Carb question: Vac secondary vs Mech secondary

[Purple440]

I'm currently running an Eddy AVS Thunder Series 800cfm on my 440 and have been pondering the effect of the vacuum secondary versus a mechanical secondary.  I'm wondering if the air/fuel distribution is equal between the front and back cylinders before the vacuum is enough to start opening the secondaries, since the primaries are not restricted, are the back cylinders fighting harder to get the mix when I punch it?  Theory being that the primaries are closer to the front cylinders and would feed them better initially.  Is that true?

Also wondering if I'd be better off with a smaller, closer to stock (650cfm) Holley, with mechanical secondaries and double pumper.  Because high velocity provides better atomization (correct?), and on my 440 I'm sure I'd max it out, it seems I'd have better performance.  On my AVS, I'm detuning the secondaries to make it act like a smaller carb.   Reason being: stock 452 heads with 3-angle valve job, no porting.  Cam is nothing special performance wise, just a longer duration, hydra flat tappet.

Setup:
446
452 stock heads w/ valve job.
Cam - Lunati Bracket Master II, 230@.50, 480 lift.
727 trans
2800-3000 A&A Converter
3:55's
Eddy 800AVS
Eddy RPM intake
Headers


Any thoughts?  Am I over thinking this?  My theory at the moment is that with the nice converter I now have, the engine can rev up quickly before it hooks and might like a tight carb with another pump.

Thanks,
- Doug

[elacruze]

The whole point of a vacuum secondary carb is to maintain the velocity through the venturis by controlling the vacuum drop while the secondaries open. My experience is that a mechanical secondary carburetor 'feels' faster or better, but rarely exceeds the performance of a well-set up vacuum carb. With 446cu. in. and a dual plane manifold, I'd say you will give up a significant amount of performance with the smaller carb. If you take the time to learn how to tune the secondaries and acquire all the tuning pieces, you won't be disappointed.

[Ghoste]

A smaller carb will give a crisper throttle response off idle for all the reasons you already mention but as elacruze said, you will give up a lot at the upper end of your powerband.  With the things you've done to the car I wouldn't want a smaller carb personally but ultimately, your driving habits have to dictate it.  If you are mainly just driving around town and rarely ever use the car in a WOT situation then maybe a smaller carb is the right choice for you.
If ultimate performance is your goal, then mechanical secondaries are a good choice but again, based on your driving habits you need to decide that.  Just guessing I am thinking that vacuum secondaries are the way to go for you.  You might want to go with the Holley in any case.  I am generally a proponent of Carter style carbs and the AVS is a great one but one of the reasons I like them for a street car is the fact that they are largely plug and play.  The limited adjustment in them means you can set them and just drive.  If you like to tinker and get the last bit of howl from them though, the Holley will make your life much easier.
Another issue I have with the Edelbrock Carters clones is that seem to be calibrated on the lean side.  This is probably a great for Edelbrock so they can get the blessing of the CARB and EPA Nazis but it doesn't help a lot in real life.
I think you should get the old tried and true Holley 750 with vacuum secondaries and have at her.

[firefighter3931]

That combo will easily accomodate a 750cfm double pumper style carb.  Once you've tuned a double pumper you'll never go back to a vacuum secondary carb...ever. 



Ron

[Ghoste]

You may go insane setting it up, but yeah.

[Cooter]

Agreed...Double pumper's forever..

[Purple440]

Good feedback, thx.  Ghoste are you suggesting I should try jetting up?  My plugs look pretty good and even, but I haven't done the W.O.T. test that Ron suggests to test the secondaries.

Ron if I was a holley tunning guru like yourself, I'd probably get one tomorrow 

- Doug

[Ghoste]

I don't think it's just rods and jets, I think the Eddy carbs are calibrated lean internally as well.  Let me be clear that this is NOT something I know as fact and I don't know enough about carbs to be mnuch more specific than to say that it just seems that way to me.  So you have to take that for what it's worth.

[375instroke]

How are you "detuning the secondaries"?  My stock 440 has the tiny Edelbrock AFB installed by the previous owner, and it sucks.  Bogs off the line, and dies on the top.  Doesn't help with gas mileage, either.  You have 8 holes the size of coffee cans you need to fill.  Whole you trying to fool?  I have the correct 4618S now that I'm rebuilding.  Why would someone put a smaller than stock carb on a car like this, a '69 Charger R/T, and throw away the original?  The cylinders are still sucking from the primary and secondary barrels regardless if the cylinder is in the front or rear.  I think the AVS has the fastest opening secondaries, because the full engine vacuum is drawing fuel out the discharge nozzles when you stab the throttle.  It doesn't have to wait for air velocity to create a pressure drop across a booster venturi.  How big are your throttle bores on that Edelbrock 800?  Mine is 1-11/16" primary and secondary, and most say it's 750CFM.

[Ghoste]

The AVS has an adjustable "air door" that can alter the rate at which the engine sees the secondaries opening.  Do you have a model number on that AFB 375instroke?

[Cooter]

The AVS has an adjustable "air door" that can alter the rate at which the engine sees the secondaries opening.  Do you have a model number on that AFB 375instroke?

I was thinking the same thing...Eddy carb's are AVS style aren't they?

[Ghoste]

The Thunder series are, the Performers are AFB clones.

[375instroke]

It's an Edelbrock 1405.  Manual choke 600CFM AFB.  They had a tiny electric fuel pump on it, too.  The thing was like 2" square.  I don't think it could feed a VW Bug.

[Ghoste]

The 1405 is definitely listed as a 600.  Whoever told you thats a 750 must be thinking of something else?

[375instroke]

No.  I'm talking about the Carter 4618S with 1-11/16" primary and secondary venturies is around 750CFM, not the Edelbrock 1405.  What's the bore size of the 800CFM Edelbrock 1813?

[Ghoste]

Sorry about that, I thought you were talking about the other one.  That spec on the 4618 is for the throttle bore not the venturi diameter but you are right it would be the same as the ones Eddy rates at 750.  The 800 cfm one has 1-3/4 bores.

[Purple440]

How are you "detuning the secondaries"?

Just tightening the secondary spring to make it act like a smaller carb until the RPM's are up, which is a nice feature on the AVS. 

Maybe your stock 440 needs a smaller carb.  Stock 440's came with a 650cfm Carter correct?  If you're dying on the high end maybe you're not getting enough fuel. 

I had an old racer up at BIR tell me that my sluggish takeoff problem was too big of a carb...he was partially correct. 



- Doug

[Ghoste]

I have always thought the stock 440 carb was about 625-650 too but that point 375stroke made about bore size corresponds with the Eddy carb at 750.  Of course that doesn't take into account how they were flowed and we all know how that works.

[Cooter]

The Basic thing to remember here is the carb size needs to be matched to your combo..Remember, the mid 70's 440 had an 850 TQ on it...


Running a Mopar "Purple" shaft .484/284(241 @ .050) cam, with STOCK everything else including carb (750 CFM) will NOT run correctly...You have to match the combo. Now you take the same set up, dump a set of headers on it, TUNE that 750, aftermarket intake, ported heads, increase in compression, more stall converter, you will see that "Too big" carb perform correctly...I've seen "Street" engines with 1050 Dominators running correctly..It's all in the set up..You didn't mention the compression of that 446 Cu.In. motor either...That 230 @.050 Cam is alot of duration for a typical, Low comp.,street motor..

[375instroke]

How are you "detuning the secondaries"?

Just tightening the secondary spring to make it act like a smaller carb until the RPM's are up, which is a nice feature on the AVS. 

Maybe your stock 440 needs a smaller carb.  Stock 440's came with a 650cfm Carter correct?  If you're dying on the high end maybe you're not getting enough fuel. 

I had an old racer up at BIR tell me that my sluggish takeoff problem was too big of a carb...he was partially correct. 




- Doug

I don't think tightening the secondary spring on an AVS would work that way.  It does on Holleys and AFBs, but on an AVS, the door is on top, and the discharge nozzles are under it, so they are exposed to manifold vacuum.  They don't have venturies on the secondary side.  If you tighten the door too much, the excess vacuum will just draw too much fuel out the nozzles.

Whatever the stock 440 carb was rated at, it was bigger than what I have now.  I'm just putting the stock carb, a 4618S, back on it, whatever size it is.

I wasn't getting enough fuel before with the tiny electric pump.  I have the stock Carter MS-4434S now.  It felt like someone was turning the key on and off before when I got on it.

[Ghoste]

Actually that's exactly what tightening the air door is for.  The air door spring tension is specifically adjustable so that you can determine the points where the secondary side of the AVS comes in.

[375instroke]

What I'm trying to say is that If you go too tight on the AVS spring, you will get too rich of a mixture because manifold vacuum will be drawing excessive fuel out the discharge nozzles.  This doesn't happen with an AFB or Holley.

[Ghoste]

I know what you're trying to say, I'm just disagreeing with it.  It is no different than in an AFB or Holley when the secondaries are closed and manifold vacuum is high.  The AVS still has throttle blades on the secondary side so the discharge nozzles aren't seeing that much pull.

[375instroke]

Holley secondaries are opened by a vacuum servo.  The AFB has throttle plates that open on demand, like a double pumper, but there is a counterweighted air door between them and the booster venturis that requires vacuum and air velocity to open.  If it opens too soon, there won't be enough air flow through the boosters to give the engine enough fuel.  If it opens too slow, there won't be a problem with the air/fuel ratio.  Just a slower response.  On an AVS, the secondaries are opened on demand, also.  It doesn't have venturies, or booster venturies.  The air door is on the top of the carb, above the discharge nozzles.  This gives the added vacuum signal to the discharge nozzles that booster venturies give to other carb designs.  It also could cause the carb to flood the motor with fuel if the secondary air door doesn't open early enough.  Where am I wrong here?

[Ghoste]

How much vacuum would it need to draw that amount of fuel?  Maybe it's a worthwhile experiment to tighten an air door down on an AVS so that it can't open at all and see what point it draws enough fuel out the discharge to cause a problem.  I still understand what you're saying but I'm not properly making my point.
I realize that as the secondaries on the AVS open, the manifold vacuum is supposed to get fuel coming from the discharge and that there is no venturi or booster in there.  It's all manifold pressure drop.  To my thinking, by the time the throttle blade is opening in the secondary you would also be at a point where vacuum level is dropping and having less effect on the discharge.  There is a point in the operation where air velocity takes over to keep fuel flowing throught the discharge.  That manifold vacuum draw is only meant to be a transition flow.
Truthfull, I have never tried to tighten an AVS down so much that it couldn't open so I don't even know that you can.  I would hope that Carter engineers would take that into account and design so that at some point, the door MUST open before a rich condition becomes a problem.  But I guess, yeah, at this point without knowing that I have to accept that it could happen.
Does anyone know if the AVS air door can be tightened to the point of non function or even nearly so?

[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!