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AVS vs. Vacuum sacondary carburetors.
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FSJunkie View Drop Down
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    Posted: Mar/08/2013 at 4:29pm
Vacuum secondary carburetors monitor primary venturi vacuum to initiate secondary throttle opening once venturi vacuum (restriction) reaches a threshold.  As the secondaries open, another port monitors secondary venturi vacuum for a reason I have not yet figured out. -Presumably to aid the secondaries to open more than primary vacuum alone can achieve. 
 
Air Valve Secondary (AVS) carburetors open the secondary throttles mechanicaly when the primaries are 2/3 open and progressively increase the opening rate so the secondaries reach fully open when the primaries do. A damper valve over the entrance to the secondaries controls how much airflow is avalible depending on how hard the engine is sucking on the carburetor (manifold vacuum).
 
Both methods achieve the same goal of preventing secondary operation untill the primary bores have restricted airflow to a threshold and then regulate the secondary flow to prevent over-carburetion and the compromise of booster signal and atomization.
 
What are the benefits of one design over the other? How do they feel different in the way they drive?  Let us discuss the pros and cons of each design, their tuning, and the experience in driving them.
 
 
Note: this is not another "Edelbrock vs. Holley" debate.  This involves all carburetors of each secondary design: Holleys, Demons, WCFBs, AFBs, AVSs, Quadrajets, Thermoquads, etc.


Edited by FSJunkie - Mar/08/2013 at 4:32pm
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My understanding of the secondary vacuum passage that "T"'s into the vac signal is maybe it keeps the secondaries from closing if you back off the throttle or hesitate getting on it after hitting it.  I have driven both types, but don't see any difference.  I have had both that the secondaries were not quite right or did not open but that was a bent linkage or tuning problem and not a design flaw.  Working properly, not really a big difference.
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Seems to me like the AVS would be better for acceleration since the secondaries can respond instantly and right when they're needed, but the vacuum secondary carb would be better for steady state cruising since the secondaries can move freely as needed like when climbing up a hill.

For the holley on the 327, I don't have to move my foot much when going up and down hills as the secondaries freely open and close to maintain engine output as the load changes even though the primaries are only 1/3-1/2 open.  A tiny movement of the primaries can envoge a much greater change in secondary opening. It makes it easier to drive for someone who likes to maintain constant speed like me.
 
That effect on an AVS carb like a Motorcraft 4300 would not happen since the secondaries are not open untill 2/3-3/4 throttle minimum.  Even then, any increase in primary throttle would lower manifold vacuum and tend to close the damper valve untill the primaries were closed back down and manifold vacuum raised enough to open the damper. On the other hand, the primary throttles will try to counteract both of those conditions.  
 
 
 


Edited by FSJunkie - Mar/08/2013 at 5:56pm
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AFBs are counterbalanced air valve carbs and do not rely on manifold vacume to open the secondary dampers.  They are airflow regulated.
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WCFBs, AFBs, AVSs, Quadrajets, Thermoquads  are air flow regulated . Holleys and demons use venturi vacume .
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Originally posted by 69 ambassador 390 69 ambassador 390 wrote:

AFBs are counterbalanced air valve carbs and do not rely on manifold vacume to open the secondary dampers.  They are airflow regulated.
Yes, airflow enduced by vacuum in the intake manifold. Stronger vacuum, greater airflow.  Vacuum on the underside of the damper to open it against atmospheric pressure above.
 
We're saying the same thing.
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Vacuum Secondary Carbs can also be made to respond virtually instantaneously by putting a lighter spring in the secondary pod.   Adding an aftermarket "quick-change" top to the pod greatly increases tuneability.  With AVS Carbs, you need to disassemble and remove material from the counterweight to accomplish the same thing.

Win: Vac. 2nd-ary.
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AVS/Q-Jet you adjust the secondary air door with spring tension. AFB adjust it by adding or grinding weight off of the secondary counterweight.
 
 
As far as Holley, I had the great fortune of personally knowing a Holley engineer. I've shared some of his info in the past to only have it refuted by someone who read a book they bought at Summit, soooooo, I won't even go there anymore.
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Originally posted by FSJunkie FSJunkie wrote:

Originally posted by 69 ambassador 390 69 ambassador 390 wrote:

AFBs are counterbalanced air valve carbs and do not rely on manifold vacume to open the secondary dampers.  They are airflow regulated.
Yes, airflow enduced by vacuum in the intake manifold. Stronger vacuum, greater airflow.  Vacuum on the underside of the damper to open it against atmospheric pressure above.
 
We're saying the same thing.
Actually thats exactly backwards from the truth.  As the pressure drops in the inlet side, the air valve is pushed open by atmosheric pressure.  But, if there is not enough airflow the valve will not be acted upon by enough air mass to open.  Has nothing to do with vacume under the throttle plates at all.  Because the secandary air valves are counter balanced and also shaped to help lift themselves open they are not directly proportionate to vacume or pressure differential acting upon the total surface area presented to the incoming airstream.  The air valves are airfoils and are designed in such a way as to mantain a certain angle of attack to the airstream at certain flow rates.  The center of lift is offset from the center of moment so that they are rotated open in relation to the amount of lift they are generating.  When the air valves are fully open, they are presenting nearly no surface to the "manifold vacume" to be acted upon.  Read up on a guy named Bernoulli a bit and also check out a few articles on fluid dynamics and eddy currents. 

Edited by 69 ambassador 390 - Mar/08/2013 at 9:11pm
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Originally posted by 69 ambassador 390 69 ambassador 390 wrote:

Originally posted by FSJunkie FSJunkie wrote:

Originally posted by 69 ambassador 390 69 ambassador 390 wrote:

AFBs are counterbalanced air valve carbs and do not rely on manifold vacume to open the secondary dampers.  They are airflow regulated.
Yes, airflow enduced by vacuum in the intake manifold. Stronger vacuum, greater airflow.  Vacuum on the underside of the damper to open it against atmospheric pressure above.
 
We're saying the same thing.
Actually thats exactly backwards from the truth.  As the pressure drops in the inlet side, the air valve is pushed open by atmosheric pressure.  But, if there is not enough airflow the valve will not be acted upon by enough air mass to open.  Has nothing to do with vacume under the throttle plates at all.  Because the secandary air valves are counter balanced and also shaped to help lift themselves open they are not directly proportionate to vacume or pressure differential acting upon the total surface area presented to the incoming airstream.  The air valves are airfoils and are designed in such a way as to mantain a certain angle of attack to the airstream at certain flow rates.  The center of lift is offset from the center of moment so that they are rotated open in relation to the amount of lift they are generating.  When the air valves are fully open, they are presenting nearly no surface to the "manifold vacume" to be acted upon.  Read up on a guy named Bernoulli a bit and also check out a few articles on fluid dynamics and eddy currents. 
Right, and I understand completely what you're saying.
 
So regardless of engine vacuum, as soon as the secondary throttle blades open the rush of airflow will open the damper valves to regulate the airflow and by a related effect, regulate manifold vacuum and power output, right? isn't it also true then that the higher engine manifold vacuum is, the more airflow will be pulled through the secondaries and therefore, open the damper valve?
 
I think we're just getting all caught up in termenology and wording. 
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