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Vacuum secondary or double pumper?

My vote would be a vacuum secondary carb, specifically the Edelbrock AVS2 series. Much easier to tune for me than a Holley and they work great on the street.
 
I've run several cars and even a couple of pickups with DP's and the ones that benefited the most were ones with 3.91 gears or more, loose converters or stick cars. But, one 4 speed car had 2.80 gears! That small block Ferd got lousy mileage in town but got 20-21 out on the highway and the very mild 289 pushed a 66 Mudstain Basfack 15.00 on the 1/4 mile. Not too shabby for 1975. However, a TQ on a 71 340 Cuda was the best and dang near maintenance free while the DP's needed tuning a lot more often....
 
BSB67 said:
Kudos to you. That is exactly what I would ask. Despite not being from Missouri, I'm a show me guy too.

Data, maybe some somewhere. But at some point you need to choose what you want to believe based on the source. I'm fascinated by how there can be a ground swell of miss information that cannot be unravelled.

Generally I don't talk about the road I've traveled in my hot rodding history to get where I'm at today. I'll just hit a few highlights.

Almost all my experience is on street cars. Probably over 1000 track passes with my own stuff, and that pales in comparison to all the street racing I did. Along with that, there was a core group of us, usually 4 or 5 other guys that were doing the the same thing, kinda of a competition to out do each other on a week to week basis. I would change camshafts on Saturday mornings. We tried and traded carburetors all the time. All the: is a Holley better or a Carter, is big better than small, is vacuum better than DP was tested and concluded 40 years ago. But even so, today I've still got about 12 carburetors sitting around here, including a 3310-1 as you know is actually a 780 with the down leg secondary booster and rear metering body.

The stuff I post always comes from real personal experience, or observation paired with experience.

At the risk of sounding arrogant, when I go to the track, most folks that come by are very surprised what my cars run considering what they are.

Take it for what it's worth.
Click to expand...

BSB67,
Thank you for your candour. I'm a registered professional engineer, who, throughout my career depended on accuracy and documentation of all of the projects l was involved with....from small process requirements to building green field plants with all processes and equipment. Documentation and details became a part of my life. That was the reason for the request.
The original Holley 4150 R-3310 780 CFM, eveloped for the Corvette L88 application was a great carb. It was easily adaptable. The R-3310-1 model 4150 780 CFM with the secondary metering block, down leg secondary boosters was (is) easily tuned for just about all applications Later, Holley slightly modified it to a 4160 design and rated it at 750 CFM. I've used DP designs b4...they work quite well when applied to the right engine / car combination....free breathing engine, high numeric rear axle ratio light weight car, light weight flywheel (std trans) or high stall speed converter (auto trans). For street use in a slightly modified engine, low numeric rear axle ratio in s heavy car, I'll stick with the vacuum operated secondaries. Just my opinion of course.
Bob Renton
 
I'd go VS for a mostly street car. Which direction you go is often best decided on what you already have available. Do you have any tuning or rebuild kits for Carterbrock or Holley already?

When I sold the six pack that was on the 440 in my 64 last year, I had a 4160 750cfm carb on the shelf and spare parts and jets so put that on, then upgraded it with a quick change spring, replaced the rear metering plate with one that had jets, I also got jet extension tubes and the requisite rear float. Works pretty good.
 
threewood said:
My vote would be a vacuum secondary carb, specifically the Edelbrock AVS2 series. Much easier to tune for me than a Holley and they work great on the street.
Click to expand...


But the AVS is not a vacuum secondary carb. Ron
 
383man said:
But the AVS is not a vacuum secondary carb. Ron
Click to expand...

Sure it is.
Adjustable Vacuum Secondary (AVS)
The secondary plates are mechanical but they are operated off of engine demand (vacuum) not throttle position.
 
threewood said:
Sure it is.
Adjustable Vacuum Secondary (AVS)
The secondary plates are mechanical but they are operated off of engine demand (vacuum) not throttle position.
Click to expand...
Gentlemen,
AVS stands for Air Valve Secondary....this is Carter's terminology. The adjustable secondary air valve responds to AIR FLOW thru the secondary butterfly / fuel admission nozzles. This is IDENTICAL to Holley's vacuum operated secondaries, which operate in response to the air flow thru the PRIMARY venturii by measuring the pressure drop thru the venturi. This response is adjustable via the SPRING TENSION on the secondary operating diaphragm, which equates to adjusting the tension on the secondary Air Valve's operating spring. Are both carbs on demand types.....YES. The methodology of achieving the end result is slightly different but the end result is the same.... an adjustable secondary system.
This is just my opinion.
Bob Renton
 
RJRENTON said:
Gentlemen,
AVS stands for Air Valve Secondary....this is Carter's terminology. The adjustable secondary air valve responds to AIR FLOW thru the secondary butterfly / fuel admission nozzles. This is IDENTICAL to Holley's vacuum operated secondaries, which operate in response to the air flow thru the PRIMARY venturii by measuring the pressure drop thru the venturi. This response is adjustable via the SPRING TENSION on the secondary operating diaphragm, which equates to adjusting the tension on the secondary Air Valve's operating spring. Are both carbs on demand types.....YES. The methodology of achieving the end result is slightly different but the end result is the same.... an adjustable secondary system.
This is just my opinion.
Bob Renton
Click to expand...


That's correct but the AVS is not considered a vacuum operated carb since the throttle plates open mechanically. And AVS stands for Air valve Secondary. Only the air valve above the throttle plates open by eng demand by the low pressure under the air valve and the atmospheric pressure above the air valve. Pretty much all mechanical secondary four barrel carbs use some type of an air valve to control eng bog and stumble when you floor it. The AFB's use a weighted air valve in them. The Quadrajet uses a secondary air valve just like the Thermoquad does that its opening is controlled by spring tension and the choke pull-off diaphragm. The Ford Autolite carb uses an air valve that is controlled by spring tension and a damper piston in the fuel chamber. And even the old Rochester 4GC carb is a mechanical secondary carb that uses an air valve.The only carbs that I can think of that are considered vacuum operated secondary four barrel's are the Holley's and the old Autolite carb used in the 60's that also used a vacuum pod to open the secondaries. A vacuum secondary carb means the secondary throttle plates are opened by a vacuum pod. All the carbs that I mentioned that use an air valve are considered mechanical operated secondaries because the throttle plates are opened mechanically and not by a vacuum pod. Only the Holleys and the old Autolite carb that use the vacuum pod to open the secondary throttle plates are considered as vacuum opperated secondary carbs. Ron
 
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383man said:
That's correct but the AVS is not considered a vacuum operated carb since the throttle plates open mechanically. And AVS stands for Air valve Secondary. Only the air valve above the throttle plates open by eng demand by the low pressure under the air valve and the atmospheric pressure above the air valve. Pretty much all mechanical secondary four barrel carbs use some type of an air valve to control eng bog and stumble when you floor it. The AFB's use a weighted air valve in them. The Quadrajet uses a secondary air valve just like the Thermoquad does that its opening is controlled by spring tension and the choke pull-off diaphragm. The Ford Autolite carb uses an air valve that is controlled by spring tension and a damper piston in the fuel chamber. And even the old Rochester 4GC carb is a mechanical secondary carb that uses an air valve.The only carbs that I can think of that are considered vacuum operated secondary four barrel's are the Holley's and the old Autolite carb used in the 60's that also used a vacuum pod to open the secondaries. A vacuum secondary carb means the secondary throttle plates are opened by a vacuum pod. All the carbs that I mentioned that use an air valve are considered mechanical operated secondaries because the throttle plates are opened mechanically and not by a vacuum pod. Only the Holleys and the old Autolite carb that use the vacuum pod to open the secondary throttle plates are considered as vacuum opperated secondary carbs. Ron
Click to expand...
Ron,
I think the METHODOLOGY of introducing additional air flow/fuel flow is a matter of semantics. Yes, Holley and the old Ford /Motorcraft 4100 series measured primary venturii pressure to achieve the secondary opening method; once the air flow begins, the fuel feeding systems begin to function. The AVS, Q-JET, THERMOQUAD, MOTORCRAFT 4300, etal, operate as you described BUT the important criteria is the fuel is added to the air stream as the demand for it increases as determined by the engine air flow. In short, it can be said: "the difference is the same"....seamless operation.
Bob Renton
 
RJRENTON said:
Ron,
I think the METHODOLOGY of introducing additional air flow/fuel flow is a matter of semantics. Yes, Holley and the old Ford /Motorcraft 4100 series measured primary venturii pressure to achieve the secondary opening method; once the air flow begins, the fuel feeding systems begin to function. The AVS, Q-JET, THERMOQUAD, MOTORCRAFT 4300, etal, operate as you described BUT the important criteria is the fuel is added to the air stream as the demand for it increases as determined by the engine air flow. In short, it can be said: "the difference is the same"....seamless operation.
Bob Renton
Click to expand...

I agree the method is different, results essentially the same. Still very different than the true mechanical secondary style.
 
Basically what I am saying is all my life everyone I know always called only the carbs that use a vacuum pod to open the secondaries a vacuum operated carb. And I guess thats because the others with air valve secondaries all use a mechanical method to open the secondary throttle plates. Even any book I have read says the same. Sure the air valve will work based on eng demand and spring pressure and the choke pull-off bleed off against air flow. But I have just never heard anyone call them a vacuum secondary carb. Not trying to make an issue out of it at all but I just never heard anyone call the AVS or AFB or any air valve secondary carb a vacuum operated carb. Ron
 
66Satellite47 said:
I agree the method is different, results essentially the same. Still very different than the true mechanical secondary style.
Click to expand...


Actually about the only mechanical secondary carb I can think of that don't use an air valve is the Holley double pumper carbs. And of course they have to use a secondary accel pump to eliminate any bog. Which of course is what the air valve is for on any mech secondary carb that don't have a secondary accel pump. Pretty much all mechanical secondary carbs use either an accel pump or an air valve in the secondary side to stop any bog/hesitation. Ron
 
383man said:
Actually about the only mechanical secondary carb I can think of that don't use an air valve is the Holley double pumper carbs. And of course they have to use a secondary accel pump to eliminate any bog. Which of course is what the air valve is for on any mech secondary carb that don't have a secondary accel pump. Pretty much all mechanical secondary carbs use either an accel pump or an air valve in the secondary side to stop any bog/hesitation. Ron
Click to expand...

Actually, certain models of Q-Jet carbs, I believe Chevy and Pontiac applications , use a POE (Pull Over Enrichment) feature with calibrated orifices in the air horn, directly above the secondary air valve, which, when exposed to vacuum, just as the air valve blade starts to open, begin to feed fuel (similar to an accelerator pump's operation) to eliminate any momentary lean condition, until the main nozzles begin feeding fuel. Perhaps the "best" of both designs....double pump vs AVS designs. Just my opinion of course.
Bob Renton
 
RJRENTON said:
Actually, certain models of Q-Jet carbs, I believe Chevy and Pontiac applications , use a POE (Pull Over Enrichment) feature with calibrated orifices in the air horn, directly above the secondary air valve, which, when exposed to vacuum, just as the air valve blade starts to open, begin to feed fuel (similar to an accelerator pump's operation) to eliminate any momentary lean condition, until the main nozzles begin feeding fuel. Perhaps the "best" of both designs....double pump vs AVS designs. Just my opinion of course.
Bob Renton
Click to expand...


Yes that's right as some Quadrajet books call that a seconday accel pump system even though its no pump it just pulls the fuel out as the air valve opens. The air valve also raises the secondary metering rods as it opens on the Quadrajet also. The newer Eddy carbs also use a system like that on some. I cant remember if its the AFB or AVS or both as I have to look it up again. Ron
 
I have a 750 holley dp on my '74 motorhome 440 with headman hedders and dominator intake. only had to turn the screws about 3/4 out but it runs good.
 
66Satellite47 said:
The Carters lift the rods too.
Click to expand...

No they don't lift the metering rods by the air valve. Only the Thermoquad has a mechanical help in lifting its primary metering rods but they are not lifted by the air valve and they are the primary metering rods that are worked by a vacuum piston. Then they have a plate on the carb base in the center that has a cam that lifts the plate as you step on the throttle to assist the vacuum operated primary metering rods.
The Quadrajet also has a vacuum piston that works its primary metering rods as the secondary ones are the ones that the air valve pulls up as it opens. The Thermoquad is the only Carter four barrel I have ever seen with the vacuum/mechanical primary rods as the AVS and AFB's are vacuum only one their primary rods. They like the thermoquad don't use secondary rods. Ron
 
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Ron, OK, guess I mis-read your post. My aftermarket AFB has rods on the PRIMARY only. Sorry for the confusion. I'm still a Holley guy, for better or worse.
 
66Satellite47 said:
Ron, OK, guess I mis-read your post. My aftermarket AFB has rods on the PRIMARY only. Sorry for the confusion. I'm still a Holley guy, for better or worse.
Click to expand...


No big deal as I enjoy talking cars or carbs or whatever. And many times I learn more things about out cool Mopars. Ron
 
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