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Old 09-01-2009, 03:36 PM
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Vac Sec. Carbs and superchargers

Can vacuum secondary carbs like the Holley 3310 be used on a 6-71 blower on a BBC? Or would it have to have mechanical secondaries and double pumps?

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Old 09-01-2009, 03:53 PM
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I have a vac secondary 750 on my blown "T" bucket. Daily driven. No problems. Cept the rear tires smoke when the secondaries open..
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Old 09-01-2009, 07:00 PM
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Quote:
Originally Posted by BOBCRMAN@aol.com
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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Old 09-01-2009, 07:18 PM
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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.

Last edited by ericnova72; 09-01-2009 at 09:12 PM.
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Old 09-01-2009, 07:38 PM
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Quote:
Originally Posted by ericnova72
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.
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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Old 09-01-2009, 09:02 PM
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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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Old 09-02-2009, 11:52 AM
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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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Old 09-02-2009, 12:12 PM
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Quote:
Originally Posted by ericnova72
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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Old 09-03-2009, 08:56 AM
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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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Old 09-03-2009, 09:23 AM
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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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Old 09-03-2009, 10:09 AM
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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?
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Old 09-03-2009, 10:25 AM
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Quote:
Originally Posted by Overdriv
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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Old 09-03-2009, 11:38 AM
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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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Old 09-03-2009, 12:28 PM
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Quote:
Originally Posted by ericnova72
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/sh...cat/500/page/2
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Old 09-03-2009, 01:14 PM
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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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