Despite the popularity of...
Despite the popularity of the electronic fuel-injection era, many enthusiasts rely on the carburetor to supply the proper mixture of air and gas. Today, you'll find an ample supply of aftermarket suppliers who can tailor a carb to your engine's specific needs. In some cases, this can be worth 20 or more horsepower.
Lookin' down the barrels of...
Lookin' down the barrels of a 4150-style Holley four-barrel. This one has mechanical secondaries, dual pumps, and dual feeds. It's one of the most common carburetors you'll find on the street or track.
Carburetors have many parts...
Carburetors have many parts and a few circuits. The series of circuits is necessary to supply the correct mixture in all driving conditions. This is the main body of a Holley HP-series carburetor. It houses the four venturi, or barrels, as they are commonly called-as well as a series of passages (called bleeds) for the air and fuel to flow through.
This is the baseplate, which...
This is the baseplate, which mounts to the underside of the body. It contains the throttle blades, or butterflies and linkage, which controls the flow of air into the engine.
There are different-style...
There are different-style carburetors on the market, but they function the same regardless of brand or type. Most Holley, Demon, and other aftermarket carbs use a metering block (shown) between each float bowl and the body. Metering blocks house the main jets and power enrichment valve, which both feed fuel to the main metering system. This is a billet metering block from Quick Fuel Technology.
Is EFI really the end-all be-all for controlling the air and fuel mixture? Has it left the carburetor in the dust? While this is one sizzlin' topic for debate, we're not going there today. Instead, we'll explore the world of carburetion and take a look at some of the recent technology in the world of non-electronic air/fuel mixing.
The four-barrel carb is a classic performance piece that has been used in OE and aftermarket form. It's prominent in professional- and sportsman-level classes, despite the fact that the factories haven't used them since the mid-'80s. What makes carburetion so attractive is the simplicity (i.e., lack of wiring, software, and a computer), along with cost, which is often a lot less than EFI.
But picking a suitable carb can be difficult, so to defuse the situation, we'll look right down the barrels and expose some important theory behind basic operation, sizing, tuning, and for good measure, we'll throw in a few tips from the pros.
The first thing to learn is that the carburetor is just one piece of the performance puzzle. To get the most from your carb, it must be matched to the entire combination. Unfortunately, this is one area where many miss the boat. Know that rear gearing, transmission type, transmission ratios, vehicle weight, and driving style will affect the carb, along with the overall performance and driveability of your vehicle. Therefore, under-standing the details about your combination is the first step you should take. It will allow you to zone in on the best possible carburetor with the least amount of trial and error. Next, be realistic about what you plan to do with the car.
Many of you know the carburetor meters the flow of air and gas, but do you know how this happens? To function effectively under all driving conditions, the carburetor has systems or circuits that allow it to accept incoming fuel, store a small amount of fuel, control and meter airflow (breathe), and then supply fuel under the plethora of driving conditions such as cold-start, warm-start, idle, tip-in acceleration, kick-down, cruise, and wide-open throttle (WOT). All of a sudden, this simple device is not so simple.
Thankfully, carburetors work off basic principals of physics that are relatively easy to understand. These principals revolve around pressure, which is often called vacuum. Once the driver fires the engine, there will be a constant flow of air through the carburetor, even if the throttle blades are shut. Air is ingested due to the pumping action of the pistons, which creates a pressure drop in the manifold. We often call this "manifold vacuum." Since the atmospheric pressure is greater outside the engine than it is in the manifold, air rushes in. And when it does, air enters the carb and flows through each venturi (or barrel). A venturi is a simple tube with an aerodynamic restriction designed into it. The restriction is the necked-down area, which is shaped like an hourglass.
The venturi causes a pressure drop. The fuel supply is channeled to the venturi so fuel is drawn in to fill the void. Since the air pressure resting above the fuel bowls is at atmospheric, fuel is forced in because the pressure on the fuel is greater that the pressure in the tube that connects the fuel to the venturi. Thus, fuel is also pushed into the boosters (once there is enough pressure drop), and the boosters spray fuel (or allow it to be sheared) into the airstream.
Fuel enters the venturi through the booster, which is the main fuel supplier. The booster is really a tiny venturi with a passage that connects to the main fuel well and the fuel bowl(s). An orifice, commonly called a jet, is placed in this route to restrict/control the flow of fuel to the booster. Changing jets, therefore, is a simple and often effective way to fine-tune the fuel flow (read: tune).
You can now conclude that the draw on the carb is created by the pumping action of the pistons, which creates a low-pressure area in the manifold. As the throttle is opened, the piston speed increases, so there is a greater pressure drop. More air enters, and more fuel is drawn in.
The carburetor must also atomize the fuel as it supplies it to the venturi, and the intake manifold must be capable of flowing the air/fuel mixture to the heads while keeping the fuel atomized and suspended. If the fuel falls out of suspension, it will puddle in the manifold, thus disturbing the air/fuel ratio, and that results in poor performance.