Vacuum advance revisited.
March 14th, 2008, 11:16 PM
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Vacuum advance revisited.
A sidebar to this thread beginning at post #4, with the following exchange:
Good post.
It continues because few, if any, of the participants fully understand the concept.
As my statement was applied to this topic, I disagree with your disagreement.
Not possible. Both positions draw the same vacuum, from the same source.
Corvettes, Pontiacs, Oldsmobiles, and Volkswagens, are all subject to the same laws of physics.
In this case, both authors advocate the use of “full time” manifold vacuum to “improve” the idle of a performance cam. Nothing but a band aid to cover a mismatched advance curve.
Norm
........ make sure both the centrifugal and vacuum advance mechanisms in the distributor are functioning correctly and that the initial timing is set right for your engine.There is a longtime running debate over vacuum advances and what they do. I agree that on race engines it is seldom used. Not to step on anyone's toes, but I respectfully disagree with broad statements like vacuum advance has nothing to do with power output. Rather I would say that it depends on what type of power you are talking about. Vacuum advance has a lot do do with part throttle power, good driveability, throttle response, and fuel economy on a street driven vehicle. If you are running near redline most of the time, then it is not necessary, centrifugal only is sufficient. Also, if the source to the advance is ported vacuum and not manifold vacuum, then the supply to the advance will not be zero under a load. This article may be of interest:
http://65corvette.nonethewiser.net/t...um_advance.pdf
It is mainly aimed at corvette owners, but good reading.
This is aimed at Pontiac owners, but also good info:
http://www.highperformancepontiac.co...uum_curve.html
I brought this topic up because the last word of the thread opener was inexpensively, and these 2 important distributor components are often overlooked. Centrifugal advances get stuck, lose weights and are easily repaired, or modified for more or quicker advance curves. Vacuum pots blow diaphrams and stop functioning entirely. Many late 60's and early 70's Olds engines had devices to limit the vacuum advance for reduced NOX emissions, ie transmission controlled spark advance and if they are malfunctioning, power output will be down and fuel economy will suffer. If you are driving a hot rod, chances are you have already bypassed these devices, but if you are driving a stocker you may want to investigate whether they are working correctly given that gas is hovering at 4.00/gallon.
http://65corvette.nonethewiser.net/t...um_advance.pdf
It is mainly aimed at corvette owners, but good reading.
This is aimed at Pontiac owners, but also good info:
http://www.highperformancepontiac.co...uum_curve.html
I brought this topic up because the last word of the thread opener was inexpensively, and these 2 important distributor components are often overlooked. Centrifugal advances get stuck, lose weights and are easily repaired, or modified for more or quicker advance curves. Vacuum pots blow diaphrams and stop functioning entirely. Many late 60's and early 70's Olds engines had devices to limit the vacuum advance for reduced NOX emissions, ie transmission controlled spark advance and if they are malfunctioning, power output will be down and fuel economy will suffer. If you are driving a hot rod, chances are you have already bypassed these devices, but if you are driving a stocker you may want to investigate whether they are working correctly given that gas is hovering at 4.00/gallon.
In this case, both authors advocate the use of “full time” manifold vacuum to “improve” the idle of a performance cam. Nothing but a band aid to cover a mismatched advance curve.
Norm
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djcrook73
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February 25th, 2012 10:37 AM
