Quick Question...what are vacuum secondaries?
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,713
Likes: 31
From: Shreveport LA
LA Events Coordinator
Quick Question...what are vacuum secondaries?
I have a Holley 4bbl "Street Avenger" 670cfm with "vacuum secondaries"...What exactly is that. Whenever I have the air cleaner off & look into the throat of the carb and rev the engine quickly to 3500-4000rpm, the primaries shoot a small stream of gasoline down the carb...but nothing on the back 2 barrels... 
Burning Brakes
Joined: Feb 2004
Posts: 958
Likes: 1
From: In the Beautiful Shenandoah Valley of VA
Vacuum secondaries means that the rear two barrels of the carburetor aren't mechanically connected to the throttle, they are controlled by a vacuum operated servo that opens based on engine air flow volume, usually controlled by venturi vacuum, but not always. Because they open slowly, as needed, rather than being slammed open by your foot, an accelerator pump shot isn't needed to cover a bog inducing situation. Holley originally developed the double pumper (accelerator pump in the secondaries) to overcome the lack of airflow on mechanically operated secondaries.
In many cases, since the secondaries are more demand controlled, a smoother off idle hard throttle condition can be seen. It can take less tuning to get a vacuum secondary carburetor running properly under those conditions than a mechanical secondary carburetor. (flame suit now on)
Steve
In many cases, since the secondaries are more demand controlled, a smoother off idle hard throttle condition can be seen. It can take less tuning to get a vacuum secondary carburetor running properly under those conditions than a mechanical secondary carburetor. (flame suit now on)
Steve
Pro
Joined: Apr 2004
Posts: 687
Likes: 0
From: Atascadero Ca
if you read barry grant's site they reccomend you buy the vacuum secondary carbs if you have an auto and go with mechanical if you shift your own gears.
There was a thread i read today that went more into the details of this...
Scroll about 2/3 of the way down and start readin.
There was a thread i read today that went more into the details of this...
Scroll about 2/3 of the way down and start readin.
Le Mans Master
Joined: Mar 2004
Posts: 7,464
Likes: 1,485
From: Little Rock AR
Mechanical secondaries open when you decide to open them. That can be too much airflow at low speeds and they use a lot of gas when you drive them hard. They generaly work best for stick shift cars where economy is not a factor.
Vacuum secondaries will only open when the engine is under high load conditions so the effect is better metering under most all driving conditions. These are recomended for street use especialy for automatic tranny. Stick can use vacuum secondary too but you do not get the same kick from reving up in low gear and then punching it like you do with mechanical secondaries.
Vacuum secondaries are no slouch if they are set up right.
Reving it in the driveway does not put a load on the engine so vacuum secondary carbs will not open that way.
-Mark.
Vacuum secondaries will only open when the engine is under high load conditions so the effect is better metering under most all driving conditions. These are recomended for street use especialy for automatic tranny. Stick can use vacuum secondary too but you do not get the same kick from reving up in low gear and then punching it like you do with mechanical secondaries.
Vacuum secondaries are no slouch if they are set up right.Reving it in the driveway does not put a load on the engine so vacuum secondary carbs will not open that way.
-Mark.
Racer
Joined: Aug 2004
Posts: 399
Likes: 0
From: Chambersburg PA
To know what it sounds like when secondaries kick in rent the origional Blues Brothers. When they approach the Illinios Natzis on the bridge the back two barrells open up. That is how they knew it time to hit the water!!!! 
Race Director
Joined: Oct 2000
Posts: 17,102
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From: Fair Oaks, OK
Originally Posted by 75rag4sp
To know what it sounds like when secondaries kick in rent the origional Blues Brothers. When they approach the Illinios Natzis on the bridge the back two barrells open up. That is how they knew it time to hit the water!!!!
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Safety Car
Joined: Jun 2002
Posts: 4,135
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the bridge your referring to is in marquette park, at the time of the movie the ***** used to hold rallys there, i believe they also had a headquarters in the area on 71st street-- as a kid we held boy scout outings in that park ,so i know exactly where that bridge is.people my age that were from that area ,,refer to it as the ''troll bridge'' (i guess because there's a troll living under there!)
Last edited by carl a; Sep 26, 2004 at 05:02 AM.
