Carb question: Vac secondary vs Mech secondary

[Purple440]

..That 230 @.050 Cam is alot of duration for a typical, Low comp.,street motor..

C/R is 9.5:1....static pressure was measured around 165-170 with one plug out at a time.

Does anyone know if the AVS air door can be tightened to the point of non function or even nearly so?

Yes, I've done that.  If it's too tight you can "feel" the secondaries open at a certain RPM.  I've tuned it so I can't feel it, which is where it's running best.  Also, if it's too loose, it hesitates on takeoff.

- Doug

[G-man]

From all my research and excessive question asking from numerous engine builders....

I dont know why they even make vacuum secondary carburettors for.

A: Mechanical (double pumpers) are more fuel efficient by design than a Vacuum secondaries because the secondaries can not open unless you make them open with your foot by pressing the accelerator pedal to the point where the secondaries do open up at. This means, a vacuum carbie may open/close/open/close of its own accord when you are cruising at 60mph while the Mechanical will not open and thus more efficient. Reason they work out less economical is because of the driver constantly playing with the secondaries (this has nothing to do with carb though) Hence the carb butterflies only open according to what your foot is doing on the accelerator as its mechanicaly controled by the distance the pedal is pressed by and thus can NOT open ever unless U make it.

B: Mechanicals get rid of that dumb choke you do not need.

C: Mechanicals get a deeper note which sounds way better

D: Mechanicals are much more responsive. To give you an example... a standard auto gearbox vs a manual gearbox... hit accelerator in both cars and the auto delays before it goes while manual goes soon you press it. Same idea here with mechanical/vacuum.

E: Mechanicals do what you want when your foot wants while vacuum has a mind of its own. Ive tried both under WOT and believe you me, the mechanicals do it soon u press the pedal to floor all 4  barrels are open while vacuum will decide a little later to do it and therefor you feel that lag.

F: Mechanicals look way better also (HP Holleys for example)

G: Mechanicals perform 2nd to none.

G: Hence I said I dont know why on earth they even make vacuums, they are not as efficient (get more miles out of a mechanical), not as responsive, dont do what u want when u want and dont look good either and dont sound as good and performance wise they got nothing on mechanicals. Less performance and more fuel cost...

Vaccums = for the bin.

[Ghoste]

I wopuld disagree with the fuel one.  A vacuum secondary should first of all be tuned to match the points where YOUR engine wants the secondary side to come in.  Keep in mind I am talking about a street driven car here.  You can also tailor the linkage on the mechanical one but wherever you set it, the secondaries will come and the second accelerator pump is going to dump fuel in no matter what the engine conditions are at the time.  I would need to see some serious proof before buying into the idea that an engine set up like that is using less fuel than a vacuum secondary coming in when vacuum levels drop (such as at or on the way to WOT) with a single accelerator pump.
As for the choke, you should look at the Holley 4150's if you think that mechanical secondary carbs don't come with a choke .
Looks and sound are pretty subjective reasons to use in an argument for superiority in a carburetor. 
But I would agree with the idea that it is responsive and when tuned properly the ability for it to be a better high performance carb than a vacuum one.

[375instroke]

From all my research and excessive question asking from numerous engine builders....

I dont know why they even make vacuum secondary carburettors for.

A: Mechanical (double pumpers) are more fuel efficient by design than a Vacuum secondaries because the secondaries can not open unless you make them open with your foot by pressing the accelerator pedal to the point where the secondaries do open up at. This means, a vacuum carbie may open/close/open/close of its own accord when you are cruising at 60mph while the Mechanical will not open and thus more efficient. Reason they work out less economical is because of the driver constantly playing with the secondaries (this has nothing to do with carb though) Hence the carb butterflies only open according to what your foot is doing on the accelerator as its mechanicaly controled by the distance the pedal is pressed by and thus can NOT open ever unless U make it.

This applies to Hollies. Not Carter AFBs, AVSs, or Thermoquads.  Those have mechanically actuated secondary throttle plates.

B: Mechanicals get rid of that dumb choke you do not need.
I guess you don't need one in warmer climates with an overly rich idle.

C: Mechanicals get a deeper note which sounds way better
Ever hear a Thermoquad?

D: Mechanicals are much more responsive. To give you an example... a standard auto gearbox vs a manual gearbox... hit accelerator in both cars and the auto delays before it goes while manual goes soon you press it. Same idea here with mechanical/vacuum.
Only with Holleys.

E: Mechanicals do what you want when your foot wants while vacuum has a mind of its own. Ive tried both under WOT and believe you me, the mechanicals do it soon u press the pedal to floor all 4  barrels are open while vacuum will decide a little later to do it and therefor you feel that lag.
This, too, only applies to Holleys.  Carters are not operated by vacuum.

F: Mechanicals look way better also (HP Holleys for example)
Again, only with Holley double pumpers compared to vacuum Holleys.  I like the look of Carters over Holleys, Rochesters, and Motorcrafts.  Purely subjective.

G: Mechanicals perform 2nd to none.

G: Hence I said I dont know why on earth they even make vacuums, they are not as efficient (get more miles out of a mechanical), not as responsive, dont do what u want when u want and dont look good either and dont sound as good and performance wise they got nothing on mechanicals. Less performance and more fuel cost...
You're comparing Holleys to Holleys again.  

Vaccums = for the bin.
This I agree with.

[Cooter]

It's of my humble opinion that if Vac. has anything to do with the fuel delivery in the secondaries, it's considered a Vac. Secondary carb., regaurdless of whether or not the butterflies open mechanically or not..One has a vac. "Door" on top, the other uses vac. operated butterflies...Either way, BOTH use some sort of Vac. to opperate the secondary side of things when Nailed at WOT..

Mechanical secondary carb's Mechanically deliver the fuel to the rear secondaries of the carb upon Nailin' it...

[Ghoste]

Excuse my ignorance here Cooter but I just want to be clear.  You are ONLY talking about the way air is admitted to the secondary side, correct?

[Cooter]

Excuse my ignorance here Cooter but I just want to be clear.  You are ONLY talking about the way air is admitted to the secondary side, correct?

Well, I was refering to the way I look at Vac. and Mechanical carb's...
I guess you could say your correct In order for air to enter the secondary side, The engines vac. must be used with the "Door" as well as opening the rear secondaries..Either way, the only thing that's different to me would be the way the fuel is delivered..Again, this is just my way of looking at it...

[Ghoste]

I get what your saying now.  So I can put words in your mouth and point out that actual fuel flow is still dependent on air moving through the carb and that it isn't being mechanically forced into the venturi?   (cuz you just know that is how someone is going to read it)

[Cooter]

You COULD look at it as Mechanical secondaries IF the set up was a "Throttle body" only without any fuel, but a Carburetor's job is to mix Fuel and air and the minute you introduce Vac. actuated this, or the secondaries require a Vac. signal from off idle to WOT in order for any fuel flow to reach the rear of the intake, it is considered a "Vac. Secondary" carb regaurdless if the Butterflies open by linkage or not.

[Ghoste]

I'm thinking you misunderstood me this time.  What I was referring to was that the fuel won't flow from the bowl to the venturi without a pressure drop inside said venturi.  In short a vacuum causes the fuel to flow regardless of which method is used to activate the secondary throttle blades opening.  The first time I was misreading what you wrote but now I think we are talking about two different things is all.

[G-man]

G: Hence I said I dont know why on earth they even make vacuums, they are not as efficient (get more miles out of a mechanical), not as responsive, dont do what u want when u want and dont look good either and dont sound as good and performance wise they got nothing on mechanicals. Less performance and more fuel cost...

You're comparing Holleys to Holleys again.

I must of been comparing Holleys since thats the only carb I classify worth using and therefor had no idea how any others are setup/operate as you could clearly tell by my post. 

Vaccums = for the bin.
This I agree with.

All else is good then 

[Runner]

so you think a 5k pound c body with 2.76 gears and a 318 should have a mechanical secondary carb on it?   there are plenty of places for a vacuum secondary carb.  there is no way a properly tuned vacuum secondary carb gets worse fuel economy than a double pumper with the same type of driveing.

[375instroke]

I guess this is how I'm thinking it all works:  When the throttle is snapped open, air is pushed into the cylinders faster than the fuel can be carried there, creating a lean condition.  An accelerator pump is used to force fuel into the air stream long enough for the fuel to flow out of the booster venturi.  This works fine with a single barrel carb since the air velocity increases rapidly.  Same with a normal size two barrel.  With a four barrel, the air velocity isn't as fast, and the pressure drop across each venturi isn't as great.  It needs something to compensate for the lack of fuel until the air velocity is great enough to allow the proper amount of fuel to make it to the cylinders.  A vacuum secondary like the Holley just slows down the opening of the secondary throttle plates, I'm guessing.  A weak spring will allow less vacuum to open it, and a stronger spring requires more vacuum to open it.  When the throttle is opened, the vacuum drops, and the secondaries don't open till enough vacuum is developed to overcome the pressure of the spring.  A double pumper, on the other hand, uses an additional accelerator pump to provide supplemental fuel during this transitional period between idle and power, and any time the throttle is opened.  This means that the pumps must supply enough volume for a long enough duration until the RPM of the engine is great enough to provide enough air velocity through the carb to get the fuel flowing out of the venturies and into the cylinders.  Things like a heavy car, low gears, low stall converter, and things like that require more supplemental fuel because the engine will be at a lower RPM for a longer period of time.  I think the engine would make more power with a double pumper because all throttles are opened, and this allows more air to enter the cylinders.  If the accelerator circuit is tuned properly, the car should be faster since it would be making more power than a vacuum secondary carb from the time the throttle is opened, till the point when the secondaries of the vacuum carb open.  I think an AFB may fall in between both styles of Holleys because the secondary discharge nozzles are under the upper air door, and the vacuum of the engine gets the fuel moving sooner than the vacuum Holley, and thus would need to open faster than the vacuum Holley.  It probably has a greater sensitivity to tuning since if the door is adjusted too tight, an over rich condition may occur until the door opens.

[Dans 68]

This is how I've read it works (taken from the HPBooks "Big-Block Mopar Performance" and edited slightly.... ) . "When all the barrels snap open, the air speed inside the venturis of the carb suddenly slows. Now, the main system only responds to airflow (and therefore speed) in the venturis. When the air speed falls momentarily, the carb thinks the engine has basically stopped running and doesn't dish out any fuel. The engine would actually stop running if not for the carb's accelerator pump. This pump shoots some raw fuel directly into the venturi of the carb to keep the engine running long enough for the main system to realize that it needs to start working. In other words, the accelerator pump covers up the air speed drop and the resulting lack of fuel delivery. If the pump can't squirt enough fuel to keep the engine running, it will bog. Most mechanical carbs are prone to bogging as they only have one accelerator pump, usually on the primaries. An additional accelerator pump on the secondaries drastically helps to increase response. These pumps can be adjusted with different opening cams and nozzle sizes to help reduce bogging."

"Vacuum secondary carbs use the engine's air speed to open the secondaries. The carb has a vacuum diaphragm that senses the load and air speed to determine the secondary opening. Because of this, it is almost impossible to open the secondaries too fast and cause bogging. If the carb senses that the engine isn't ready for the additional air flow, it won't open the secondaries too soon and cause a bog. There are diaphragm spring kits that allow the opening rate to be adjusted depending upon engine size and use. However, since the secondaries usually won't open very fast, power is lost."

Dan

[Runner]

375instroke, it seems to me that your thinking that the secondaries "snap open" on a vacuum seconday carb.  thats not the case unless the carb has been messed with.  there is a check ball in the vacuum passage to the canister.it slows down the the opening of the secondarys no matter how much signal they get hence the reason they dont need the accelerater pump.    like anything  it needs a little tuning to make it right.  no more or less than a double pumper.

[375instroke]

375instroke, it seems to me that your thinking that the secondaries "snap open" on a vacuum seconday carb.

I didn't say that, as it applies to Holley vac. carbs.  What I was saying is that on an AVS, the secondary throttle plates can be snapped open.  Here's the bottom of an AVS.  There is a mechanical linkage that opens them.  No vacuum servo like a Holley.



Here are the discharge nozzles.  Notice there aren't any venturis in the secondary side?



You can see how the secondary air door sits above the discharge nozzles.  Air velocity is what opens it, not vacuum like a Holley.  The important thing is that this arrangement gets the fuel moving sooner, and thus the upper air door can open sooner, than with a Holley vacuum secondary carb.  An AFB, on the other hand, has secondary throttle plates that open mechanically, but have a secondary air door that sits under the booster venturies, so it probably works as slow as a Holley.  Now a Thermoquad has mechanical secondary throttle plates, an air door on top that opens with air velocity, and booster venturies.  You can hear the secondaries open on a Thermoquad, and the Thermoquad I had on a 440 with 3.91 gears was no slouch as far as how fast the secondaries opened.  I'm not arguing over which is better.  This is just how I see they work.

[1Bad70Charger]

This has been one incredibly informative thread, and was one of the reasons I have my brand new Proform 750 double pumper with vacum secondaries, sitting in my basement waiting to be installed, that I was albe to get for only $485 throught Summit racing, with special 10% discount they are running if you give them a special code; although Ron (Firefigher) certainly gets the biggest credit on my going with that carb (thanks man)! 

Summit Rocks and so does this thread!