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Discussion Starter · #3 ·
I have a vac secondary 750 on my blown "T" bucket. Daily driven. No problems. Cept the rear tires smoke when the secondaries open..
The vacuum secondaries open on decreasing vacuum controlled by the spring in the diaphragm. I was under the impression there was always vacuum under the carbs with a blower.

Then the vacuum must go down low enough to open the secondaries. Have you ever put a vacuum gauge on it? What spring are you using?

Thanks for the info.
 

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Your understanding of how a vacuum secondary carb works is flawed. Manifold vacuum under the carb(or lack of vacuum) has nothing to do with the secondary opening.

The vacuum used to actuate the secondary's is venturi vacuum drawn from the tightest venturi choke diameter of the primary barrels. If you look closely at your vac secondary carb looking down into the front passenger primary barrel, at about 10 o'clock you will see a stubby little 1/8" tube(or just an 1/8" hole)located at the smallest point in the bore leading out to the vacuum pod on an angle. When airspeed is high enough through the primary side of the carb, it creates a vacuum in this passage that is used to start to draw the vacuum diaphragm up and start opening the rear barrels. There is also a similar passage drilled to the pass side rear barrel that helps to finish opening the rear blades as airspeed through the carb increases.

If Holley had just called it an Airspeed Activated Secondary people would be a lot less confused. Don't feel bad, 95 out of 100 hotrodders also don't know how they work and think that manifold vacuum is responsible for opening the carb.

A blower will actually open a vac secondary carb more fully and faster than would occur with the same secondary spring on a naturally aspirated engine because it is moving more air sooner. :thumbup:
 

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Discussion Starter · #5 ·
ericnova72 said:
Your understanding of how a vacuum secondary carb works is flawed. Manifold vacuum under the carb(or lack of vacuum) has nothing to do with the secondary opening.

The vacuum used to actuate the secondary's is venturi vacuum drawn from the tightest venturi choke diameter of the primary barrels. If you look closely at you vac secondary carb looking down into the front passenger primary barrel, at about 10 o'clock you will see a stubby little 1/8" tube(or just an 1/8" hole)located at the smallest point in the bore leading out to the vacuum pod on an angle. When airspeed is high enough through the primary side of the carb, it creates a vacuum in this passage that is used to start to draw the vacuum diaphram up and start opening the rear barrels. There is also a similar passage drilled to the pass side rear barrel that helps to finish opening the rear blades as airspeed through the carb increases.

If Holley had just called it an Airspeed Activated Secondary people would be a lot less confused. Don't feel bad, 95 out of 100 hotrodders also don't know how they work and think that manifold vacuum is responsible for opening the carb.

A blower will actually open a vac secondary carb more fully and faster than would occur with the same secondary spring on a naturally asperated engine because it is moving more air sooner. :thumbup:
You are right, I did not know that is how they operate, and I've been around Holley carb for many years. Thanks for setting me straight.

So to take this a bit farther, are not the springs rated for the amount of vacuum it takes to compress it? On second thought I think they are just called out as heavy to light by color if I remember correctly.
 

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Yes, they are just color coded heavy to light. The Holley spring kits give a chart with estimated starting and fully open rpm levels for 350, 402, and 454 and 600 or 750 carbs if I remember it right, but I never found it to be very accurate. Trial by error seems to be the only real way to find what works for each vehicle. I do remember that the black coded spring is listed as never fully open for all the combo's on the sheet :( .
 

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bentwings
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A friend has a blown big block with 2 vac sec 750's that I think are 3310's. didn't get to take the scoop off. He runs 10.90's while smoking a cigarette so they can't be all bad. I was a little unsure before I saw his car run. He drives it all the time on the street. It's almost a grocery getter.

I've got the traditional double pumpers ordered for my new blower set up but I still have second thoughts. I don't think there is any problem now. I would plan on doing some work on the opening rate and time with a spring kit.
 

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ericnova72 said:
Your understanding of how a vacuum secondary carb works is flawed. Manifold vacuum under the carb(or lack of vacuum) has nothing to do with the secondary opening.
You are correct, but the space between the carb throttle plate and the blower inlet is essentially "manifold vacuum" in any case. For example, you could plumb your power brake booster there.
 

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Discussion Starter · #9 ·
Wow guys, as usual just great information from this forum, from very knowledgeable people. There is no substitute for experience.

I suppose both carbs should be jetted the same and with the same spring so they work together. Or is there a better way? Should they be set up on progressive linkage?
 

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You will need the supercharger/tunnel ram/two fours type of tops for the vacuum secondary housings. These have a small vacuum hose nipple on them that is used to connect the two carbs vacuum secondary's together with a short hose so that they open at the same rate and not one before the other. Same tension or color springs in both pods.

Primary side throttle linkage should be 1:1.
 

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Overdriv said:
Thanks Ericnova. I guess these tops can be purchased from places like Holley and Summit?

What dual carb fuel lines do you or anyone use?
Yes, from Holley or several other carb specialist like Quick Fuel, etc. Summit can probably hook you up too.

I make my own custom fuel lines depending on each application from Stainless or Aluminum hard line and AN fittings , and/or braided hose. I don't like the generic or pre-made stuff, it is always a compromise on fit or location or aesthetics.
 

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bentwings
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you can get the Hilborn type scoops with aircleaners from Summit at least 2 sizes, Big Al's Toybox has BDS, Hampton blowers has his own without aircleaners.
These vary from about $150 to $350.

Hampton tells me they don't use aircleaners out there but here in Minn you will be asking for trouble as we get dirt and sand blowing all the time. Plus there are always a few dorks that like to flick butts and gum into the scoop while you are parked.

Good question; what do you do if you are caught in a rain stom?? We get them all the time here. I'm thinking of making a small plastic deflector that would clip to the scoop or under the lip of the hood. If you had time you could just turn the scoop around too. It isn't like you would need maximun air flow in a rain storm. haha

I'm going to make my own fuel lines for the same reason. I like hard lines so SS or aluminum will be the way to go.

It's hard to beat the standard ball bearing throttle linkage. I think Enderle makes them for everybody. About $130 from Summit. Links both carbs 1:1.
 

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ericnova72 said:
You will need the supercharger/tunnel ram/two fours type of tops for the vacuum secondary housings. These have a small vacuum hose nipple on them that is used to connect the two carbs vacuum secondary's together with a short hose so that they open at the same rate and not one before the other. Same tension or color springs in both pods.

Primary side throttle linkage should be 1:1.
http://www.hotrodders.com/gallery/showphoto.php/photo/36900/cat/500/page/2
 

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Not to hijack this thread, but could you guys answer this then since I'm curious myself - because of the vacuum secondaries, would that make MPG and overall drivability better? I am assuming mechanical secondaries make things more simpler at the cost of suffering gas mileage, but better throttle response?

Considering the fact it's a blown application - are there any real drawbacks to using one or the other? I'm just curious, because I feel like I'm missing out on something important here. Generally speaking there's a huge price-gap difference in the mechanical carbs, and I wanted to understand why (aside from supply and demand). :)
 

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For one thing you will get better MPG with vacuum secondary and when it comes to CFM, vacuum secondary are more forgiving. You will never overcarb a motor. It will use so much CFM what the motor needs. You can adjust the opening rate on the sec. by changing the springs. Vacuum secondary carb are more street friendly.
 

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lg1969 said:
For one thing you will get better MPG with vacuum secondary and when it comes to CFM, vacuum secondary are more forgiving. You will never overcarb a motor. It will use so much CFM what the motor needs. You can adjust the opening rate on the sec. by changing the springs. Vacuum secondary carb are more street friendly.
Why doesn't everyone use them, then? That sounds alot more user-friendly.
 

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DeltaElite121 said:
Why doesn't everyone use them, then? That sounds alot more user-friendly.
The take a little time to react. track racers will usually use a mechanical secondary, because it opens mechanically and most are also double pumpers. You cant really double pump a vacuum secondary carb as the the throttle position does not dictate when the secondaries will open, load does.

Many people have thought their vacuum secondary carb was not opening the secondaries because they sit in park and rev teh engine and the secondaries dont open. They never will.
 

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bentwings
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I think in a lot of cases the DP just looks and sounds (appears) to be the way to go. Common sense and logic are thrown out the window.

On a 6-71 type application on the street, full throttle will cause a demand air/fuel flow nearly instantly that is far higher than an unblown application. Untill recently I thought that the vacuum sec might not work too well even though I've used the Carter AVS carbs on one blower application I did years ago. It worked very well on a 454 BBC with a manual trans in a Camaro. Edelbrock has a blown crate motor with one of these and it makes 500+ right out of the crate. Recently a friend who has a 502 BBC with an 8-71 and Holley vac sec. carbs ran it and ran 10.90 without even tuning it up after driving it all over on the street. Watching the carbs from the pass seat, the carbs open almost as quickly as directly linked. You can't feel the transition nor hear it. ( it has mufflers). I was concerned that you may get a backfire due to a momentary lean but the vac sec has fuel ready to be released on demand at a demand rate so as long as you are jetted properly they work great. Overdrv states very well just how the vac sec works and you can see that the very high demand that a blower will make on the primary will insure a very quick opening of the sec. A 50cc pump is usually run with vac sec so you can get extra fuel required quickly.

On a race application where you might have a bigger motor and run the blower in a large overdrive so you get a lot more boost (air flow) even quicker it might be better to run the double pumper so you can flood the AF ratio at the initial hit. This is what the mechanical FI units do. There is a lot of tuning in this area on these units but we are talking street stuff so we can leave that animal rest.

I really think the biggest problem running vac sec is that you will need to tune them yourself or do some dyno work to really get them set up properly. This is opposed the simply buying blower carbs already to go. These will only need minor jet changes for the most part.

As for mpg, well, you will be playing with the big boys now and mpg is way down the food chain. A streetrod buddy does get 20+ with his blown 3 window but he doesn't step on it very often. If you put on 3000 miles during the summer at 10 mpg it's $720 for gas. Not a lot considering what you have in the blower, engine and trans modifications. Pluss the fun factor simply is priceless. haha
 
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