Increasing Fuel Pressure: Can Smaller Lines Boost Performance?

The relationship between fuel line size and fuel pressure is an important consideration when designing a fuel delivery system. Fuel lines that are too small or too large can lead to pressure loss, which affects the performance of the engine. In carbureted fuel delivery systems, smaller fuel lines can cause excessive pressure drop, inhibiting the proper function of the carburetor. On the other hand, larger fuel lines may not significantly impact the pressure drop but could increase fuel consumption and put additional strain on the fuel pump. It is crucial to carefully analyse the fuel line diameter and length, taking into account the engine's requirements and whether the system is carbureted or EFI-based.

Fuel line size and fuel pressure relationship

It is important to understand the relationship between fuel line size and fuel pressure when designing a fuel delivery system. Fuel systems can be incorrectly designed if the pressure loss attributed to the length of the fuel lines is not considered.

The longer the fuel line, the more friction incurred, and the higher the flow rate, the more friction there is. This friction causes a drop in pressure. Every fuel line experiences some pressure loss depending on the fuel line length and fuel flow rate.

Fuel lines that feed pressure gauges experience the least amount of loss, as the flow rate through the gauge line is very low. This is why gauges typically use 3/16" or 1/4" line size, as they will not affect performance due to pressure loss.

When it comes to carbureted fuel pumps, the maximum possible flow rate is at free-flow, but the maximum flow rate during actual operation is lower. If the minimum required fuel system pressure for a given example is 6 PSI (not accounting for other losses), the maximum flow rate to be considered is 110 GPH.

Comparing -6AN and -8AN supply fuel lines, both 14' long, the amount of pressure drop that can be expected for 14' fuel lines at 110 GPH is:

• 6AN Fuel Line: nearly 4 PSID
• 8AN Fuel Line: nearly 1.5 PSID

While a "couple of PSI" may not seem like a significant difference, it could be a problem depending on how much pressure loss the system can tolerate before performance is affected. For instance, adding just two PSI of line drop to the example pump lowers the available flow rate from 110 GPH to 80 GPH (a loss of over a fourth of the total capacity). This example demonstrates that carbureted systems are very sensitive to fuel line size.

On the other hand, EFI fuel pumps do not experience the same pressure loss issues as carburetor fuel pumps. When comparing the performance of an EFI fuel pump to a carbureted fuel pump at higher pressure, the fuel flow rate for the carbureted pump is much lower. If we add 3 PSI of pressure difference, the EFI pump flow rate performance may drop by less than 3%, while the carbureted pump flow rate loss is more than 33%. This shows that EFI systems are not as sensitive to fuel line size as carbureted systems.

Therefore, when planning a fuel system, it is crucial to consider not only the flow rate and line length when determining fuel line size but also whether the fuel delivery system is carburetor or EFI-based.

Pressure loss and fuel line length

When planning a fuel delivery system, it is crucial to understand the relationship between fuel line size and fuel pressure. Failing to account for the pressure loss attributed to the length of the fuel lines can lead to incorrect designs. Excessive pressure drop in the fuel lines can hinder the proper functioning of the carburetor or EFI system, and in the case of a bypass or return-style regulator, it can affect the regulator's ability to operate correctly. Therefore, careful consideration of fuel line diameter and length is essential to ensure the correct flow rate and pressure.

The source of pressure loss in fuel lines is friction. As fluids pass through pipes and hoses, they experience friction, which reduces the pressure under which the fluid is delivered. Two factors influence the increase in friction and, consequently, fuel pressure drop: the length of the fuel line and the flow rate. The longer the fuel line, the more friction incurred, and as the flow rate increases, so does friction. Every fuel line experiences pressure loss depending on its length and the fuel flow rate.

The sensitivity to pressure drop varies between carbureted and EFI systems. Carbureted systems are more sensitive to fuel line size, while EFI fuel pumps don't experience the same pressure loss issues as carburetor fuel pumps. When determining the appropriate fuel line size, it is crucial to consider whether the fuel delivery system is carburetor or EFI-based.

In summary, while increasing the size of the fuel line can improve performance by allowing a greater volume of fuel to flow to the engine, it may also lead to increased fuel consumption and additional strain on the fuel pump and other components. It is important to carefully weigh the trade-offs and consult professionals before making any modifications to the fuel line size.

Fuel flow rate and fuel pressure drop

The relationship between fuel line size and fuel pressure is important to understand when planning a fuel delivery system. Fuel systems can be incorrectly designed if the pressure loss attributed to the length of the fuel lines is not taken into account. Excessive pressure drop in the fuel lines can inhibit their proper function and can squelch the regulator's ability to operate correctly. Therefore, careful consideration of fuel line diameter and length is crucial to deliver fuel at the correct flow rate and pressure.

Fluids experience friction as they pass through pipes and hoses, and this friction reduces the pressure under which the fluid is delivered. In fuel lines, two factors increase friction and, consequently, fuel pressure drop: the length of the fuel line and the flow rate. As the length of the line increases, so does friction, and as the flow rate is increased, so is friction.

When determining the appropriate fuel line size, it is important to consider whether the fuel delivery system is carbureted or EFI-based. Carbureted systems are more sensitive to fuel line size than EFI systems. In carbureted systems, increasing the size of the fuel line may be necessary to reduce pressure loss and maintain the required flow rate. However, in EFI systems, the same pressure loss issues may not occur, and larger fuel lines may not provide significant benefits.

In summary, while increasing the size of the fuel line can improve performance by allowing a greater volume of fuel to flow to the engine, it may also increase fuel consumption and put additional strain on the fuel pump. It is important to carefully consider the trade-offs and consult professionals before making any modifications to the fuel delivery system.

Fuel delivery system design considerations

When designing a fuel delivery system, there are several key considerations to keep in mind. Firstly, it is crucial to select a fuel line with a sufficient diameter to supply the required amount of fuel to the engine. Increasing the size of the fuel line can lead to improved performance and increased power and acceleration. However, there are potential drawbacks, such as increased fuel consumption and additional strain on the fuel pump. Therefore, careful consideration is necessary when choosing the appropriate fuel line size, taking into account factors like engine size, horsepower, and desired performance level.

The type of fuel pump is another important factor. Mechanical fuel pumps, in-tank pumps, and inline electric pumps each have their advantages and disadvantages. Mechanical pumps are operated by the turning of the engine, while in-tank electric pumps are submerged in fuel, staying cool during extended use. Inline electric pumps, on the other hand, offer the advantage of easy inspection and servicing.

The fuel pressure regulator is a critical component in maintaining the correct fuel pressure for the engine. Different types of engines, such as carbureted or EFI systems, have specific fuel pressure requirements. For example, vintage carburetors should not exceed 3 psi to prevent flooding, while modern EFI systems use fuel pressures ranging from 43.5 to 65 psi.

Additionally, the material and routing of the fuel lines are important design considerations. Rubber lines have more internal friction than steel or aluminum, affecting fuel flow. It is also essential to route the lines away from heat sources like the exhaust to maintain optimal performance.

Lastly, the fittings and finishes of the fuel lines play a significant role in fuel delivery. Smooth transitions and straight connections are ideal for maximizing flow. AN-style connections, for instance, offer improved flow over barbed fittings or plumbing-style connections.

Fuel line size sensitivity in carbureted vs. EFI systems

The relationship between fuel line size and fuel pressure is an important consideration when planning a fuel delivery system. Fuel systems can be incorrectly designed if the pressure loss attributed to the length of the fuel lines is not taken into account. This can affect the performance of both carbureted and EFI systems and impact the way a bypass or return-style regulator operates.

In the early days of aftermarket EFI systems, experts in carbureted fuel delivery systems were sceptical about the smaller EFI return fuel lines, believing that they would not be able to deliver enough fuel. However, carbureted and EFI systems operate differently due to their sensitivity to pressure drop.

When it comes to fuel line size, carbureted systems are more sensitive than EFI systems. Fluids experience friction as they pass through pipes and hoses, which reduces the pressure under which the fluid is delivered. In fuel lines, the length of the line and the flow rate influence the amount of friction and, consequently, the fuel pressure drop.

In a carbureted system, the maximum flow rate and minimum required fuel system pressure should be considered. Different fuel line sizes can significantly impact pressure drop, which in turn affects the available flow rate. For example, a slight increase in pressure loss can result in a substantial decrease in flow rate, impacting the performance of the carbureted system.

On the other hand, EFI fuel pumps do not experience the same pressure loss issues as carbureted fuel pumps. When comparing the performance of an EFI fuel pump to a carbureted fuel pump over a similar pressure range, the EFI pump exhibits a much smaller drop in flow rate, indicating that EFI systems are less sensitive to fuel line size.

In summary, while increasing the size of the fuel line can improve engine performance by allowing a greater volume of fuel to flow to the engine, it is important to carefully consider the trade-offs. The appropriate fuel line size depends on factors such as engine size, horsepower, and desired performance level. Consulting with a professional or conducting thorough research is essential before making any modifications to the fuel delivery system.

Frequently asked questions