Fuel Pressure For Carburetor: How Much Is Too Much?

Carbureted engines require a specific amount of fuel pressure to function optimally, and this pressure can vary depending on the type of carburetor and engine. The fuel pressure for a carburetor is typically set between 6 and 8 psi (pounds per square inch) for street engines, with higher pressure for race engines. However, it's important to note that fuel pressure and volume are inversely proportional, meaning that as fuel pressure rises, the pump's volume decreases.

Most carburetors only require around 6 psi to operate effectively. This ensures that the fuel bowls remain filled with fuel without exceeding the pressure limit that could blow the needle off the seat, causing fuel leakage. The relationship between fuel pressure and volume is crucial, and a balance must be maintained to ensure the carburetor receives the necessary fuel supply without compromising engine performance.

Additionally, the type of fuel pump, whether mechanical or electric, can impact the required fuel pressure. Low-pressure electric fuel pumps, such as Holley's classic Red vertical pump, are designed to produce sufficient fuel pressure for mild carbureted applications without the need for a regulator. On the other hand, high-performance engines generating 450 horsepower or more typically require a regulator to accurately set the desired fuel pressure.

It's worth mentioning that factors such as engine speed, g-forces, and friction in the fuel delivery system can also influence the optimal fuel pressure for a carburetor. Therefore, it is essential to refer to the specific carburetor's owner's manual and consult with experts for precise fuel pressure settings.

Fuel pressure vs volume

Fuel pressure and volume are two key considerations when designing a fuel system for a carbureted engine. While the fuel pump plays a crucial role in delivering the required amount of fuel, the pressure and volume of the fuel also need to be carefully regulated to ensure optimal engine performance.

Fuel Pressure

Fuel pressure refers to the amount of pressure required to deliver the fuel from the pump to the carburetor or injectors. The optimal fuel pressure depends on the type of engine and its power output. For example, low-pressure carbureted engines typically operate at around 5-7 psi, while high-pressure EFI applications require much higher pressures of 40-70 psi.

Fuel Volume

Fuel volume, or flow rate, refers to the amount of fuel that the pump can deliver. It is usually measured in liters per hour (lph) or pounds per hour (lbs/hr). The required fuel volume depends on the engine's power output and its fuel efficiency, known as Brake Specific Fuel Consumption (BSFC). Most gasoline engines use less than 1 pound of fuel to produce 1 horsepower for 1 hour, resulting in a BSFC of less than 1.

The Relationship Between Fuel Pressure and Volume

It is important to understand that fuel pressure and volume are inversely proportional. As fuel pressure increases, the fuel pump's volume decreases. This means that if a pump is rated to deliver a certain amount of fuel at a specific pressure, the actual volume of fuel delivered may be lower if the pressure is increased. For example, a pump rated at 800 horsepower at 6-8 psi may only deliver 600 horsepower when used with fuel injection, which requires higher pressures of 43-60 psi.

Factors Affecting Fuel Pressure and Volume

In addition to engine power and type, there are other factors that can impact fuel pressure and volume. These include:

• G-forces: The force of a car moving forward can impede fuel delivery to the carburetor.
• Friction: The fuel encounters friction as it passes through the lines and fittings, which can affect the volume and pressure at the carburetor.
• Line size and bends: Using lines with adequate diameter and avoiding sharp bends can help maintain fuel volume and pressure.
• Voltage: For electric fuel pumps, the operating voltage can impact the pump's performance. Insufficient voltage can lead to reduced volume and pressure.
• Filter type and placement: A restrictive fuel filter can decrease fuel volume and increase pressure drop.

Choosing the Right Regulator

To maintain the desired fuel pressure and volume, it is crucial to select the appropriate fuel pressure regulator. Deadhead regulators are commonly used for carbureted engines and are simple and cost-effective. However, they place additional load on the fuel pump and can lead to higher fuel temperatures. On the other hand, return-style regulators are often used for EFI applications and offer more precise pressure control by returning excess fuel to the tank.

In conclusion, when designing a fuel system for a carbureted engine, it is essential to consider both fuel pressure and volume. By selecting the right fuel pump, regulator, and taking into account various factors affecting pressure and volume, one can ensure optimal engine performance, efficiency, and reliability.

Fuel pump selection

When selecting a fuel pump for a carbureted engine, there are several factors to consider. Firstly, determine the engine's maximum horsepower, either through dyno-testing or by referring to the manufacturer's advertised rating. It is recommended to stay on the high side to ensure sufficient fuel delivery. Next, consider the brake-specific fuel consumption (BSFC), which indicates how much fuel is consumed per unit of power produced. Most naturally aspirated gasoline engines operate at 0.4-0.5 pounds per horsepower per hour, while nitrous engines typically use slightly more fuel, resulting in a BSFC of 0.5-0.6. Forced-induction engines, such as supercharged or turbocharged engines, have the highest BSFC, ranging from 0.6 to 0.75.

Another critical factor is the operating pressure required for your carburetor, which typically ranges from 2 to 9 psi. The fuel pressure has an inverse relationship with the fuel volume; as fuel pressure increases, the flow rate decreases. Additionally, the type of fuel used can impact fuel consumption. For example, ethanol engines use 30-35% more fuel than gasoline engines, while methanol (alcohol) engines can use up to 200% more.

When choosing between an in-tank or inline-style setup, consider the benefits of each. In-tank pumps benefit from the cooling effects of being submerged in fuel, while inline pumps are generally easier to replace and tend to be more cost-effective. If you opt for an in-tank electric pump, ensure that a bypass regulator is used to return excess fuel to the tank, as EFI fuel pumps produce a higher flow rate than what is needed for carbureted engines. Most carburetors only require around 6 psi to function properly.

It is also important to select a fuel pump that matches the fuel type and system-pressure requirements. Fuel pump manufacturers specify the free-flow rate in gallons per hour, liters per hour, or pounds per hour. Additionally, consider the voltage requirements of the pump and ensure that your vehicle's electrical system can meet those needs.

Lastly, filtration is an important component of any fuel system. Pre-filters, such as a 100-micron high-flow pre-filter, are standard in the industry. For post-filters, a 10-micron filter is typically adequate for EFI setups, while a 40-micron filter is recommended for carbureted applications.

Fuel filter type and placement

The type of fuel filter used is crucial, and the selection should be based on the micron rating and element type. Micron ratings for filters typically range from 2 to 100, with lower numbers indicating the ability to filter out smaller particles. For carbureted engines, a 40-micron filter is generally recommended after the fuel pump, along with a 100-micron pre-filter for added protection. This setup ensures that larger debris is captured before reaching the pump, preventing clogs in the post-pump filter.

When it comes to element types, there are a few options to consider. Paper filters, also known as cellulose filters, are disposable but should not be used with methanol or fuels containing over 20% ethanol. Stainless-steel filters, on the other hand, are compatible with all types of fuel and can be cleaned and reused. Micro Glass filters are intended for post-pump use only and are disposable.

In addition to filter type and placement, it is important to consider the fuel line routing. Hard lines or stainless braided AN lines should be used, and sharp-radius turns should be avoided. The fuel line should be routed away from heat sources, suspension components, jacking points, and potential impact areas.

Furthermore, the fuel pump's mounting position is crucial. It should be mounted where gravity assists in drawing fuel to the inlet, typically at the rear, lower side of the tank. If this arrangement is not feasible, the pump should be mounted near the tank, as electric fuel pumps function better as pushers than pullers.

Carburetor tuning

Step 1: Check Fuel Pressure and Ignition Timing

Before beginning any adjustments, ensure that the carburetor is receiving the correct fuel pressure by using a gauge. Also, confirm that the ignition timing for the engine is accurate.

Step 2: Warm Up the Engine

Start the engine and allow it to reach its normal operating temperature. This will ensure that any adjustments made are done so with the engine at its typical performance level.

Step 3: Check Vacuum at Idle

Using a port from the intake or below the throttle blades on the carburetor, check the vacuum at idle. Confirm that the engine's idle speed is within the typical range of 800-1000 rpms. Record or memorize this vacuum reading for future reference.

Step 4: Adjust Idle Mixture Screws

On Holley carburetors, the idle mixture screws are located on the metering blocks on either side of the main body, and there are usually two or four screws. For Edelbrock carburetors, these screws are located on the front side. The goal is to adjust these screws to the "lean-best" setting. To do this, turn one of the screws clockwise while the engine is idling and warmed up, using a vacuum gauge connected to engine vacuum. Turn the screw until you achieve the highest vacuum reading. Then, turn it counter-clockwise just slightly. Repeat this process for the other idle mixture screw, as adjusting one screw affects the other. Continue this process until both screws are adjusted to achieve the maximum vacuum reading. If adjusting the screw doesn't change the vacuum, the passageway may be blocked and needs to be cleaned. Record the final engine vacuum reading.

Step 5: Adjust Step-Up Springs or Power Valve

Using the idle vacuum reading from the previous step, divide it by two. This value should match with either the power valve (for Holley-style carburetors) or the step-up springs (for Edelbrock carburetors). If you are between two options, select the higher number. In Edelbrock carburetors, the step-up springs pull the tapered metering rods out of the jet holes when there is less vacuum than the spring is rated for, providing metered fuel during acceleration. Holley power valves, on the other hand, use a rubber diaphragm to enrichen the fuel mixture.

Step 6: Adjust the Float

For Holley adjustable float carburetors, loosen the nut on top of the fuel bowl and turn the screw until fuel reaches the bottom of the sight plug hole. Tighten the nut and reinstall the sight plug. This prevents flooding or fuel starvation. Edelbrock carburetors require removing the top of the carburetor to adjust the floats. Compare the float height to the specifications in the owner's manual and make adjustments as necessary.

Step 7: Check Carburetor Jetting

If, after completing the previous tuning steps, the carburetor is still providing too much or too little fuel during idle, part throttle, or wide-open throttle, the jets likely need to be changed. Refer to the owner's manual for instructions on proper jet selection. Edelbrock carburetors use a specific combination of primary jets and metering rods, while single metering block Holley carburetors may require jet replacement or metering plate adjustment for the secondaries.

By following these steps, you can effectively tune a carburetor to ensure optimal fuel delivery and engine performance.

Fuel system maintenance

Removal and Installation:

Before removing the carburetor, ensure the fuel shut-off valve is closed. Disconnect the throttle and mixture control linkages, and lockwire the throttle valve. Disconnect the fuel inlet line and all related lines. Remove the carburetor carefully, and cover the engine mounting flange to prevent debris from falling into the engine. When reinstalling, check the carburetor for proper lockwiring and ensure all shipping plugs are removed. Place the gasket in position, ensuring the bleed hole aligns with the passage in the mounting flange. Inspect the induction passages for any foreign material.

Fuel and Air Delivery:

To maximize horsepower, ensure the carburetor receives cool and dense air. Minimize restrictions in the inlet path, and consider installing a hood scoop or external air intake. Maintain a minimum clearance of three inches between the top of the venturis and the hood scoop. Use the tallest possible air cleaner element, preferably four inches. Seal the carburetor to the scoop to prevent air from bypassing the inlet tract, which can lead to fuel siphoning and the engine running lean.

Fuel Delivery Issues:

Many race cars encounter fuel delivery issues, even if they don't exhibit noticeable problems. The production of horsepower depends on the efficient conversion of fuel into energy. Therefore, ensure your vehicle is receiving the necessary amount of fuel to achieve maximum power. For example, a 600-horsepower engine requires 300 pounds of gasoline per hour. This fuel must pass through the needles, seats, and fuel pressure regulator, while also contending with 'G' forces that can stall fuel flow.

Testing and Adjustments:

Use a one-gallon gas can to test your fuel system. Measure the time it takes to fill the can with the system switched on. This test can help diagnose potential issues with your fuel volume. Ensure you are using racing-specific fuel filters, placed before the fuel pump to safeguard the system from contaminants.

For a gasoline carburetor, set the fuel pressure within the range of 6 to 8 psi. For an alcohol carburetor, the requirements differ, typically needing 4 to 5 psi at idle and 9 to 12 psi at wide-open throttle. Remember, fuel pressure cannot replace fuel volume; it merely indicates the restriction within the fuel system.

Regulators and Bypasses:

Most electrical fuel pump systems require a fuel pressure regulator. High-horsepower engines may need two regulators to prevent excessive fuel restriction. Mechanical fuel pumps and certain electrical pumps often work better with a bypass system instead of a regulator. Consistent fuel pressure may be achieved with a diaphragm bypass, with or without an idle bleed, depending on the pump type.

Idle Mixture and Speed:

Adjust the idle mixture screws to achieve the best idle quality. If adjustments are challenging, the idle circuits may need modification, as the cam may not generate enough idle vacuum. Ensure equal adjustment of primary and secondary butterfly openings. The transfer slot should have no more than 0.040" exposed. Adjust the idle speed to the specified rpm in the maintenance manual.

Carb Jetting:

Adjust the jet size to achieve maximum performance, increasing or decreasing by two numbers as needed. Continue adjustments in the same direction until performance worsens, then adjust one jet size at a time in the opposite direction. Do not rely solely on spark plug color as a gauge, especially with high-energy ignitions.

Accelerator Pump Lever:

Fine-tune the accelerator pump for optimal performance. For drag racing, adjust the pump lever override spring so that fuel discharge begins at an engine speed slightly lower than the launch RPM. This ensures a more powerful launch.

Float Level Adjustment:

Properly adjust the carburetor float position during assembly or reinstallation. For a Holley-style carburetor, the recommended float setting is approximately 0.450" from the top of the bowl. Recheck the float level while the engine is running and with the sight plugs removed. The fuel level should lightly wet the outside of the bowl.

Power Valves:

Power valves provide additional fuel during high-load, wide-open throttle situations. Select a power valve with a rating that corresponds to a vacuum level of at least 1.5-2.0 in/Hg below the engine's idle vacuum. This ensures timely fuel enrichment.

Throttle Plates and Linkage:

Regularly check the throttle opening to ensure it reaches the wide-open position. The throttle linkage should operate smoothly without binding or obstructions. Install an auxiliary return spring to ensure the throttle closes securely.

Climate Changes:

Variations in air density due to temperature, barometric pressure, and humidity directly impact engine performance. Adjust the fuel mixture as needed to account for these changes. Spacers and plenum dividers can help alter the intake tract configuration and improve performance.

Cleaning and Maintenance:

Keep the carburetor clean by regularly spraying the air bleeds with carburetor spray or WD-40. When storing for an extended period, turn the engine over with the ignition off and throttle open, then spray WD-40 down the venturi to protect against rust. Rebuild the carburetor at least once a year or more frequently if operating in dirty conditions.

For alcohol fuel systems, proper maintenance is crucial as alcohol is highly corrosive. Drain and flush the system regularly, typically using gasoline. Run the engine until it idles and stalls to effectively flush the system.

By following these maintenance procedures, you can help ensure your carburetor fuel system is operating at its best, maximizing performance, fuel efficiency, and engine longevity.

Frequently asked questions