How do vacume secondaries work? The rear butterflys are still controlled by your foot, right?

 

No. A vacuum signal from the primary venturi acts on a diaphram to open the secondary throttles.

BigBlockk

Later.....

 

So the rear butterflys are accually vacume controlled, not mechanically linked? Is that correct. do they work very well? Mechanical secondaries better?

 

They are vacuum actuated. There is nothing wrong with vacuum secondaries.

BigBlockk

Later.....

 

Sounds like you get less controll with them.

 

For street cars they are the best way to increase low end performance and mpg. There are only openned when they need to be allow for better low end performance. For cars that are not setup for racing, if you open the seconadries to soon you will feel a bogg. This is caused by allowing too much air to pass through the carb at one time and the fuel has to play catchup.

 

an exception is the holley DoubleP. 4165.
it works awesome, even on a 283. it is the Qjet upgrade. once u try it, u won't go back.

 

BigBlockk has already answered your question correctly, but thought I'd add some additional clarification:

The vacuum secondaries on a vac sec carb are controlled and opened solely by vacuum signal - they cannnot be opened by simply pressing the pedal to the floor. This vacuum signal is produced by venturi vacuum through one of the primary venturies: There is actually a small hole drilled through the wall of the passenger-side primary venturi which applies vacuum to the opposite side of a spring-loaded diaphragm. Vacuum is generated in a venturi due to the "Bernoullli Principle," where the vacuum (or low pressure) becomes greater with greater mass flow (velocity) through the venturi. Note that this is completely independent of, and non-associated with, manifold vacuum (since there is no manifold vacuum at wide open throttle). Once the vacuum secondaries get cracked open and moving from the primary venturi vacuum, they are also supplemented with a vacuum force from the secondary venturi vacuum. The amount that the secondaries open is determined by the airflow requirements of the engine.

Since the vacuum applied to open the secondaries is supplied based on airflow through the primary side, you do have very good control over whether or not the secondaries open: they open based on how far you have your foot into the carb and the rpm you're spinning the engine. The secondaries don't just randomly open due to some random vacuum signal - they open based on your throttle input. The vacuum feauture simply prevents them from opening prematurely, before the engine needs the airflow. For this reason, they work very well with auotmatic trans cars and street driven performance cars. The opening point of the secondaries can be tuned through use of different tension springs on the vacuum diaphragm, thus making the carb very responsive to the specific application.

 

Thank you.
Wasn't sure how they worked,, now I know.
Chris.