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?