Vacuum Secondary vs Mechanical Secondary

[Z Man]

I have an ERA Cobra with a 427 FE and dual 4160 Holleys (600CFMs). After lots of tuning attempts, I feel I may be over carbureted and want to make a change. Holley offers 2 smaller carbs, 390 CFM with vacuum secondaries and 450 CFM with mechanical secondaries. I have zero experience with the mechanical secondaries. Any thoughts? Thanks!!

[Tommy]

I run a single Holley four barrel with mechanical secondaries on my 410 ci custom built Windsor. It is a race setup with no choke. ... I'm told that vacuum secondaries compensate for overly rapid throttle opening by delaying the opening of the secondaries until the vacuum signal says conditions are right. The pro is that it prevents an overly aggressive driver from dumping a lot of fuel into the engines from the secondary power valves before the air flow from rising RPM can catch up. The con is that there is a slight delay in when the secondaries open that is not under the direct control of the driver. The optimum for race applications is mechanical secondaries AND a driver who can match the opening of the throttle with engine RPM and conditions.

I've driven mine so long I don't think about or notice the opening of the secondaries. And I don't have enough traction below 80 MPH to abruptly give it full throttle without swapping ends. So it has not been an issue for me. If you like to abruptly go to full throttle without regard to engine RPM, then vacuum secondaries might be a better choice. But if you seldom abruptly go to full throttle, it probably doesn't make much difference which you choose.

[wkooiman]

2 Holley 600 vacuum secondary carburetors are not too much. The secondaries only open if they are needed, so it's basically the same as a 600 mechanical secondary with an extra 600cfm if you need it.

That's assuming you have the proper BJ/BK, BC/BD, AC/AD, etc. carburetors and progressive linkage. Ford spent quite a bit of time getting these right. They weren't too much carburetor in the 60's, so why would they be now?

If you aren't using a stock style setup, all bets are off.

I previously ran off-the-shelf 1850s (600cfm) on a 428 that were reworked by QFT. They ran like a dream.

BTW, if you google this subject, you will find several threads. In one thread, someone had lots of problems with 2 390cfm carburetors. Stick with what works, and you'll be happier.

[Z Man]

Thanks guys! My issues are a pig rich idle and off idle. I do not have a BJ/BK, etc pair. Just 2 equal 1850s. I have machined the choke housing off, progressive linkage is correct, float levels are right, main jets have been swapped between 60 down to 52. Power valves have been swapped between 4.5 & 6.5. Every combination and the plugs are black in 5 minutes. I am relatively new to carb tuning and learning more than I ever thought I would need to! I love it, but it's getting frustrating.

[Gaz64]

Two 600 vac secondaries are fine, twin 390s would be a little small.

Your idle feed restrictions need to be smaller, but first thing to check is the idle air bleeds are clean of any foreign debris.

A blown power valve diaphragm will also give you this condition.

Gary

[DanEC]

You should be able to tune the idle circuit with the mixture screws provided your cam isn't too big and you have a reasonable idle vacuum reading. Are you tuning the idle mixture on both carbs together to maximize idle speed - it's a rather tedious back and forth process?

I start by closing the front throttle blades until the cam is just barely making contact with the idle screw. Then open both carb idle settings about 1-1/2 turns equally and start the car to make sure it has a reasonable idle speed. If it needs to increase or decrease, then make adjustments equally on the idle speed screws for both carbs. Then I use a tach and a vacuum gage to start tuning the mixture screws on one carb at a time to maximize the readings - go to the other carb and repeat - go back to the first carb and readjust - then to the second carb again. There is going to be a sort of dead zone in the mixture settings that don't seem to make any difference in the idle speed or vacuum readings and I tend to set final adjustments towards the lower side of that zone to attempt to be slightly lean - but I'm not sure it really works that way or makes any difference. The system just may not allow such fine adjustments. After you have the idle mixtures optimized, turn both idle speed screws equally to adjust for the curb idle speed that you want. As long as your idle mixture circuits respond - roll off in rpm on the rich side, and roll off in rpm on the lean side - then all should be good with that.

However, the moment you crack the throttle the transition circuit comes into play (not the accelerator pump circuit) and it is pre-set in Holleys by non-adjustable air bleeds. And if you have a big cam and a high idle speed for it, the transition circuits may even be contributing at idle and if so, other than lowering idle speed, there isn't much you can do about that without extensive carb mods. These transition circuit bleesds are set as a compromise by Holley to work with just about anything that you might bolt that carb on from a 350 up to a 482 stroked FE. That is what is probably rich on your car. There are means of modifying the bleeds to lean out the transition circuit - you can do some Googling. They run from inserting pieces of wire in them, to drilling and tapping for replacement bleeds, to relocating the bleeds. Or some of the premium Holley knock-offs like Quick Fuel had replaceable bleeds - but Holley has bought most of them out.

But no matter how much you work on one of these, a carb set up is never (well, that may be too strong) going to idle as cleanly as a modern fuel injection system.

[patrickt]

Originally Posted by Z Man

I have an ERA Cobra with a 427 FE and dual 4160 Holleys (600CFMs). !

Z Man, a perfectly adjusted vacuum secondary will meet, or exceed, a mechanical secondary in almost all respects -- certainly for a predominantly street driven car. The problem is that changing the springs in the vacuum secondary, and then clearly observing the effect on the secondaries opening, is a real PITA. Add to that the fact that any change in the air cleaner, or running without one, completely changes the equation, and it becomes real easy to just throw in the towel on vacuum secondaries. All of that could be easily remedied if the carb manufacturer just included a simple opening adjustment and a gauge to measure the opening of the secondaries... but they don't. Fortunately, you can do that if you're so inclined. I have a single vacuum secondary and I have installed an easily-adjustable vacuum canister, along with a tell-tale zip tie to measure secondary openings. It makes adjusting your secondaries about as easy as adjusting your idle. Here's the thread on it: Theoretical Vacuum Secondary Question

[saltshaker]

The linkage position on the 1850's is different then the Blk/by carbs. You can google it on the "fordfe" forumn. When I set mine up it was suggested to disconnect the linkage and get the front carb tuned and then reconnect it and adjust the rear carb. It also helps to connect the vacumn caps together ( need a special cap) so both of the carbs rear barrels open at the same time. This was how Ford had set it up.
I have a couple of those threads from the forumn ....send me a pm with you email and I will froward them.
Jon

[moore_rb]

Originally Posted by Z Man

Thanks guys! My issues are a pig rich idle and off idle. have been swapped between 60 down to 52. Power valves have been swapped between 4.5 & 6.5.

How is your throttle response immediately off-idle? Do the RPM's jump up crisply? or is there a stumble? Any black puffs of smoke out of the pipes when you rap the throttle? If so, then your accelerator pumps (one per carb) might be too rich, either via the duration cam (which makes the pump shot last longer or shorter) or via the squirter size (which changes the overall volume of fuel being delivered during the pump shot)

If the throttle response is good, and there's no black smoke when you hit it, then the accelerator pumps are probably ok, and your black plugs are probably either
A) Too much fuel from the idle mixture screws,
B) Too little spark advance at idle (the flame is still burning when the exhaust valve starts to open, leaving unburned residue in the combustion chamber), or
C) The spark plug heat range is too cold.

If you are confident that your overall tune is good (idle mixture is set at the point where maximum idle vacuum is created, and initial timing is set where you want it) then try going up one heat range on your plugs, and see if that helps to keep them cleaner...

DanEC's post above offers some excellent pointers on tuning in the idle on a dual carb setup...

Originally Posted by Z Man

Holley offers 2 smaller carbs, 390 CFM with vacuum secondaries and 450 CFM with mechanical secondaries. I have zero experience with the mechanical secondaries. Any thoughts? Thanks!!

My thoughts about this are: Stay away from mechanical secondaries in a dual carb setup. Actually, I agree with Patrick above - Stay away from mechanical secondaries altogether, unless your car is primarily a drag racer.

When the day comes that the double pumper that came with my car finally craps out, I'm gonna have a well tuned vacuum secondary carb on there faster than you say "less is sometimes more"

From a performance standpoint, my personal opinion is that two carbs are overkill to begin with, unless your car revs to 15 grand. But, I understand the aesthetic, and the originality aspect of having a dual carb setup on a Cobra...

Now, some of the old-school popular wisdom I've read and heard several times, suggests that twin carbs actually flow at about 85% of their combined max CFM rating, so a pair of 390s = 780cfm, x .85 =~ 663 CFM in the real world, which would flow adequately to feed a well tuned 427 inch engine up to about 5300rpm... which might be a little light.

[MKS427]

Your rich at idle condition may not be rich at all if you have a big cam. Higher degrees of lobe separation cause raw fuel to get into the exhaust, so it may not actually be rich at idle, not in a menner that can be tuned away at the carb. I spent a long time chasing this before I found out it is not going to change.
As far as the vacuum secondaries go, a big cam affects engine vacuum. Performance engine builders recommend mechanical secondaries over vacuum due to tuning issues.
The advice I got and followed was to go efi. The car is less "rich" (raw fuel)at idle. Low speed 1300-1800 is significantly better. Better throttle response/control. MPG up to 15 from 10.
I went with fitech. They do have a dual quad application that may interest you.
Good luck

[*13*]

If you are oversized in the carb, you are less likely to have the vacuum to pull open the secondaries, you'll have issues. With mechanical secondaries you have a second squirter nozzle that pumps when you kick open the blades. For racing purposes, sure! But, it is a cobra with dual quads. Not exactly a commuter. .

[patrickt]

Vacuum secondaries have nothing to do with manifold vacuum. They run off of venturi vacuum, which is related to the velocity of the air/fuel mixture down through the carb. A big cam with low manifold vacuum will do just fine with a vacuum secondary carb. If your engine builder, or engine tuner, doesn't understand the difference between the two then you shouldn't let him even look at your engine.

[DanEC]

Have to go with Patrick on this. Even the Chev L88 used a vacuum secondary carb in most factory installed applications and some of the L88 cams were up around 275 deg @ .050. They were nasty and wild. My 427 Vette runs a 256/266/,579/.605 cam, pulls 8 inch vacuum at idle on a good day and does fine with a vacuum secondary Holley. My dual quad 459 FE is considerably calmer with a 230/236 cam but it is very smooth and responsive with 1850 carbs. The reproduction progressive linkage works very smoothly on it. It even seems to be pretty easy on gas although I've never actually calculated the mileage.

[Z Man]

Thanks for everyone's input. I am going to stick with what I have and start from scratch.

wkooiman, You mentioned your carbs were reworked by QFT. Did yo actually send the carbs to QuickFuel?

Thanks!

[dcdoug]

Originally Posted by patrickt

Vacuum secondaries have nothing to do with manifold vacuum. They run off of venturi vacuum, which is related to the velocity of the air/fuel mixture down through the carb. A big cam with low manifold vacuum will do just fine with a vacuum secondary carb. If your engine builder, or engine tuner, doesn't understand the difference between the two then you shouldn't let him even look at your engine.

You can test this with an air compressor. Direct air past the venturi in the primary to simulate vacuum load. Secondaries will open if you do it right.

[moore_rb]

Originally Posted by patrickt

Vacuum secondaries have nothing to do with manifold vacuum. They run off of venturi vacuum, which is related to the velocity of the air/fuel mixture down through the carb. A big cam with low manifold vacuum will do just fine with a vacuum secondary carb.

True Dat. Vacuum secondaries actually work OPPOSITE from manifold vacuum.... The secondaries open when manifold vacuum is low, and they close at idle and cruise conditions, when engine vacuum is high....

EDIT: Whoops- Posted this before I read DanEC and DCDoug's followup posts on the topic... Dead horse. I'll put my stick down, now.

[DanEC]

Originally Posted by Z Man

Thanks for everyone's input. I am going to stick with what I have and start from scratch.

wkooiman, You mentioned your carbs were reworked by QFT. Did yo actually send the carbs to QuickFuel?

Thanks!

You might PM Brent (Blykins) as it seems I recall he had someone who custom tuned carbs and was an ex-Quick Fuel or Demon technician (not sure exactly).

[davids2toys]

DCDOG....Great pic, really understand it now. As they say, a good pic is worth a thousand words!!!

[undy]

I was the BETA for Barry's dual QuickFuels that he sells now. Actually, Barry liked my carb setup so well he ordered them and started selling them on his web site.

I've got a Pond aluminum KC 482" side oiler and was running a Tunnel Wedge 2x4 intake manifold. I contacted QuickFuel about building me 2 750 carbs for it. I gave them my displacement, cam profile, targeted HP and RPM. The built me 2ea 4160s with the secondarys modified to accept replaceable jets. Long story short, they idled at 14.7 AFR with 12.5 at WOT on the chassis dyno which was PERFECT. I went with them after hearing horror stories about the QC on the new BK/BJ and BC/BD reproduction carbs and the (potential) nightmares of using off the shelf 4160 600s.

Best money spent on my build...