one more about ported or unported dist advance, but maybe for simplier ?

[Nacho-RT74]

ok... I know this has being discussed several times everywhere, but I want to talk of this from my point of view to check if I'm right or wrong. As far I have known there are two kind of vacuum moments and both vacuum advance setups were set for emissions. Let me know If I'm right on this:

I know Chrysler set it up to a ported source, what makes it work just at the initial throttle moment or while throttle is opening. When throttle is steady open or closed, there is not or just a bit of vacuum. right ?

Then I know GM set it up to a manifold vacuum source, which is the same than carb unported sources, right ?. This makes it work opposite to Chrysler, while throttle is closed or steady open. As soon you gives throttle, the vacuum is gone. This gives more initial time at iddle, but then also at a steady RPMs... right ?

before continue with my "doubts" please confirm these statements or correct me if I'm wrong and will continue with some questions.

[c00nhunterjoe]

Correct on terms. When it comes to modified engines, i have found its best to throw the book out the window. Some like ported, some like manifold, others run best on just mechanical advance. Depends on the combination.

[John_Kunkel]

I know Chrysler set it up to a ported source, what makes it work just at the initial throttle moment or while throttle is opening. When throttle is steady open or closed, there is not or just a bit of vacuum. right ?

When the throttle is closed there is little or no ported vacuum but ported vacuum increases with airflow through the venturi so, with the throttle steady open, there is ported vacuum and the amount of vacuum will depend on how much the throttle is open.

BTW, Chrysler wasn't the only one to use ported vacuum, it's all about emissions and they all had to deal with that.

[69wannabe]

I haven't had mine hooked up since 07. Mine runs great on just mechanical advance so I just do without the vacuum advance. I used to block the vacuum line off with a ball and still put it on the distributor and carb just to make it look like it's hooked up but I got lazy and just plugged it off.....

[Nacho-RT74]

ok... let's continue.

If was made for emissions and gas mileage, why the diff on both setups being both completelly opposite. From the iddle point of view, I can see logic the unported source giving advance to save from rich mixture giving time to a full burn of gas. But then, this source is released giving throttle and just mech works, so, to Chrysler the ported source works allong with mech advance and you get gas mileage and save a bit of rich mixture given by the carb pump.

Or something like that.

Two diff theories on how must work the vacuum dist for emissions and mileage. I have seen this debate once and again and again. Sometimes I understand better why one or the other and sometimes not.

I'm stuck at this moment with the Chrysler ported design, for no other reason than it is what it how was made my car

[BLK 68 R/T]

Did some searching on vacuum advance and came across this article.

http://www.superchevy.com/how-to/additional-tech/1601-everything-you-wanted-to-know-about-vacuum-advance-and-ignition-timing/

[Nacho-RT74]

there is a nice thread being talked at FBBO, but I haven't detailed it completelly. I'm trying to be simplier as posible

[Nacho-RT74]

ok... let's continue.

If was made for emissions and gas mileage, why the diff on both setups being both completelly opposite. From the iddle point of view, I can see logic the unported source giving advance to save from rich mixture giving time to a full burn of gas. But then, this source is released giving throttle and just mech works, so, to Chrysler the ported source works allong with mech advance and you get gas mileage and save a bit of rich mixture given by the carb pump.

Or something like that.

Two diff theories on how must work the vacuum dist for emissions and mileage. I have seen this debate once and again and again. Sometimes I understand better why one or the other and sometimes not.

I'm stuck at this moment with the Chrysler ported design, for no other reason than it is what it how was made my car

I told rich, but is lean...

[Nacho-RT74]

Did some searching on vacuum advance and came across this article.

http://www.superchevy.com/how-to/additional-tech/1601-everything-you-wanted-to-know-about-vacuum-advance-and-ignition-timing/

and being Chevy mag/website of course is toward to manifold ( unported ) source

[Nacho-RT74]

here is the FBBO thread, which also get the initial post based on GMC theory

https://www.forbbodiesonly.com/moparforum/threads/ported-or-constant-vacuum-source-again.145207/

Now also wondering about pinging, timing and vacuum source. If pinging uses to come on more with lean mixtures, why I need to retard the timing to save that when advancing could save the pinging? ( or it's at least what I recall, advance is not helpfull for pinging, right ? ).

next thought.

Did Chrysler allways use ported sources or did sometime change to manifold sourced based on this theory allong the years ?

Next...

when you give throttle, carb gas pump enriches the mixture, so... you will be "breaking" emissions on that moment... on this moment, Chrysler advances being ported.

sorry... lot of questions coming out in disorder out of my mind

[375instroke]

I took emissions training in high school in the late '80s as part of auto shop, and I remember them talking about all the ways people try to f*** their car up thinking something was done for emissions, so they change it.  One is vacuum advance.  Race cars don't have it, so my car must be faster without it.  Moving ported to manifold was another.  At idle, if you're connected to manifold vacuum, you get a lot more advance, and probably have to retard the initial to compensate, but you also get a feedback loop caused by the idle speed changing vacuum, vacuum changing timing, and timing changing idle speed, and you end up hunting back and forth your idle mixture, timing, idle speed, and get this surging.  Also, if you ever drive on the street, vacuum advance gives you around 50° cruising, causing a cooler running engine, and less gas washing your rings.  Why don't you want that?  It hurts absolutely nothing.

[Nacho-RT74]

  At idle, if you're connected to manifold vacuum, you get a lot more advance, and probably have to retard the initial to compensate, but you also get a feedback loop caused by the idle speed changing vacuum, vacuum changing timing, and timing changing idle speed, and you end up hunting back and forth your idle mixture, timing, idle speed, and get this surging.

that's a good debating point I have allways thought

[flyinlow]

I would use vacuum advance on a street car. Set up the base timing and centrifugal advance first. Then add what ever vacuum advance you can for highest manifold vacuum readings at idle and part throttle.

First choice would be manifold vacuum to the distributor (GM style). Second choice would be ported vacuum to the distributor (Mopar style).

I run 24* base plus 12* centrifugal at 2500rpm. ( 36* total mechanical) plus ported vacuum  with the carb and manifold vacuum after I switched to EFI. I had to restrict the vacuum advance unit to about 8* for the best running. This is about 1/2 of the factory amount.