Holley 4-barrel carb - Car Tech Books

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There is much to consider when choosing a carburetor for your vehicle.

Airflow, choke style, and even the physical height of your carburetor will come into play.

But what about the barrels? How do they work? And what type of secondaries do you want?

Here are the basics you need to know about primary and secondary barrels.

What’s the Difference Between a Carburetor’s Primary and Secondary Barrels?

A four-barrel carburetor has two primary and two secondary barrels.

At idle and low-rpm driving, only the primary barrels open. The throttle blades on the secondary barrels stay closed.

As rpm increases, the engine requires more air and fuel to make power, and this is when the secondary barrels kick in. As the secondary barrels begin to open, more air travels through the carburetor, supplying the engine with the air and fuel it needs.

What are the Different Secondary Types?

Vacuum Secondary

The secondary blades are attached to a vacuum diaphragm by a rod. As rpm increases, so does the vacuum in the intake manifold. The added vacuum moves the diaphragm. This moves the rod and opens the secondary throttle blades.

Mechanical Secondary

These secondaries open with a mechanical linkage. The more you push the pedal, the more they open.

Air Valve

These secondaries have an air valve, similar to a choke blade, above the barrels. As engine vacuum increases, it opens the air valve. Some air valves are counter weighted. Others are controlled by a spring.

How Do the Different Secondary Styles Affect Performance?

It is important for the secondary barrels to open at the right time. It is also important that they open at the optimum rate to precisely meter the air and fuel the engine needs.

If they open too slowly—or too late—it will starve the engine of air and fuel. The end result is less top end power.

If they open too fast—or too soon—the engine will get bogged down. This lets in too much air and not enough fuel. This lean condition will result in poor throttle response and slow acceleration.

Vacuum Secondary Carbs are a good choice for street cars. Because they are controlled by vacuum, they respond automatically to the engine’s needs. Adjusting the secondaries is as easy as changing the spring inside the diaphragm.

Mechanical Secondary Carbs are used in race cars that operate at max throttle. They can also be used to give the driver more control. Tuning the accelerator pump is required to prevent a bog. This is more complicated than a simple spring change.

Air Valve Carbs provide the best throttle response on street cars. Air valve-style carbs with counterweighted doors aren’t adjustable. However, adjusting a spring-controlled air valve is as simple as turning a screw.

Author: Brian Nutter

After a stint in the U.S. Air Force, Brian Nutter studied at the Houston, TX-based School of Automotive Machinists in 1997. The early part of his automotive career included working for engine builders Scott Shafiroff and C.J. Batten, followed by several years developing performance pistons at Wiseco Piston Co. Today, Brian develops performance parts for Summit Racing Equipment and is a regular OnAllCylinders contributor. For fun, he runs his 427-powered C5 Z06 in ECTA land-speed racing, at OPTIMA® street car events, and at a mix of autocross, drag racing, and track days.