Holley Power Valve Sizes: Everything There is to Know

In your vehicle, a power valve has numerous functions. It can improve the drivability, manage the fuel better, and increase the air-to-fuel ratio. But to fit it properly in your vehicle, you need to know the proper size for it.

This is why tons of people ask, what are the Holley power valve sizes? It depends on the vacuum reading. As they’re reliable brands, they have bunch of sizes. So with a vacuum reading of 13 inches, you divide it by two and get 6.5. If you get an even number, subtract it by 0.5. This should be the size of your Holley power valve.

Here’s a table of the common vacuum readings with the appropriate power valve sizes —

Vacuum Reading Power Valve Size
13 6.5
10 4.5
8 3.5

There’s a lot more to it, and it might be difficult to understand which valve size to pick first. So today, we’ll cover all about the perfect power valve size for your Holley carburetor, and much more. Let’s get straight into it.           

Holley Power Valve Sizes

As mentioned above, it’s incredibly important to know the right power valve size for your Holley carburetor, or else things can’t work as efficiently. The intake vacuum manifold is expressed in “Hg”, and in most Holley power valves it’s 6.5 Hg.

This means that the power valve will be opened when the manifold vacuum is dropped down to 6.5 Hg or lower.

Most engines that are working smoothly will have an idle vacuum of around 12Hg or 14Hg, but sometimes, it might get as low as 9 Hg. However, you need to divide the idle reading by 2 to find out what size the power valve needs to be.

So with a vacuum reading of 13, you’ll need a 6.5 Hg power valve. If it falls on an even number, such as a vacuum reading of 14, you’ll just need to do the .5 lowest possible option. Thus, if you divide 14 by 2, it would be 7, but instead of 7, you should pick 6.5 as it’s the closest value.

Sometimes, the vacuum reading might even be as low as 8. In that case, you’ll need to pick the 3.5 Hg option for the power valve. So by doing this, you can find what size power valve you need for the carburetor.

How To Properly Tune Power Valve in A Holley Carburetor

The power valve relates to the tuning, but there are some other aspects to look at when tuning a Holley carburetor. So what are the basic things you need to look for if you want to properly tune your carburetor, excluding the power valve as we already covered earlier-

Fuel Pressure

As we know, the power valve also deals with the fuel and air mixture ratio. And the fuel pressure actually plays quite a big role in a Holley carburetor too, and it should be the right PSI.

The optimal pressure is considered 6.5 PSI according to most experts, but the maximum you could go is 7 PSI. With a pressure regular that is adjustable, though, a 6.5 PSI is the only amount you should go for as it works perfectly.

If you’re looking for the best way to find the fuel pressure, take it from idle to full operating temperature.

Float Level

The float automatically regulates the liquid supply to your carburetor, so it’s essential to make sure you have the proper float level. You can adjust this to make sure it works perfectly, and you do this by simply making the locking screws a bit looser or tighter until it works well.

Get The Right Second Throttle Opening

The second throttle opening can allow more air inside the carburetor when needed for power, so it’s important to know when the second throttle should open. But to find the right timing, it’s all about trial and error.

Just adjust the screws until the second throttle opens and your engine bogs, and then turn the screw 0.25 of a full turn. It won’t always be perfect, so just keep adjusting the screw.

Frequently Asked Questions (FAQs)

The Holley power valve sizes can depend on the vacuum reading, and you’ll need to divide it by two. But if you have similar questions, you can check out our most frequently asked questions. Reading these can help clear any further confusion —

How does the vacuum reading relate to the power valve size?

Skipping the technical details, the engine vacuum essentially operates the Holley power valve. The spring inside a power valve only opens at very specific vacuum readings, and the spring pressure can change the operating range for your power valve.
This is why when picking the right size for your power valve, you need to check the vacuum reading first to make sure the springs will be properly compatible.
If you want to know how to properly check the vacuum reading, you can check out this article by MOTOR.

How much can a Holley power valve cost?

The cost of a Holley power valve can depend on the size, but generally, most valves cost around $15. Sometimes it could be a bit over $20. However, most of the time, it’ll cost around $14 to $15.

Conclusion

The Holley power valve sizes are dependent on the vacuum reading, so the size can differ a lot. But to find the size, you first need to get the vacuum reading and divide it by two.

A vacuum rating you might often get is 13, and if you divide it you’ll get 6.5Hg which is going to be your power valve size. But if the vacuum reading is for example an even number such as 8, you’ll need to subtract it with 0.5.

So instead of taking 4Hg as the size, you take a 3.5Hg power valve. This works for all even numbers and sizes. With that said, cheers!

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