Lets talk about Vacuum Advance. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

jorgensensc · 10-19-2008, 08:34 PM

When you hook the Vacuum advance up to the carb, and vacuum is created, does that advance the timing and if so help me understand the concept. Full vacuum is at idle right? So at idle does it advance the timing, then reduce timing as you drive/ accelerate? This doesn't make sense to me (not much does I think). If you over advance your dizzy when you are setting timing, It will have a hard time cranking, but it tends to run better at speed and has quicker response (Unless you way over advance it and then you get the popping in the carb, etc). So why would you want the timing advanced at idle and reduced at speed? Or am I completely Backward in my thinking. Also on the carb I have always been told that the dizzy advance is supposed to be plugged in to the carb port that is NOT Manifold vacuum (usually on Edelbrocks the drivers-side), however I have always been told to use the opposite port (Passenger-side), the constant manifold port for better performance. Can someone please shed some light on this?
Shawn

Cyclone · 10-19-2008, 08:38 PM

Should be no vac at idle, then vac as you increase throttle, its called ported vacuum. should not be connected to manifold (constant) vacuum.
Get a cheapy vac guage and read all your ports at idle and full and partial throttle, you'll find the ported one.

jorgensensc · 10-19-2008, 08:47 PM

Okay, here is where I am lost. Does the ported vacuum work opposite of the other ports? I always thought there was vacuum at idle and as the throttle went up the vacuum went down. Am I wrong?

Cyclone · 10-19-2008, 09:01 PM

There is ported vacuum only when throttle is opened. Manifold vacuum will be there at idle/cruise and if you
crack the throttle quickly you will see the vacuum drop alot then retur to whatever your idle vacuum was. Clear as mud huh?

68gmsee · 10-19-2008, 09:29 PM

I read about vacuum advance sometime back. Here's a couple of sites to explain it.

http://www.2quicknovas.com/happytiming.html

http://www.highperformancepontiac.co...ech/index.html

Here's a short piece...

The carburetor is designed to provide minimal amounts of fuel and air in light-load conditions such as part-throttle cruise. But because cylinder volume doesn't change, the molecules within the chamber are not as tightly compacted, resulting in a less combustible mixture. Not only is such a mixture more difficult to ignite, the flame spreads slower, delaying peak pressure. For maximum performance in these conditions, spark must occur even earlier, sometimes in excess of 50 degrees BTDC.

10-19-2008, 08:34 PM	#1
jorgensensc Registered User Join Date: Jul 2003 Location: Cypress, Tx Posts: 4,005	Lets talk about Vacuum Advance. When you hook the Vacuum advance up to the carb, and vacuum is created, does that advance the timing and if so help me understand the concept. Full vacuum is at idle right? So at idle does it advance the timing, then reduce timing as you drive/ accelerate? This doesn't make sense to me (not much does I think). If you over advance your dizzy when you are setting timing, It will have a hard time cranking, but it tends to run better at speed and has quicker response (Unless you way over advance it and then you get the popping in the carb, etc). So why would you want the timing advanced at idle and reduced at speed? Or am I completely Backward in my thinking. Also on the carb I have always been told that the dizzy advance is supposed to be plugged in to the carb port that is NOT Manifold vacuum (usually on Edelbrocks the drivers-side), however I have always been told to use the opposite port (Passenger-side), the constant manifold port for better performance. Can someone please shed some light on this? Shawn __________________ 1972 C20 Suburban- Big Blue Betty '56 Chevy Bel Air Sedan- Frame up Restoration -What would you attempt to achieve if you knew you could not fail?- -I Refuse To Tiptoe Through Life, Only To Arrive Safely At Death's Door- R.I.P. EAST SIDE LOW LIFE

10-19-2008, 08:38 PM	#2
Cyclone Registered User Join Date: Oct 2004 Location: Sonoma, CA Posts: 318	Re: Lets talk about Vacuum Advance. Should be no vac at idle, then vac as you increase throttle, its called ported vacuum. should not be connected to manifold (constant) vacuum. Get a cheapy vac guage and read all your ports at idle and full and partial throttle, you'll find the ported one. Last edited by Cyclone; 10-19-2008 at 08:39 PM.

10-19-2008, 08:47 PM	#3
jorgensensc Registered User Join Date: Jul 2003 Location: Cypress, Tx Posts: 4,005	Re: Lets talk about Vacuum Advance. Okay, here is where I am lost. Does the ported vacuum work opposite of the other ports? I always thought there was vacuum at idle and as the throttle went up the vacuum went down. Am I wrong? __________________ 1972 C20 Suburban- Big Blue Betty '56 Chevy Bel Air Sedan- Frame up Restoration -What would you attempt to achieve if you knew you could not fail?- -I Refuse To Tiptoe Through Life, Only To Arrive Safely At Death's Door- R.I.P. EAST SIDE LOW LIFE

10-19-2008, 09:01 PM	#4
Cyclone Registered User Join Date: Oct 2004 Location: Sonoma, CA Posts: 318	Re: Lets talk about Vacuum Advance. There is ported vacuum only when throttle is opened. Manifold vacuum will be there at idle/cruise and if you crack the throttle quickly you will see the vacuum drop alot then retur to whatever your idle vacuum was. Clear as mud huh?

10-19-2008, 09:29 PM	#5
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Lets talk about Vacuum Advance. I read about vacuum advance sometime back. Here's a couple of sites to explain it. http://www.2quicknovas.com/happytiming.html http://www.highperformancepontiac.co...ech/index.html Here's a short piece... The carburetor is designed to provide minimal amounts of fuel and air in light-load conditions such as part-throttle cruise. But because cylinder volume doesn't change, the molecules within the chamber are not as tightly compacted, resulting in a less combustible mixture. Not only is such a mixture more difficult to ignite, the flame spreads slower, delaying peak pressure. For maximum performance in these conditions, spark must occur even earlier, sometimes in excess of 50 degrees BTDC.



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