Recently there has been a lot of discussion regarding
vacuum advance.
While there seems to be a pretty good understanding
of the functions, in believe that some of the confusion
could be removed by clarification of the basics.
Correct me if I am wrong but:
VACUUM RETARD
On the 455 the distributor has a vacuum advance
unit.
If the vacuum pot sees vacuum it gives you additional
advance.
If the vacuum pot sees no vacuum it gives you no
additional advance. =
It would be clearer if the word retard was not used
since on these engines the vacuum advance unit never
"retards" the advance.
The vacuum advance unit may, if it so chooses provide
additional advance, (or not) but it never "retards".
PORTED VERSUS MANIFOLD VACUUM
I think you would find that ported vacuum is just a way
of keeping the vacuum advance unit from seeing vacuum
at very small throttle positions.
If I remember correctly this was done for polution control
reasons.
Therefore with Ported Vacuum, when the engine is seeing
very small throttle openings (butterflys at the idle position)
the vacuum advance unit does not see any vacuum.
This prevents high advance not only at idle but also at very
high vacuum conditions like going down a hill with your foot
off the throttle.
Additionally (If I remember correctly) Ported vacuum does
not do anything else to the vacuum. When ported vacuum
allows vacuum to the distributor it is the vacuum created in
the intake manifold.
Ported vacuum is not a vacuum generator or anything it is
simply a way of shutting off the vacuum at very small throttle
openings.
I hope I got it straight.
It has been a long time since I studied the theory.
And if I got it wrong I may have started additional confusion.
Regards
Gary Zingle
1973 GMC 26 foot
vacuum advance.
While there seems to be a pretty good understanding
of the functions, in believe that some of the confusion
could be removed by clarification of the basics.
Correct me if I am wrong but:
VACUUM RETARD
On the 455 the distributor has a vacuum advance
unit.
If the vacuum pot sees vacuum it gives you additional
advance.
If the vacuum pot sees no vacuum it gives you no
additional advance. =
It would be clearer if the word retard was not used
since on these engines the vacuum advance unit never
"retards" the advance.
The vacuum advance unit may, if it so chooses provide
additional advance, (or not) but it never "retards".
PORTED VERSUS MANIFOLD VACUUM
I think you would find that ported vacuum is just a way
of keeping the vacuum advance unit from seeing vacuum
at very small throttle positions.
If I remember correctly this was done for polution control
reasons.
Therefore with Ported Vacuum, when the engine is seeing
very small throttle openings (butterflys at the idle position)
the vacuum advance unit does not see any vacuum.
This prevents high advance not only at idle but also at very
high vacuum conditions like going down a hill with your foot
off the throttle.
Additionally (If I remember correctly) Ported vacuum does
not do anything else to the vacuum. When ported vacuum
allows vacuum to the distributor it is the vacuum created in
the intake manifold.
Ported vacuum is not a vacuum generator or anything it is
simply a way of shutting off the vacuum at very small throttle
openings.
I hope I got it straight.
It has been a long time since I studied the theory.
And if I got it wrong I may have started additional confusion.
Regards
Gary Zingle
1973 GMC 26 foot
