There is nothing in the cap or rotor to store the discharge of the coil. You are right that an excessive gap between rotor and cap may exceed the 'gap jumping' capabilities of a weak system, but that's got nothing to do with this subject.

 

That goes back to what I said to start with. The fire flies when the spark from the coil sees the next pole on the distributor cap (or, another path to ground, whichever comes first).

The points opening get the coil ready to fire but it has to see a ground path before it can do so.

I am .................exhausted!

 

So why do we bother turning the entire distributor to adjust timing- much simpler to just turn the cap.

 

And since the rotor to shaft relative position never changes, and since the cap to housing relative position never changes, how does the vacuum or mechanical advance seem to do it's job?

 

The cap/rotor relationship does change and the vacuum moves the breaker plate, not the rotor, cap, shaft or distributor body.

You're going down for the third time.

 

You've yet to land any blows so there's no 'going down'. All you've done is stirred up the wind- amongst other things.

I do have to thank you for several things and want to buy you a beer(s) next time we meet up as a token of gratitude. First, you got me thinking and wondering if what I had lurnded back in skool many years ago and believed as the gospel was in fact incorrect. I do still miss some facets of my former job, in particular understanding the workings of a complex device such as a distributor with both mechanical and vacuum advance and understanding the interaction of both inputs and how they affect the output. I did learn a few new things (see below). Second, by digging into my dist. yesterday, I found that in addition some sticky weights that were not returning to home position, my vacuum advance was completely inop due to a ruptured diaphragm. I had wondered about the different 'feel' to the engine recently and believe that the combination of no advance and a vacuum leak contributed to this. New canister is arriving this afternoon from my FLAPS. Same guy that found me the correct date coded water pump. Nice guy even if he does drive a Ford


So, my findings relative to this thread and the OP's question:

1) with NO advance (either mechanical or vacuum), the tip of the rotor points dead centre at the posts in the cap when the points open. This was verified by physical measurement.

2) Upon inspection of the inside of the (well-used) cap, electrical erosion marks are visible on each of the eight towers, and are most pronounced at the centre of the electrode. No erosion is visible at the outer edges.

3) by manually moving the advance weights outwards, the relationship between the cap and the rotor does change- so you were correct on this one. However, the weight movement also causes the points cam to advance it's position meaning that the points open earlier. This supports my contention that it is the points that control the firing, not the rotor/cap relative position. The combined width of the rotor tip electrode and the cap towers is sufficient to ensure that they are still 'aligned' even at full advance, although I could not accurately measure the net angular change.

4) the vacuum advance canister moves ONLY the breaker plate that supports the points. There is no angular change between the rotor tip and the cap. This means that it is purely the function of the points opening earlier that causes the change in timing. It was at this point that I found my canister was defective, BTW.

5) With the engine running at various steady RPMs, I twisted the cap back and forth to each extreme in an attempt to induce a timing variation. Nothing, nada, nyet. I was tempted to take an old cap and file off the locating lug to be able to move it even more, but ran out of time.

 

I agree, the rotor and the breaker cam have a constant relationship with the cap. It's also constant with the breaker plate. That is, until vacuum starts to move the breaker plate. At that point the breaker plate relationship with the rest of the timing/ assembly is disconnected.

I gotta' go clean out my barn. Be back later.

 

If you guys have any further questions, try reading the following set of posts (starting with #22) from back in 2003.

http://forums.corvetteforum.com/c1-a...-dimple-2.html

Clicking the pictures in post #28 still works too (showing how the vacuum advance changes the rotor relationship to the dist. cap terminal).

Plasticman

 

I remember the thread. I had a very small piece of it. One of the more enlightening threads I've seen. I didn't disagree with your conclusions then and I don't disagree now.

Mikey took my comments slightly out of context but I'll still buy him a beer next time I see him.

As for as the OP's question, he asked ""will cap movement (pivoting around that locator tab) change timing". I still maintain it will to some degree. How much, I don't know.


Thanks for posting that old thread. That just about wore me out reading it. I didn't go back to the first part of it but I think I remember it started because somebody suggested instead of changing the shaft gear to orient the distributor, just change the wires on the cap?

 

I remembered the thread too once I started into it. I guess we'll have to agree to disagree. Both my analysis and plasticmans seems to indicate that the timing will not change by rotating the cap, as the rotor tip is still pointing at the cap electrode.

 

I think that this sums it up. If the spark has a path to ground when the points open the event will occur at that time. The width of the rotor tip and the cap electrode is great enough that there is sure to be an alignment between the two for many degrees of distributor shaft rotation. From the time the leading edge of the rotor lines up with the cap electrode edge to the time when the trailing edge passes by the electrode the spark will jump. More actually because the spark will usually jump farther than that minimum gap.

 

I agree with this too. However, this is not addressing the original disagreement between Mikey and me. He said, the "points determine when the coil fires". I said, "the coil won't fire until it has a path to ground". Splitting hairs? Yes. But you have to know Mikey and myself to understand. Especially, Mikey.

I would point out that using Dukes example with the Calif. distributor and a mis-adjusted dwell setting as Plasticman pointed out, you could develop a misfire (coil discharge) due to the path to ground (rotor/cap) being too large a gap to fire.

 

I can't help adding a point here:

"As for as the OP's question, he asked ""will cap movement (pivoting around that locator tab) change timing". I still maintain it will to some degree. How much, I don't know."

As already stated, the spark jumps when the points open, so if the rotor tip is .090" before or after center of the cap lug, the spark occurs at the same "timing". If you think that a small change in that distance will effect engine timing, then so will a different sparkplug gap.

 

Agreed except about the points controlling the spark. As I stated, the points only ALLOW the coil to fire. It has to find a path to ground to release the spark.

There's also those out there that vehemently insist that indexing the spark plugs so the gap on the plug and the ensuing spark is facing toward the combustion chamber will result in more even firing between cylinders as will having your spark plug wires all the same length.

How far would you like to take this?

 

I don't need to take it any further Mike. I just wanted to make the point that a slight distance change won't effect the timing in any measurable amount. I suppose if you wrapped a plug wire around your garage a few times before connecting it to the plug the proper instrument might detect a few nanoseconds of difference in firing, but the real world timing difference is "zero".
As for indexing plugs, I've only done that due to a tight quench area so the piston didn't ram the ground electrode closed. That was on a set of Booth-Arons ZL-1 angle milled heads (109cc).

 

What was Dr. End's, Dr. Rear End's diagnosis ??

 

Yes very true. I have long believed that MikeM would have been best suited for a career in the the FAA. I learned early in my own career that arguing with them is like wrestling with a pig in the mud. Sooner or later you figure out that the pig is enjoying it. Part of the technique (not to show anybody's cards here) is to bob and weave enough to never quite thrash an argument to death, meaning that the 'dead horse' smiley can't be used as a checkmate. The bs smiley can be used whether relevant or not, and is most effective when used as a decoy or distraction away from the main point.

There's also no use in getting frustrated, exasperated or angry, ise using: , as this is like blood in the water to a shark. The shark in this case is more of a gummi bear, but again I'm showing too many cards. This leaves the old conundrum of 'irresistible force meets immovable object'. If either of these two were not true, I wouldn't be writing this.

So, on with the show:

Jane, you ignorant ****, uhhh sorry that's the wrong show. Mike, you make a perfect point that there will be no spark until there is a path to ground. The evidence above indicates that some part of the rotor is always in alignment with some part of the cap tower when the points open, irrespective of any influence of ignition advance or cap misalignment, deliberate or not. So, no variation in availability of ground at this point.

The concern comes down to varying gap between the rotor tip and cap tower due to a relative concentricity problem, again deliberate or not. Using the worst case example mentioned above, specifically at 3 and 9 o'clock, it could be theorized that the plugs close to the side with the minimum gap would fire sooner than the plugs on the opposite side with maximum gap. Plugs near 6 and 12 o'clock would be unaffected. The problem is disguised by the fact that the average timing of the eight cylinders is exactly the same as an engine with a cap in perfect concentricity.

This (until now) hidden problem seems to be a perfect business opportunity and I'm amazed that the Jegs/Summit/Speed Channel crowd has not jumped on it. How about we go into business making adapter rings that eliminate the slop between the cap and the housing? These could be marketed at next years Carlisle show, alongside the VH water turbine powered alternators and my soon-to-be-released line of petrified voodoo chicken bones (guaranteed to prevent bad things from happening).

Sounds like a sure fire hit, at least with the bling and oil additive crowd. Watcha say?

 

We can argue this some more under JR's tent in a couple months.

 

JR's #1 rule is that we don't get rowdy and scare away potential victi..... uhh customers. Don't forget!

 

Thank you everyone for your responses, especially Mike and Mike. I learned a lot.

The reason for my original question: my ignition timing seems to flutuate a few degrees when I check my total timing. I wondered if the cap movement could cause this, but from your responses I don't think so. I have about .025" of play in my distributor shaft that I need to shim.

My engine is an original '67 L79, has never been apart (original timing chain) and has approx 75k miles on it. It runs good.

Thanks again,
Mark