How does the power valve system in a Holley carburetor work?

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Terms to know to make this explanation easier to understand.

Main Jet: Controls the amount of fuel that enters the Main Well of the metering block.
Main Well: The passage that carries fuel from the jets to the Boosters.
Booster: The ring that is visible when looking down the throttle bore.
Power Valve: A vacuum operated device that controls the activation of the power valve system (more on this later).
Power Valve Channel Restrictions (PVCRs from now on): Two small holes, one for each Main Well, that act like jets to control the amount of fuel delivered from the Power Valve System.

First let's start with the Power Valve itself.

A Power Valve is little more than a vacuum operated on/off switch for fuel. It operates using a vacuum diaghram and a spring, which determines when the valve opens. I believe this is best explained using an example. Let's use an engine that makes 13" manifold vacuum at idle, a typical lightly modified performer. Knowing that value, we know that a 6.5" rated power valve would be used (typical power valve selection is 1/2 the amount of vacuum at idle, in our case 13/2=6.5). So, during idle and light cruising (above 6.5" vacuum) the power valve will be in the closed position ("off"), but during moderate to full throttle operation the vacuum will drop to below 6.5" and will cause the valve to be open ("on"). The power valve itself doesn't control the amount of fuel delivered, that's the PVCR's job.

Now to the Power Valve System (PVS from now on).

The easiest way to explain the PVS is that it functions like a vacuum actuated set of jets built inside the metering block. What this means is that when the power valve is activated ("on") it will allow fuel to flow through the PVCRs into each main well (one for each main jet), on it's way to the boosters, thus making the fuel mixture "richer". This system is one of the reasons Holley carburetors are so tuneable. It allows the carb to be jetted for maximum economy / driveability, while at the same time making it possible to tune for full throttle performance without having to rely soley on the secondaries. Remember the PVCRs control the amount of fuel delivered, not the power valve.

Why does the carburetor need a PVS?

It all comes down to economy vs driveability vs performance. For the sake of simplicity, I'll leave out the secondary system and focus solely on the primary side. If there were no PVS, all of the fuel required would need to be delivered by the main jets. This is a problem because as engine vacuum decreases the need far a larger jet arises since the booster signal weakens. What this means is that if a carburetor was jetted for idle / cruising situations, the moderate / full throttle performance would suffer, and be too lean. On the other hand, if the carburetor was jetted for maximum full throttle performance it would run too rich at idle / cruise, since the higher vacuum draws a stronger signal. The PVS was designed to combat this very problem, and does it very well.

Removing (blocking) the Power Valve from the secondary block.

This is something that is very common practice among racers. The reason it works in the secondaries and not in the primary is because you never use the secondary circuit while just idling or cruising around, so the problem listed above doesn't exist. Just remember that if the secondary power valve is removed (blocked) the jets must be increased to compensate of the lack of fuel. Usually an increase anywhere from 6-10 sizes is needed, depending on PVCR size and carburetor cfm.

I didn't expect to write so much but I hope this gives a good general understanding of how the system works and why it's needed.
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