The world is changing. It’s been three decades since the last Big-Three production engines employed a carburetor to mix fuel. Today, even lawn mowers and garden tractors feature electronic fuel injection. Yet the carburetor – especially the ubiquitous four-barrel fuel mixer – is still very much alive. With a whole generation of enthusiasts far more familiar with laptops, tuning by keystroke, and speaking in pulse-width, this seemed a good time for a look at how carburetors work.
This is not necessarily a look backward, but rather a more up-close look inward at how carburetors work. The naysayers call these fuel mixers nothing more than carbon-builders or “calibrated leaks,” which is actually exactly how a carburetor functions. Bleed holes, emulsion tubes, and calibrated restrictions are the precision-drilled passages that make all carburetors work, and over the years, those precise tuning combinations are capable of some fairly sophisticated metering of air and fuel.
So we will start this series at its logical launch point by looking first at the arena of four-barrel carburetors. We’re not going to get into everything – Rochester two-barrel, Stromberg, and Weber fans will have to search their information elsewhere. This series is not intended to be all-inclusive. While the basics of mixing air with fuel are the same for all carburetors, we’re going to focus, at least for the near future, on the classic performance four-barrels. There’s plenty of information to cover so let’s get started.
All four-barrel carburetors are rated in terms of their airflow. The most common form of sizing is to rate these carburetors in terms of cubic feet per minute (CFM) of airflow. Most carburetor companies rate the capacity of their four-barrel carburetors using a test depression of 1.5 inches of mercury (inHg). Where it gets a little confusing is two-barrel carburetors are rated at 3.0 inHg, because they are slightly more restrictive – so the CFM numbers are not directly comparable.
The classic and best known four-barrel is of course the Holley modular that has morphed into a myriad of spin-offs, models, and sizes. In addition, there are several companies that build their own version of the traditional Holley – Demon being among the best known along with a newcomer – FST Carburetors, in addition to other front-runners.
Back in the ‘50s, the first four-barrel carburetor bolted to a production car was found on a Buick straight 8 and was called the Will Carter Four-Barrel (WCFB) that was followed in ’57 by the aluminum four-barrel (AFB) Carter that became extremely popular, especially with Mopars. Those early versions were soon followed by the air-valve secondary or AVS versions that evolved into larger CFM sizes. There was even a somewhat less well known Carter Thermoquad four-barrel popular in the ‘70s that used a plastic main body to minimize the transfer of heat into the fuel. These are still available as rebuilt units through Summit Racing.
GM soon followed with the 4GC four-barrel that predated the Quadrajet by roughly a decade. The Q-jet is by far the most popular four-barrel carb from Rochester rated at 750 cfm with a few 800 cfm units used on 455ci Buicks for example. All Q-jets employ a spring-loaded air valve door over the mechanical secondary linkage and combine that with a sophisticated collaboration of jets and metering rods to deliver fuel.
There have been a few other notable aftermarket efforts that enjoyed notoriety. The Predator was a large-CFM, variable-venturi carburetor that had some success. Other unique styles included Motorcraft’s inline four-barrel carburetor that now occupies a unique footnote in the hierarchy of performance carburetors. There’s enough important information we’ll try to impart on the most popular line of carbs that we can save the esoteric stuff for another time.
Among other designs that are worth noting is an earlier Holley carb line that departed from the traditional modular design to minimize external leaks. These were called the 4010 and 4011 models and offered a very efficient annular booster placed in a main body with a top lid that placed the sealing lid gasket above the fuel line. These were decent carburetors that just never gained any real popularity. With some subsequent improvements to its aluminum body, it has resurfaced in the Summit carburetor line and is easy to work on because it uses several traditional Holley tuning parts like jets and the accelerator pump diaphragm.
When It Comes To Carbs, Size Matters
Size plays a big part in the game of choosing a four-barrel carburetor and CFM ranges for four-barrel carburetors can go from smaller 350-400 cfm devices all the way up to the giant Holley Dominator series of carburetors that can deliver off-the-shelf numbers as large as 1,425 cfm, with custom versions easily exceeding this number. However, bigger is often not better, when it comes to selecting the proper carburetor for a specific application. This is where the educated blend of art and science are combined to create the proper fuel mixer for a specific application.
Beyond just CFM ratings, there are plenty of different configurations that can affect the carb of choice. For example, we can start with how these carburetors actuate the secondary set of barrels. The simplest form of secondary operation is purely mechanical. This often requires the addition a separate accelerator pump circuit to compensate with additional fuel when all four barrels are slammed open – especially at low engine speeds when the engine’s airflow demands are far below the carburetor’s airflow capacity.
But mechanicals are not the only choice. We alluded earlier to the Carter AVS that uses a weighted air valve door over mechanical secondaries. In this application, if the throttle is slammed opened at low engine speeds, the air valve will only open as the airflow past the primary throttle creates sufficient velocity to force the door open. This is the same concept employed by the Rochester Q-jet.
In that application, the secondary air valve rotates on an offset air valve door. As airflow increases pressure on the offset valve, it opens against an adjustable spring pressure and allows air to enter the carburetor. This air valve door also operates the secondary metering rods that meter the fuel for the secondary side of the carburetor. It’s an ingenious system that works extremely well when properly tuned.
Another way to control the secondary opening point on a four-barrel carb is with a vacuum diaphragm that uses primary airflow velocity to create a vacuum signal in a diaphragm module. This is the process most commonly used on Holley carburetors. This signal pulls against light spring pressure to open the secondary throttle blades as the engine demands more air. This eliminates the need for a secondary accelerator pump circuit as the gradual opening rate maintains sufficient air velocity to generate a signal to pull fuel from the boosters.
It’s Not All About Airflow
But not everything with regard to carburetors has to do with airflow. There’s also the critical nature of how the carburetor mixes fuel with the air. This mostly has to do with the design of the main metering circuit. Within that discussion, there’s an entire story just on booster design and how the booster introduces the fuel into the venturi. We’ll leave that to a future story and as a little teaser on the art of carburetor design.
We’ve really just rubbed the outer crust of what is a deep archaeological dig into all the metering variables of different carburetor designs and applications. The monster-sized Dominator carbs that your favorite No-Prep hero currently uses would likely not be a great choice for a 450-horsepower, 383ci small-block that spends all its time cruising the neighborhood. In fact, a good Q-jet would be an excellent choice based on its small primaries. As you might guess, personal preferences, brand loyalties, and image also play their respective roles in carburetor selection.
The world of carburetors offers just about anything you could imagine and, not surprisingly, the nuances often make a huge difference in how the carb responds when you tickle the throttle pedal. We’ll try and lay out some of those nuances in subsequent episodes of this carburetor quest. As they say, it would be best to stay tuned, because while modern technology continues to rapidly advance, the carburetor is still maintaining its relevance in the performance world.