for instance the

q-jet
Edelbrock-Carter
Holley VS

opens up the secondaries when there is ? CFM of airflow
in the primaries?

Just before the secondaries open,
how much HP is 'lost' by
having only the primaries open?

ciao
contactpatch

 

Assuming properly sized and tuned, it's air demand, so therefore CFM. The Holley is the only one that opens the secondaries based on flow through the primaries. The others are basically vacuum under the air valve.

Power lost is something less that the factor of flow with secondaries vs. without.

 

does anyone have any numbers on what vacuum
is needed for the Edelbrock air valve to start opening?

the reason I ask is,
people will say something like...
'I own this or that carb, and i can hear, or,
I can feel the secondaries opening.,

I would think that {feeling more power} would be some
other issue, as I would think that by the time
there is a ten percent loss of power,
the secondaries would open, but I don't know that.

perhaps someone would like to comment about
how much 'time delay', there is in the
q-jet or the Holley VS type carbs.

I know that anything can vary,
any info appreciated.

 

It isn't manifold vacuum that opens the secondaries, it's air flow.

Feeling the secondaries opening is a sign that the carb is mistuned. Ideally, the secondaries should be opening perfectly with demand, and you shouldn't feel a thing. Feeling a surge when the secondaries open usually means that they were opening too soon causing a flat spot.

In the Performer/AFB series carbs, the opening of the air valve is controlled by the counterweights on the air valve. It isn't tunable without disassembling the carb and modifying the weights.

In the Thunder/AVS series, Quadrajet and Thermoquad, the opening of the air valve is controlled by the tension of the air valve spring.

The Holley/Demon VS carbs are more unique in that venturi vacuum is used to sense air flow and open the secondaries. They're still tuned by spring tension though, but it's tension against the secondary vacuum diaphragm.

 

Remove the top of your Edelbrock carb. Lift out the secondary velocity valve. Reinstall the top of the carb.

Go for a ride and try it at WOT.

Now go back and reinstall the secondary velocity valve and
Leave it Alone!!

 

yah most of the time with holleys with the vacuum secondaries, if you can feel the secondaries open up i.e. sudden boost of power...then you have too much spring tension and need to step it down to a spring with less tension, that will let the secondaries open up quicker.....