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Old 07-30-2005, 10:58 AM
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Power loss with vacuum secondaries?

I was always under the impression that vacuum secondaries are better for street driving than double pumpers, but at the expense of a power loss at higher RPM. The secondaries need a partial vacuum to stay open, causing reduced cylinder filling as compared to a near-zero vacuum double pumper setup. Is this true? Is it also true that vacuum secondary carbs run a bit leaner and get better mileage than the double pumpers? I've heard some rumors floating around local gearhead hangouts that vacuum secondaries are crap for performance use, so I thought I would clear this issue up.

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Old 07-30-2005, 11:36 AM
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Vacuum vs Mechanical

Vacuum secondaries are designed to work as engine demand requires, so when tuned properly, they only start to open when the engine vacuum gives a strong enough signal to make them function. While mechanical secondaries are only open at WOT.
The problem with running a mechanical secondary on the street, or drag racing, especially with an automatic trans is that you can easlily over carburet your engine.
I found this from experience.
Example;
In the early 70's I had a 69 Mustang with a 302, that was mildy warmed up. I was running a 650 DP on it. Ran hi 14's in 1/4 mile. Due the the gas crunch in 73, I opted to put the old 500 Holley 2 bbl back on it that I had run on the stock intake, using an adaptor plate, for street driving.
In the course of events, returning to the drag strip with the car, with the 2 bbl still in place, I actually ran 1/2 second quicker than with the 650 DP.
The reason being is that launching at wide open throttle at 2000 RPM, the 650 was too much carb, and was actually over feeding my engine, and had to get up to near 4500 RPM for this carb to really be useful.
About the point where the 650 DP was coming into play, the 500 2 bbl was starting to lay down on me, but in the meantime, I was moving a lot quicker off the line with the 2 bbl so in comparison, the 650 was having to play catch up to the 500.
Eventually, I went to a 600 CFM 4 bbl, with vacuum secondaries, set so the secondaries were starting to open at about 3500 RPM, and got down to 14 flat and even ran a couple 13.90 times.
Im not sure if you are familiar with Variable Venturi carbs, but the theory used in them is exactly the same as used with the vacuum secondaries. That portion of the carburetor, only opens to the demands and needs of the engine.
If you are running an automatic transmission, and when dragracing, put the throttle to the firewall, and load up against the torque converter with the brakes locked up, at the line, or torque braking, a vacuum secondary carb is almost essential for getting peak performance.
Given two carburetors, of the same design, and CFM rating, there is no difference in the maximum flow between a vacuum secondary and a mechanical secondary model.
Vacuum secondaries dont run any leaner or richer than a mechanical job, when both are tuned correctly.
A little bit is good but a lot aint always gooder.

Last edited by Max Keith; 07-30-2005 at 11:45 AM.
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Old 07-30-2005, 11:39 AM
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There is no difference between a vacuum carb and a mechanical carb once the secondaries are all the way open. There will be no power gain froom the double pumper in that respect.

Double pumpers when sized and tuned correctly will pick up th RPMs faster than a vacuum secondary. Vacuum secondary carbs allow more room for user error. They also allow more for changing road conditions, gas, driving techniques, etc.
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Old 07-30-2005, 12:11 PM
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Quote:
Originally Posted by Mad Maggot
I was always under the impression that vacuum secondaries are better for street driving than double pumpers, but at the expense of a power loss at higher RPM. The secondaries need a partial vacuum to stay open, causing reduced cylinder filling as compared to a near-zero vacuum double pumper setup. Is this true? Is it also true that vacuum secondary carbs run a bit leaner and get better mileage than the double pumpers? I've heard some rumors floating around local gearhead hangouts that vacuum secondaries are crap for performance use, so I thought I would clear this issue up.
The vacuum secondary uses a small diaphragm and 2 inch line. There is no vacuum loss. They are used for heavy cars and are harder to tune because you may have to change the diaphragm spring to a different size.
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Old 07-30-2005, 12:18 PM
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lluciano77,

Please explain why the DP will pick up more RPM over the vac sec, would this not equate to quicker overall acceleration as well?

Also, is it not a bad idea to use one size larger carb with a vac sec, since it will only draw as much air as it needs? I'm thinking it may help the top end slightly to do this.

I also know that BG offers easily replaceable springs in their vacuum secondary carbs, so this might make tuning a little easier.
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Old 07-30-2005, 12:48 PM
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DPs were developed to serve drag racers. Never intended for any use but WOT blasts down the quarter mile. Of course, since the "If it is good for the drag racer it must be good for me" mantra (RARELY TRUE!!) pertains, they get put on a lot of street/strip warriors. IMHO, in every case a properly tuned vac secondary will out perform ANY DP in a situation where the throttle must be modulated. Witness the monster Holley 950cfm 3-bbl I run on my 354 hemi. They were built in the mid 60s and intended for NASCAR full race use only. Of course, some idiots like me bought them for the street. Even with that pedigree they come with vacuum secondaries. Unless you are running against Jegs Coghlin or Warren Johnson full time, go vacuum and learn how to tune it properly. You will always be ahead of the game.
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Old 07-30-2005, 02:12 PM
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well said willys.

i have tried both and i like the vac sec a whole lot.

--willys i got to pm you about a old post of yours. something about a MSD ignition adapter for a externally voltage regulated mopar. how its the best way to feed your mopars ignition system.
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Old 07-30-2005, 02:37 PM
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Quote:
Originally Posted by willys36@aol.com
DPs were developed to serve drag racers. Never intended for any use but WOT blasts down the quarter mile. Of course, since the "If it is good for the drag racer it must be good for me" mantra (RARELY TRUE!!) pertains, they get put on a lot of street/strip warriors. IMHO, in every case a properly tuned vac secondary will out perform ANY DP in a situation where the throttle must be modulated. Witness the monster Holley 950cfm 3-bbl I run on my 354 hemi. They were built in the mid 60s and intended for NASCAR full race use only. Of course, some idiots like me bought them for the street. Even with that pedigree they come with vacuum secondaries. Unless you are running against Jegs Coghlin or Warren Johnson full time, go vacuum and learn how to tune it properly. You will always be ahead of the game.
Actually, I have a book that has a section where Vic Edelbrock was testing the Holley 3 bbl.. Vic got better gas mileage and more torque than he did with his old school 3310-1. He said the 3 bbl. had better street manners too.
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Old 07-30-2005, 04:25 PM
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i like them cause you can have control over the timing of the opening of the back 2 barrels...lighter springs..etc...is there such thing as a dp vac carb?
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Old 07-30-2005, 04:38 PM
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vacumn secondaries

MM
The guy who writes for Chevy high performance mag said they tested both types of carb and that the vac carb NEVER opened up fully, NEVER. I have a 57 chevy with a 350 that had a vac secondary 750 on it. It never felt like it opened up so after reading this I switched to a 700 mechanical secondary. Big difference. I didnt need the 50 more cfms any more. When I crack the peddle now responce is right there, no waiting by winding it out. I could not believe the difference in responce. My old carb worked O.K. but not like this. If I hadnt tried it I would not believe it myself.
Good luck, steve
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Old 07-30-2005, 07:11 PM
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woundbroom, what kind of carb did you have?

I have a hard time believing that a brand new carb like a Demon would not let the vacuum secondaries open all the way up. If they never did, they probably would not sell very many. What kind of carb did CHP do this test on?
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Old 07-30-2005, 07:47 PM
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Quote:
Originally Posted by lluciano77
Actually, I have a book that has a section where Vic Edelbrock was testing the Holley 3 bbl.. Vic got better gas mileage and more torque than he did with his old school 3310-1. He said the 3 bbl. had better street manners too.
Yes, with the huge secondaries and proper spring, they run like a Rochester Quadra-jet. Primaries are pretty big so I made aluminum inserts for mine but it really is the best of both worlds. Great bottom end drive-ability and more top end than I have the guts to find!!

Incidentally, I put a little micro-switch on the arm of the secondary lever that lights an LED on my dash to help tune the diaphragm spring. And woundbroom is 100% correct - no matter which spring I use, the secondaries RARELY even open and NEVER fully open. The carb runs as a 2-bbl 99.99% of the time even with the weakest spring they make. And that is just what I want - if they ever did open fully (950cfm carb on a 354 engine) I would lose performance 'cause that is WAY too much carb for my engine unless I am @ +8,000rpm which I never will be!! Again bigger isn't better when it comes to carbs. For the combination of low speed responsiveness, high speed power, fuel economy, and overall good performance, vacuum secondaries win hands down. again if you are just interested in WOT, max rpm performance, by all means get the DP. All the vac stuff is just wasted in that case.
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Old 07-30-2005, 08:31 PM
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If it is true that the secondaries never open all the way, does that affect the CFM rating of the carb?
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Old 07-30-2005, 09:11 PM
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Nope. The secondaries will open as far as the engine demand requires. If you have a 500cfm Caddy @ 6000rpm, a 780 will probably open full. When fully open it will flow exactly the same rate as a double pumper 780. However RARELY does a 350 need that much flow area. Again the vac secondary unit is the best of both worlds - small flow area for good throttle response and big WOT when the engine needs it.

Sounds like you are thinking that it is the flow past the secondary butterflies that opens them and that causes excessive pressure drop & low CFM rating. That is not the case. The secondaries are actuated by the diaphragm in the vacuum chamber. The vacuum signal to that diaphragm come from a pin-hole in the narrowest section in the primary venturi. Thus as flow in the primaries becomes high enough, pressure at the venturi constriction falls to the point that it actuates the diaphragm which in turn opens the secondary butterflies. That opening rate is tuned with a bias spring which regulates how quickly they open. Leave out that spring and your secondaries will open wide at just off idle! If you are suffering power loss on top end using vacuum secondaries vs a like sized DP carb, your vacuum secondary carb is not properly tuned.
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Old 07-30-2005, 09:23 PM
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If your vacume secondary carb is sized properly it will open all the way. The vacume signal for opening the secondaries is a product from the air rushing through your right front primary throttle bore. If the carb is too large when the secondary opens it'll kill the vacume created on the primary side and will stop the secondary from opening all the way. Double pumpers are jetted differently than vacume secondary carbs, vacume secondary carbs have a larger fuel enrichment circuit and less main jet for a little better fuel economy at part throttle, double pumpers are jetter richer and are designed primarily for competition. I'll take a DP over a VS anyday- especially if the car is geared right.
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