Carburettor Specifications

Carburettors have long been essential components in internal combustion engines, especially before the widespread use of fuel injection. Despite being largely replaced by more modern technologies in everyday vehicles, carburettors remain prevalent in classic cars, motorcycles, and performance vehicles. For car enthusiasts and those involved in tuning or restoring classic vehicles, understanding how to read carburettor specifications and what the numbers mean is crucial.

1. Carburettor Basics: What It Is and How It Works

Before diving into carburettor specifications, it’s helpful to have a basic understanding of what a carburettor is and its function. A carburettor is a device that mixes air and fuel in the proper ratio for internal combustion. It ensures that the engine receives the right amount of air and fuel, regardless of engine speed or load. This process is critical for the engine to run smoothly, and the carburettor must adjust to the varying demands of the engine.

The primary components of a carburettor include:

• Air Intake: Air flows into the carburettor, passing through the air filter.
• Venturi: This section of the carburettor narrows the airflow, increasing its speed and creating a vacuum that draws fuel into the airstream.
• Jets: Small orifices that control the flow of fuel into the air stream.
• Throttle Plate: Controls the amount of air and fuel entering the engine.
• Float Bowl: Holds a reservoir of fuel, ensuring a steady flow to the jets.
• Needles and Seats: Regulate fuel flow and maintain a constant level in the float bowl.

The carburettor adjusts fuel flow based on the throttle input, engine speed, and load. Different carburettor models offer varying features, such as multiple barrels, different jet sizes, and secondary systems, to improve performance and fuel efficiency.

2. Carburettor Specifications: What Do the Numbers Mean?

Carburettor specifications vary widely depending on the make and model, and understanding these numbers can help you choose the right carburettor for your engine. These specifications typically include a combination of measurements related to airflow, fuel delivery, and design features. Here’s a breakdown of the key numbers you’ll find on carburettor specifications and what they mean:

2.1 CFM (Cubic Feet per Minute)

The most common specification listed on a carburettor is the CFM rating (Cubic Feet per Minute). This measurement refers to the volume of air a carburettor can flow per minute at a specific pressure and temperature. CFM is directly related to the engine’s airflow capacity, and choosing the correct CFM for your engine is vital to ensure the carburettor performs optimally.

• Why CFM Matters: The CFM rating is crucial because it determines the amount of air that the carburetor can provide to the engine. If the carburetor has too low of a CFM rating, the engine may starve for air, resulting in poor performance, particularly at higher RPMs. Conversely, if the carburetor has too high of a CFM rating, the air-fuel mixture may become too lean at lower RPMs, causing rough idling and poor throttle response.
• How to Choose the Right CFM: A general guideline for determining the CFM needed for your engine is based on engine size and RPM. Here’s a basic formula:CFM=(EngineDisplacement×MaxRPM)÷3456CFM = (Engine Displacement \times Max RPM) \div 3456CFM=(EngineDisplacement×MaxRPM)÷3456
  • Engine Displacement: The total volume of all cylinders in the engine (measured in cubic inches or liters).
  • Max RPM: The maximum RPM the engine is capable of achieving.
  • 3456: A constant used in the formula.

For example, if you have a 350 cubic-inch engine and you plan to rev it to 6,000 RPM, you’d calculate:CFM=(350×6000)÷3456=608CFMCFM = (350 \times 6000) \div 3456 = 608 CFMCFM=(350×6000)÷3456=608CFM

This means you would need a carburettor rated around 600 CFM for optimal performance. However, for engines with significant performance modifications, you may need a carburettor with a higher CFM.

2.2 Jets and Main Jet Sizes

Jets are critical components in carburettors that control the amount of fuel delivered to the engine. The main jet regulates fuel delivery at higher RPMs and is an essential factor in tuning the air-fuel mixture. Jet size is typically denoted in numbers, with larger numbers representing larger jets that allow more fuel to flow.

• Main Jet Sizes: The size of the main jet impacts the engine’s performance, particularly under load and at high RPMs. A larger jet will provide more fuel to the engine, making it richer and delivering more power, but it can also lead to increased fuel consumption and reduced efficiency.
• How to Choose the Right Jet Size: The correct jet size depends on several factors, including engine displacement, performance modifications, altitude, and intended use. Generally, the rule of thumb is to start with the factory-recommended jet sizes for a given carburettor and then adjust as needed based on performance testing.

For example, a typical 600 CFM Holley carburettor might come with main jets sized between 65-75 (meaning the jet’s diameter is 0.065 to 0.075 inches), but for performance applications, you may need to go to a larger size to accommodate higher fuel demands.

2.3 Venturi Size

The Venturi size refers to the diameter of the narrowest part of the carburettor, where airspeed increases and fuel is drawn into the airflow. Venturi size is an important factor in controlling airflow and fuel delivery.

• How Venturi Size Affects Performance: A larger Venturi allows more air to flow through the carburetor, which can help higher-performance engines make more power at high RPMs. However, too large of a Venturi size can result in poor fuel atomization and less throttle response at low RPMs, making the engine feel sluggish.
• Choosing the Right Venturi Size: As a rule of thumb, a larger Venturi is suitable for high-performance applications, while a smaller Venturi is better for street-driven vehicles that require smooth throttle response and fuel efficiency. Common Venturi sizes for carburettors range from 1.0 inches to 1.5 inches.

2.4 Throttle Bore Diameter

The throttle bore diameter refers to the diameter of the opening through which air enters the carburettor. This is another factor in determining how much air the carburettor can flow.

• How Throttle Bore Diameter Affects Performance: Larger throttle bores allow for greater air volume to be delivered to the engine, which increases overall airflow and enables more power production at high RPMs. However, similar to Venturi size, a larger throttle bore can result in poor low-end response and throttle lag.
• Choosing the Right Throttle Bore Size: Throttle bore sizes typically range from 1.0 inches to 2.0 inches or more for high-performance carburettors. For a street-driven car, a carburettor with a 1.5-inch throttle bore may be ideal, while performance engines may require 1.75 inches or more.

2.5 Power Valve

The power valve is a key component in many carburettors, especially those designed for high-performance applications. It’s used to deliver additional fuel under heavy load or at high RPMs, ensuring the engine receives the fuel it needs during hard acceleration or full throttle.

• Power Valve Ratings: Power valves are rated in inches of mercury (inHg) and open at a specific vacuum threshold. For example, a 6.5-inch power valve opens when the intake vacuum falls below 6.5 inHg, enriching the fuel mixture for higher RPMs and loads.
• Choosing the Right Power Valve: The power valve rating should be chosen based on the engine’s operating characteristics. For an engine with a moderate camshaft and moderate vacuum, a 6.5-inch power valve may be appropriate. For higher-performance engines with more aggressive camshafts, a lower-rated power valve (such as 4.5 inches) may be needed, while lower vacuum engines may require a higher-rated power valve (such as 8.5 inches).

2.6 Accelerator Pump

The accelerator pump is responsible for delivering an extra shot of fuel when the throttle is opened rapidly. This helps eliminate hesitation and improves throttle response.

• Accelerator Pump Sizes and Adjustment: Accelerator pumps come in different sizes (measured in cc’s of fuel per stroke) and can be adjusted for a quicker or slower fuel shot. A larger pump or a more aggressive pump shot may be necessary for high-performance applications to ensure smooth acceleration without hesitation.
• Choosing the Right Accelerator Pump: For vehicles that need a fast throttle response, a larger accelerator pump may be necessary. For engines with mild tuning or those requiring smoother operation, a standard or smaller accelerator pump may suffice.

2.7 Carburettor Configuration: Single, Dual, and Multi-Barrel Carburettors

Carburettors come in a variety of configurations, from single-barrel to multi-barrel designs. Understanding these configurations is essential in determining the right carburettor for your engine.

• Single-Barrel Carburettors: These carburettors have one barrel through which air and fuel flow. They are generally used on smaller, less powerful engines and are found on many economy cars. Single-barrel carburettors tend to have lower CFM ratings and are suitable for engines with low performance demands.
• Two-Barrel Carburettors: Two-barrel carburettors, such as the Holley 4160, offer better performance than single-barrel carburettors by providing more fuel and air to the engine. These carburettors typically feature a progressive linkage, meaning that one barrel operates at lower RPMs, while the second barrel opens up under higher loads and RPMs.
• Four-Barrel Carburettors: Four-barrel carburettors, such as the Holley 4150, are ideal for performance and racing applications. These carburettors provide a significant increase in air and fuel delivery, allowing for higher RPM operation and greater power output.
• Multi-Barrel Carburettors: Multi-barrel carburettors, which include configurations with more than four barrels (e.g., six-barrel carburettors), are typically used in high-performance or racing engines. These carburettors are optimized for extremely high airflow and are designed for engines with significant performance modifications.

3. Carburettor Brands and Models

Now that we understand how to read carburettor specifications, let’s take a look at some of the most popular carburettor brands and models. Each brand has its own distinct features, and understanding the numbers on these carburettors can help you choose the right one for your needs.

3.1 Holley Carburettors

Holley is one of the most recognized names in performance carburettors. Their carburettors come in a wide range of specifications, from street carburettors to race-ready models.

• Holley 4150: The Holley 4150 is a four-barrel carburettor designed for high-performance engines. It’s available in different CFM ratings (e.g., 600, 750, 800, and 1000 CFM) and is suitable for modified street engines and racing applications.
• Holley 4160: The 4160 series carburettors are similar to the 4150 but use a vacuum secondary system, offering smoother operation in street cars. They come in various CFM ratings and are ideal for moderate performance engines.

3.2 Edelbrock Carburettors

Edelbrock carburettors are known for their smooth operation and tuning flexibility. These carburettors are popular for street applications and mild performance modifications.

• Edelbrock Performer Series: The Performer Series carburettors are designed for street cars, offering improved throttle response and fuel efficiency. They are available in various sizes, with CFM ratings ranging from 500 to 800 CFM.

3.3 Demon Carburettors

Demon carburettors are designed for high-performance applications and are often used in racing. They offer great tuning flexibility and high airflow capacity.

• Demon 750 CFM: This is a high-performance carburettor that’s suitable for racing engines and modified street cars. It offers excellent airflow and throttle response.

4. Conclusion

Reading carburettor specifications is essential for selecting the right carburettor for your engine and understanding how different features impact performance. Key specifications such as CFM, jet sizes, Venturi diameter, throttle bore size, and power valve ratings all play a crucial role in how a carburettor delivers fuel and air to the engine.