The spark always fires when the rotor is in the correct position.  The points open pretty much always when the rotor is in the correct spot. Where the piston is, when the points open, is dictated by the fact that the rotor changes its relationship to the piston position due to mechanical advance. So the spark fires when the rotor is in position on the cap,  but the position where the piston is when the spark fires is what varies.

I shouldn't say that Phasing is not a thing.  It has minor relevance with vacuum advance because the "correct spot" varies by a few degrees of phasing because vacuum advance doesn't change the relationship of the rotor position to the piston position.

Also it comes into play if the distributor is not phased correctly when it is installed in the engine.

But if the distributor is mechanically sound and installed correctly,  no amount of advance is going to make that spark fire when the rotor is way off the contact.

Like I said,  that's the way I always understood it,  as explained by my grandpop.

Bob

Last edited by rbtconsultants (9/24/2014 9:47 PM)

Barnett post    "All the mechanical advance unit does is move the rotor and reluctor farther ahead of the distributor shaft so it fires sooner in relation to the pistons location. It does not change the position of the pick up or the position of the rotor to the reluctor."

If i am understanding you correctly here,  Mechanical advance does not have anything to do with advancing the spark. If it doesn't advance the spark, then it does not affect timing, and if it does not affect timing then it can't contribute to rotor phasing.  Am i understanding you correctly?  I think this is the only disparity between us at this point. 

If all mechanical advances work the way you describe, then I totally get it now. 

In my distributors (durasparks)   mechanical advance moves the reluctor.   The plate that the magnetic pickup is attached to moves with the vacuum advance.    FACT.

yup, this is the way all

Greg B wrote:

In my distributors (durasparks)   mechanical advance moves the reluctor.   The plate that the magnetic pickup is attached to moves with the vacuum advance.    FACT.

lol, yup, this is the way all those advances work
 

barnett468 wrote:

rbtconsultants wrote:

The spark always fires when the rotor is in the correct position. The points open pretty much always when the rotor is in the correct spot. Where the piston is, when the points open, is dictated by the fact that the rotor changes its relationship to the piston position due to mechanical advance. So the spark fires when the rotor is in position on the cap, but the position where the piston is when the spark fires is what varies.

I shouldn't say that Phasing is not a thing. It has minor relevance with vacuum advance because the "correct spot" varies by a few degrees of phasing because vacuum advance doesn't change the relationship of the rotor position to the piston position.

Also it comes into play if the distributor is not phased correctly when it is installed in the engine.

But if the distributor is mechanically sound and installed correctly, no amount of advance is going to make that spark fire when the rotor is way off the contact.

Like I said, that's the way I always understood it, as explained by my grandpop.

Bob

.
The spark always fires when the rotor is in the correct position.
 
No, the spark always fires when the reluctor is lined up with the pick up whether or not the timing is set correct for the engine and irregardless of where the rotor is pointing.
 
 
The points open pretty much always when the rotor is in the correct spot.
 
The points open when the points cam contacts them. Changing the points gap slightly changes the position the rotor fires in its rotation.
 
 
 Where the piston is, when the points open, is dictated by the fact that the rotor changes its relationship to the piston position due to mechanical advance.
 
Bingo as I have mentioned a few times earlier.
 
 
So the spark fires when the rotor is in position on the cap, 
 
As far as the mechanical advance goes this is correct. It is incorrect for vacuum advance.
 
 
 but the position where the piston is when the spark fires is what varies.
 
See previous answer.
 

vacuum advance doesn't change the relationship of the rotor position to the piston position.
 
Correct.
 

if the distributor is mechanically sound and installed correctly,  no amount of advance is going to make that spark fire when the rotor is way off the contact.
 
The vacuum advance will cause the spark to occur earlier in the rotors rotation, however, if the rotor was aligned with the cap post with around 10 degrees of static timing, the amount of advance available from any vacuum can would be insufficient to cause the rotor to actually fire very much before the cap post.
.

Cool. So, we agree completely. In my simplified thinking "phasing is not a thing", meaning if the distributor is installed correctly and mechanically sound, there is no "phasing" to worry about or adjust, other than the few degrees required for vacuum advance, and the spark should always fire when the rotor is pretty much under the cap contact, and that this worry about settng "phasing" to allow for full advance and  having the spark jump around in the cap because the rotor is too far from the cap contact is not a thing.

Bob 
 

rbtconsultants wrote:

barnett468 wrote:

rbtconsultants wrote:

The spark always fires when the rotor is in the correct position. The points open pretty much always when the rotor is in the correct spot. Where the piston is, when the points open, is dictated by the fact that the rotor changes its relationship to the piston position due to mechanical advance. So the spark fires when the rotor is in position on the cap, but the position where the piston is when the spark fires is what varies.

I shouldn't say that Phasing is not a thing. It has minor relevance with vacuum advance because the "correct spot" varies by a few degrees of phasing because vacuum advance doesn't change the relationship of the rotor position to the piston position.

Also it comes into play if the distributor is not phased correctly when it is installed in the engine.

But if the distributor is mechanically sound and installed correctly, no amount of advance is going to make that spark fire when the rotor is way off the contact.

Like I said, that's the way I always understood it, as explained by my grandpop.

Bob

.
The spark always fires when the rotor is in the correct position.

No, the spark always fires when the reluctor is lined up with the pick up whether or not the timing is set correct for the engine and irregardless of where the rotor is pointing.


The points open pretty much always when the rotor is in the correct spot.

The points open when the points cam contacts them. Changing the points gap slightly changes the position the rotor fires in its rotation.


Where the piston is, when the points open, is dictated by the fact that the rotor changes its relationship to the piston position due to mechanical advance.

Bingo as I have mentioned a few times earlier.


So the spark fires when the rotor is in position on the cap,

As far as the mechanical advance goes this is correct. It is incorrect for vacuum advance.


but the position where the piston is when the spark fires is what varies.

See previous answer.


vacuum advance doesn't change the relationship of the rotor position to the piston position.

Correct.


if the distributor is mechanically sound and installed correctly,  no amount of advance is going to make that spark fire when the rotor is way off the contact.

The vacuum advance will cause the spark to occur earlier in the rotors rotation, however, if the rotor was aligned with the cap post with around 10 degrees of static timing, the amount of advance available from any vacuum can would be insufficient to cause the rotor to actually fire very much before the cap post.
.

Cool. So, we agree completely. In my simplified thinking "phasing is not a thing", meaning if the distributor is installed correctly and mechanically sound, there is no "phasing" to worry about or adjust, . . .. . this is correct if the parts are made correctly or set up correctly so the rotor will be properly phased  so it is dead center of the post with around 10 degrees of advance . . the aftermarket ones are supposedly phased at 0 but this does not always happen . .  . .          e  m .  re  . . other than the few degrees required for vacuum advance, and the spark should always fire when the rotor is pretty much under the cap contact, . yes . . . . . . . and that this worry about settng "phasing" to allow for full advance and  having the spark jump around in the cap because the rotor is too far from the cap contact is not a thing.
as mentioned it can be and occasionally is but in a perfect world or perfectly  designed or set up distributor it would not be a thing or problem or issue etc lol.
Bob

............

Last edited by barnett468 (9/25/2014 3:11 PM)

Nasty65 wrote:

........ but typically in a daily driver/street car with vac advance it's not a matter of much consequence,.....

Not true.  I had a Prestolite, centrifugal advance 302 boat motor distributor that fired halfway to the next cylinder post.  It ran like crap until I re-phased it.
 

boomyal wrote:

Nasty65 wrote:

........ but typically in a daily driver/street car with vac advance it's not a matter of much consequence,.....

Not true.  I had a Prestolite, centrifugal advance 302 boat motor distributor that fired halfway to the next cylinder post.  It ran like crap until I re-phased it.
 

I said typically, that means "most of the time", there are always exceptions.  Lighten up Francise.