i know what the diffrence is and that im gonna be running a VS carb on my current project, but how do they work. i have a mechanical secondaries on my 82z and thats pretty straight foward but what does the vacuum hook up to or does it?

 

On a Holley or Demon vacuum secondary carb, the vacuum passages are internal. The vacuum diaphragm itself is external, and linked to the secondary butterfly by a rod coming out of the bottom of the diaphragm.

See page 6 of this Holley doc. http://www.holley.com/data/TechServi...ech%20Info.pdf

 

oh makes sense now thanks, so it creates its own vacuum and nothing needs to be hooked up to it. is the vacuum advance on a distributor the same way?

 

No, the diaphragm has vacuum applied to it via the internal vacuum passages in the carb's main body. Those passages draw their vacuum signal directly from the primary side main venturi (modified by a small "balancing" passage that connects to the secondary side main venturi).

That vacuum signal is what overcomes the spring tension in the secondary diaphragm and begins to open the secondary throttles.

All of this makes no sense unless you understand what the vacuum signal from the main venturi acts like. Short and sweet answer... the more air that is flowing through the venturi (due to greater throttle opening and/or higher RPMs), the higher the vacuum signal is in the venturi. Essentially, it's a good indicator of how much airflow there is going into the engine. The more the engine draws greater airflow, the more vacuum is generated in the venturis and the more it tries to open the secondary throttles to feed that airflow need. Truly, only giving you the airflow through the carb that the engine needs.

A marvel of old-school engineering. Simple, elegant and effective.

Only thing better would be a QJet.

A vacuum advance can on a distributor requires an EXTERNAL line going back to an appropriate vacuum port on the carb and that vacuum signal is NEVER directly from venturi vacuum. It's always more or less manifold vacuum which DROPS with greater throttle opening.

The basic mechanical prinicple is the same- vacuum vs. spring pressure. But the type and source of vacuum they use are completely different- almost exact opposites.

 

ok im with you so far Damon thanks for the info, so the vacuum advance on a distributor gets its air from the intake and not the carb?

 

The vacuum advance on the distributor is usually connected to a vacuum port on the carb which supplies ported or unported manifold vacuum. It's different from the venturi vacuum that operates the secondaries.

 

so if you get a vacuum advanced distributor you have to have a VS carb right? cause im building a non computer controlled 350 with with no emissions worries, so i just want plain and simple and easy to tune.

 

No, vacuum advance for the distributor has nothing at all to do with vacuum secondaries.

 

Except that they use vacuum from the engine drawing in air to determine the proper settings.

The engine drawing air through the intake is what causes vacuum. There is a low pressure in the intake at all times. On a naturally aspirated engine, it's generally always below atmospheric pressure (ie, it's vacuum). That lack of air pressure in there what sucks more air in. On forced induction cars, it's a little different, but still remarkably similar.

The key to understanding the whole thing is that vacuum, or more specifically, the amount of vacuum, is an indicator of how much load is on the engine. The amount of load on the engine is very important when it comes to when the spark should happen and how much fuel it should get. If you rev the car in neutral, there is no loadon the engine, therefore hte engine sucks in all the air it can, as fast as it can. If there is a load on the engine, then the amount of RPMs the engine can turn for a given throttle position are much, much less than in neutral, but the throttle is open the same amount as before. The difference is under load there is less air going into the motor, less low pressure in the intake manifold, etc, because the full theoretical speed of hte motor for that given throttle position is being restrained by the load on the motor.

Since engine load is so important when it comes to how much fuel and spark timing the motor needs, most of the devices that measure fuel and spark were cleverly designed so that they would adjust for vacuum, or load, on the engine. They do this over a whole range. IT's really amazing how simple the solutions are for such a complicated problem when you realy delve into it.

For instance, we've established that under load the manifold pressure increases, or the vacuum decreases, depending on how you wnat to look at it. Same thing. As the engine is loaded more, it needs more fuel. What happens is vacuum secondaries are held closed by vacuum. As the vacuum decreases, there is less suction on the blades holding them closed, and when the manifold pressure rises above a set amount, they begin to open and flow air through the secondary jets and venturis, therefore providing more fuel.

EFI systems work around all the same measurements, they just do it electronically instead of mechanically.

 

Not quite, it's venturi vacuum that operates the secondaries in a VS carb, not manifold vacuum. You can test the operation of the VS mechanism with the engine turned off, or even with the carb removed from the engine entirely, by opening the primaries and blowing compressed air across the secondary vacuum port in the air horn.

 

You're right, I was just trying to be more general about it.