Understanding Carburetors: Vacuum Secondaries vs. Mechanical Secondaries

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When you’re starting to learn about carburetors, you will be hit with enough homework to make your head spin. It not only requires studying how they work, but it also requires learning a whole new vocabulary. After all, you can’t follow instructions if you don’t know what you’re looking for or looking at, and you can’t make adjustments until you know how it all ties together. 

Take vacuum secondaries and mechanical secondaries, for example. You may need to decipher something such as the operation of the secondaries on a carburetor from a discussion on making improvements to those systems, and it can create a lot of confusion.

Understanding Carburetors: Vacuum Secondaries vs. Mechanical Secondaries
The secondary side on a vacuum secondary carburetor. Photo by Hank O’Hop

That’s what Car Bibles’ carburetor cadets are for. We’re going to talk a little bit about how the secondaries work on different four-barrel carburetors so you can better understand what you’ve got to work with. Look for more carburetor stories in the future. But for now, let’s get to it.

What Are Secondaries?

You might already be scratching your head trying to understand what exactly secondaries are, so that’s where our discussion begins. 

Understanding Carburetors: Vacuum Secondaries vs. Mechanical Secondaries
Mechanical secondaries at work. GIF by Hank O’Hop

To state the obvious, a four-barrel carburetor has four barrels. The barrels are the openings in the main body of any carburetor. These are more than a simple opening for air to flow through, though. Each barrel is designed to manipulate the airflow with what’s known as a venturi. A venturi is a narrow passage that works to increase air speed. As that air speeds up, pressure drops and creates a vacuum that works to draw fuel into the air stream.

The size and count of these barrels correlate with how much air and fuel are supplied to the engine. That’s why most performance builds call for a four-barrel carburetor.

Each barrel on the carburetor begins to supply air and fuel as the throttle blades underneath start to open. Not all four barrels open at the same time on most configurations. In most cases, you have two blades open first. After so long, the second set will open. As you can guess, the first sets to open are your primaries, and those that open later are your secondaries.

How Secondaries Work

Out of the gate, we need to mention that comparing carburetors with mechanical and vacuum secondaries tailors the discussion to Holley carburetors or those based on similar designs. Barry Grant, Proform, and Quick Fuel are some of the other carburetor manufacturers that lean on the operating systems we’re discussing.

Understanding Carburetors: Vacuum Secondaries vs. Mechanical Secondaries
Accelerator pump for mechanical secondaries. Photo by Hank O’Hop

Mechanical secondaries are the easiest to understand since the operation mirrors that of the primaries. As we said, each of the barrels on your carburetor supplies air and fuel when the throttle blades are open. The operation of the primaries on most carburetors is achieved through a mechanical linkage attached to the throttle cable. As you depress the accelerator pedal, that cable pulls the blades open.

With a mechanical secondary carburetor, the linkage on your primaries is attached to a mechanical connection on the secondaries. The link is reliant on a connecting rod that moves through a slot on the secondary linkage. As the primaries open, that rod will slide through the slot until it is stopped and then begins to pull the mechanism, opening the secondaries.

It’s important to note that fuel supply is also mechanical in this type of system. All four barrels on mechanical secondary carburetors are given fuel with the aid of an accelerator pump that’s depressed once the throttle blades start to open. There’s more to their function, but we need to understand that much to further understand how vacuum secondaries work.

Understanding Carburetors: Vacuum Secondaries vs. Mechanical Secondaries
As you can see, the throttle linkage doesn’t open the secondaries on a vacuum secondary carburetor. GIF by Hank O’Hop

To start, vacuum secondaries don’t work on a mechanically driven linkage. Remember when we talked about how the air in the venturi builds vacuum? That’s what vacuum-secondary carburetors take advantage of.

There is still a mechanical linkage between the primaries and secondaries, but it doesn’t drive the secondaries open. It simply allows them to open. Instead, a vacuum-actuated mechanism drives those secondaries open as vacuum pressure in the carburetor increases.

Understanding Carburetors: Vacuum Secondaries vs. Mechanical Secondaries
Note how the vacuum canister is linked to the secondary throttle blades. Photo by Hank O’Hop

But how does vacuum build if those blades aren’t open to begin with? A canister on the side of the carburetor taps into the vacuum signal in the carburetor. As vacuum builds, a diaphragm in the canister pulls on a rod that’s connected to the throttle blades. As opposed to the mechanical secondaries that use the aid of an accelerator pump, fuel delivery is totally reliant on airflow. As more air is pulled through, it can draw fuel along with it.

Understanding Carburetors: Vacuum Secondaries vs. Mechanical Secondaries
Photo by Hank O’Hop

What About Air Valves?

Only talking about vacuum and mechanical secondaries limits the scope of this conversation to Holley four-barrel carburetors. But it isn’t the only brand to offer four-barrel carbs, and many of the others don’t rely on either of the secondary types we’re talking about here. That leads the conversation to air-valve secondaries (AVS). Some examples of carburetor manufacturers that put this system to work are Edelbrock, Rochester, and Carter.

Understanding Carburetors: Vacuum Secondaries vs. Mechanical Secondaries
The throttle blades are mechanically driven, but there’s more than meets the eye. GIF by Hank O’Hop

An AVS carburetor relies on both mechanical and vacuum systems for the operation of the secondaries. As with a mechanical-secondary carburetor, the AVS type uses a mechanical linkage to drive the secondary throttle blades open.

Understanding Carburetors: Vacuum Secondaries vs. Mechanical Secondaries
The secondaries are reliant on the opening of this door. GIF by Hank O’Hop

But rather than allowing air to immediately flow through, an air valve attached to counterweights or springs must be overcome. The air valve looks and moves much like a choke and is opened by increasing vacuum in the carburetor. As with the vacuum-secondary system, that airflow is also responsible for increased fuel flow.

What’s the Difference Between Secondary Types? 

While both carburetor types work to achieve the same job, there are some differences in their performance. That’s not to say one is always superior to the other. All have their strong points, and the better choice generally depends on the application.

We’d first like to address some commonly held generalities you’re bound to run into when discussing secondaries. For example, some folks will state that you run mechanical secondaries with a manual transmission and vacuum secondaries with an automatic transmission. Another common thought is that street-driven vehicles should use AVS carburetors and mechanical secondaries are better for performance builds.

All of these statements hold water. Mechanical secondaries are favored by manual and performance applications because there’s no dependence on engine load, which contributes to consistency in performance when dealing with fast and high-revving applications. But because vacuum-secondary carbs are dependent on engine load, that makes them far more forgiving in daily driving scenarios. There’s less risk of giving the engine more air and fuel than it needs, ultimately keeping it from burning tires at red lights and so on. Furthermore, the fact that the engine only draws in as much fuel and air as it needs with these types of carburetors, it’s no surprise that they offer better fuel economy than carburetors with mechanical secondaries. 

However true that may be, there is a lot more to consider than just the transmission and the type of driving. The rear gear, the weight of the vehicle, the size of the cam, and a list of other factors come into play, and you’ll want to do the homework to decide which is truly the better choice for your application.

We should mention that you shouldn’t have much trouble dialing in the carburetor for any use with a stock or near-stock application. It’s simply made easier when you pick the type of carburetor with characteristics that would be a natural pair to the application.

Car Bible’s Glossary for Carburetor Secondaries

Welcome to bible school!

Barrel

Barrels are the openings in the main body of the carburetor through which air and fuel pass. The count and size of these barrels deal with how much airflow the carburetor can support. A critical detail about the design of these barrels is that they use an irregular design to create a venturi to manipulate airflow.

Venturi

A venturi is a narrow section between two wider passages that work to accelerate airflow. The barrels in the carburetor use this design to increase airflow. A benefit of the increased air speed is that pressure drops. This pressure drop is responsible for providing a continuous flow of fuel through the carburetors.

Throttle Blade

The throttle blade, otherwise known as the butterfly valve, is the valve at the bottom of each barrel that controls airflow. It’s a simple plate that sits on a rod and pivots to allow more or less air to travel through the barrels.

Secondaries

The secondaries are simply the second set of barrels on a four-barrel carburetor. They’re called secondaries as they come in after the primaries throughout the throttle cycle. Keep in mind that you can adjust the carburetor to change that. For example, adjustable linkages and vacuum diaphragms or springs will reduce the amount of time it takes for the secondaries to open.

The Car Bibles Questionnaire

Car Bibles answers all your burning questions!

Q: Which is better, mechanical or vacuum secondaries?

A: That depends on a lot of factors. For the most part, you can reference the generalities above to get you an idea of which is better for your particular application. Even then, you want to do a little homework to make sure you’re right. Remember that there’s more to consider than just what kind of transmission is in the car. Vehicle weight, cam size, and driving conditions are other major players.

Q: Can you convert vacuum secondaries to mechanical?

A: No. You cannot convert a vacuum-secondary carburetor to use mechanical secondaries. They function on entirely different systems with very few interchangeable parts. That said, you can dial in a vacuum-secondary carburetor to activate the secondaries as quickly as mechanical secondaries. This is achieved by swapping out springs in the diaphragm that work to delay the opening of the secondary throttle blades. An alternative is to install an adjustable diaphragm.

Q: What makes a carburetor a double pumper?

A: The name double pumper comes from the simple fact that two accelerator pumps are featured on that type of carburetor. The mechanical secondaries mirror the primaries for all intents and purposes, including the accelerator pump circuit used to supply a stream of fuel upon initial activation.

Q: Do Edelbrock carburetors have mechanical secondaries?

A: No. Although there is a mechanical linkage that works the secondary throttle blades, these are not to be mistaken for mechanical secondaries. The air valve works to moderate airflow through the secondaries, effectively making function dependent on that system.

Q: What’s better, a two-barrel or four-barrel carburetor?

A: This is not an easy question to answer as it ultimately depends on the application. It’s common for newcomers to think that bigger is better and that four-barrels are the better choice. The thing to understand is that a carburetor is built to match a given amount of airflow an engine will produce. You can exceed the demand with a carburetor that is too big and hurt engine performance. In short, neither configuration is universally superior as you must match the carburetor to the application.

Video on Vacuum and Mechanical Secondaries

We get it, we threw a lot out there, and you might have a hard time following along. It’s all going to be alright, though. This quick video does a great job at explaining some of the basic differences between vacuum and mechanical secondaries.

We understand that not everyone has a bunch of carburetors at their disposal. And we don’t expect everyone to know which models to consider installing when the time comes for a new one. Some of our favorites are the Holley 850 CFM Double-Pumper Carburetor, Holley Street Avenger 770 CFM Vacuum-Secondary Carburetor, and Edelbrock 1906 AVS2 650 CFM 4-Barrel Carburetor. We also know you’re going to want to keep either the Holley Renew Kit or Edelbrock 1477 carb rebuild kit on hand if you pick up any carbs from the appropriate suppliers. Don’t worry. You can use Walmart and Amazon as sources for all of these products.

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