[gibbons]

The crank turns twice for every one turn of the distributor. And on a V8 distributor cap, there's 45* between the spark plug terminals. Sooooo if you have a large centrifugal advance with vacuum advance up to, say 52* total at no-load cruise, that's 26* advance on the cap. But with only 45* between the terminals, the previous cylinder's terminal is only 19* away. Why doesn't the spark jump backwards to the closer terminal?

Just wonderin'. There has to be an embarrassing obvious explanation that I am overlooking.

 

[Mike]

The larger the diameter of the cap ,the larger the distance between terminals.
Look at the difference in terminal distance on a GM points cap vs the GM HEI cap.

 

[greg_moreira]

It doesnt fire in between posts on the cam.

When you advance/retard a distributor, you move the distributor body(including the cap and contacts in the cap), but the rotor stays in the same place.

You move the post itself closer to, or further away from the rotor...timing how soon or late the rotor hits the post and releases the spark.

If your timing is say 12 degrees at idle(before top dead center)...this doesnt mean that when the piston is at TDC, the rotor is 12 degrees before the post. It means it hit the post 12 degrees before the engine reached TDC. But yeah it shouldnt be firing in between posts or it would happen exactly like you said.

 

[MDchanic]

The instant of firing is controlled by the opening of the points, or by the triggering of the electronic module, not by the position of the distributor cap.

- Eric

 

[gibbons]

Points cap or HEI cap, it's still 45* between the terminals. Their arc lengths between their terminals is different, but the angular relationship is the same as described by my question. Cap type is irrelevant.

Sure, timing is determined by the angle you clamp the distributor body, but that's a relationship that varies. The counterweights on the centrifugal advance cam move the rotor further from the target terminal when they advance and fire.

I'm still not seeing an explanation as to why in extreme advances, the spark doesn't jump backwards to the previous terminal, which seems like it would be closer

 

[MEJ1990TM]

The spark is not being released until the rotor reaches the post for the cylinder in question. Say the timing is at 40*. The points are not opening forty degrees before the rotor reaches the #1 post. The rotor is meeting the post with the piston 40* BTDC.

You are making this much more complicated than it is.

 

[Tom Mobley]

No, gibbons would never do anything like that. Would he?

 

[Schurkey]

The crank turns twice for every one turn of the distributor. And on a V8 distributor cap, there's 45* between the spark plug terminals.

Yup. So far, so good.

Sooooo if you have a large centrifugal advance with vacuum advance up to, say 52* total at no-load cruise, that's 26* advance on the cap. But with only 45* between the terminals, the previous cylinder's terminal is only 19* away. Why doesn't the spark jump backwards to the closer terminal?

Just wonderin'. There has to be an embarrassing obvious explanation that I am overlooking.

How about several semi-obvious explanations? I'd think that would make you feel a bit better.

1. Depending on the distributor style, the rotor contact and the distributor cap contact are each ~1/4 inch wide (more or less). That provides something like 1/2 inch of rotor tip travel where the rotor is still metal-to-metal aligned with the terminal on the cap (except for rotor-to-cap air gap, of course)

2. At base timing, the rotor is firing "ahead" to the closest distributor cap terminal; at full centrifugal advance it's firing "behind" to the closest distributor cap terminal. In other words, the arc of advance degrees is somewhat centered on the distributor cap terminal, and therefore that terminal is always the closest one. It's not like the rotor is exactly aligned with the distributor cap terminal at base timing, and goes farther and farther away from alignment with additional advance.

3. Centrifugal advance causes the rotor and reluctor (HEI) or point cam to advance in relation to the cap. Rotor moves forward--in the direction of distributor shaft rotation--during advance.

4. Vacuum advance moves the pickup coil or point plate AGAINST distributor shaft rotation, the spark jumps off the rotor earlier (in degrees) than it would without vacuum advance. Therefore, the centrifugal advance tends to move the rotor ahead, but vacuum advance makes the spark jump earlier--the two "kind of" offset each other in terms of rotor position vs. spark timing.

Feel better now???

 

[461RAT]

Gibbs,youre spending WAY too much time in the laboratory with going overkill on every little speck of a cars tolerances.Turn the key,fill it up at a gas station of your choice and drive the thing.Pulling the rear tranny seal out of trans that aint leaking because you like the red color seal instead of the blue color is see sawing on the edge to insanity.Go cruising

 

[dreis454]

Gibbs,youre spending WAY too much time in the laboratory with going overkill on every little speck of a cars tolerances.Turn the key,fill it up at a gas station of your choice and drive the thing.Pulling the rear tranny seal out of trans that aint leaking because you like the red color seal instead of the blue color is see sawing on the edge to insanity.Go cruising

Ya know,its not often that I agree 100% with 461RAT but this time I do!:thumbsup:

 

[Tom Mobley]

I got an idea, let's all get together and give gibbons a hard time. Slow day anyway.

 

[steveeaz]

The crank turns twice for every one turn of the distributor. And on a V8 distributor cap, there's 45* between the spark plug terminals. Sooooo if you have a large centrifugal advance with vacuum advance up to, say 52* total at no-load cruise, that's 26* advance on the cap. But with only 45* between the terminals, the previous cylinder's terminal is only 19* away. Why doesn't the spark jump backwards to the closer terminal?

Just wonderin'. There has to be an embarrassing obvious explanation that I am overlooking.

I'll see if I can do the simple explanation. Some have already touched on the relationship between the crank and the cam (2 revs of crank per 1 rev of cam). So when you are talking 45 deg of timing advance, that is crank timing and that equates to 22.5 deg of distributor timing (2:1 relationship again).

 

[gibbons]

I got an idea, let's all get together and give gibbons a hard time. Slow day anyway.

Ha! Today would be no different than any other day when I ask one of my special questions I won't be around to defend myself, I am going skiing with my son, then I'm gunna ride my bicycle this afternoon, and then maybe get my new new E-Curve distributor set up and start the car tonight. Priorities....

BUT... based on some of the posts above, it's obvious that I'm not the only one not clear on this

 

[dreis454]

I got an idea, let's all get together and give gibbons a hard time. Slow day anyway.

I for one am NOT trying to give him a hard time...If anything I'm trying to make things EASIER on him.:thumbsup:

 

[Steve439]

Gibbs, you'll enjoy this:

http://www.msdignition.com/uploaded...port/frm28392_tech_bulletin_rotor_phasing.pdf