didn't move the rotor in sync.

Hope that makes sense.

In oversimplified terms, the ECM supplies advance electroncially by delaying or advancing when the module breaks ground and causes the coil to fire. It basically makes a scillion little time calculations per second so it knows exactly how many milisenconds to add or subtract from when the module says "now!". Man, THAT didn't come out so hot. Somebody else could probably explain electronic timing better than me.

 

No that's fine. I think I get it, thanks. That makes a lot of sense. One last question. Before electronic ignition, they had mechanical advance, vacuum advance, or both. I understand mechanical advance giving you more advance at more rpm, makes sense. Why would you want more advance with more vacuum? Wouldn't you want it the other way around? And are we talking about manifold vacuum or ported?

 

"Electronic" typically is used to refer to electronic triggering, widely used from the mid-70's to early 80's. This is in contrast to mechanical points triggering, but both used mechanical/vacuum advance. What you are calling "electronic" is defined as computer controlled.

Vacuum advance improves economy in cruise conditions. It doesn't matter whether it's manifold or ported, the concept is the same. Ported just means there is no vacuum advance at idle. The practice of using ported was introduced at the start of emissions controls era, because it reduced formation of nitrogen oxide compounds. Advance rates were tuned differently between the two, though.

Computer controlled ignitions mimic the advance rates of vacuum/mechanical with varying engine speed and load conditions. Do it much better, though.