Mercedes Mullins
Fuel line sizing is a complex topic that depends on a variety of factors, including the engine's fuel flow rate, pressure drops, pump head, and fuel type. For example, when using an External Fuel Pump, it is important to ensure that the line is not too small, or the pump may be starved for fuel. Additionally, when using fuels like E85, alcohol, or methanol, it is recommended to use a larger fuel line. In general, fuel pumps are sized to optimize engine performance by supplying enough fuel to the system. To determine the appropriate fuel line size, one must consider the engine's maximum fuel flow, RPM, and pressure values that the engine can tolerate.
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What You'll Learn
Fuel line sizing
The relationship between fuel line size and fuel pressure is crucial. A longer fuel line will result in a greater pressure drop, which can inhibit the proper function of carbureted or EFI systems. Additionally, the bend in the fuel line can also impact pressure. A straight pipe will have less pressure loss compared to an elbow or a 45-degree bend. Therefore, when determining the appropriate fuel line size, it is important to consider the overall length and the number of bends required.
The diameter of the fuel line also plays a significant role in fuel delivery. A fuel line with a larger diameter can support higher horsepower applications. For example, a 3/8" ID fuel line can easily support 600 HP with sufficient "pump head". Increasing the pump head can be achieved by using a bigger pump or higher pump voltage. However, it is important to note that most people tend to use a larger diameter fuel line than necessary.
When designing a fuel line system for alternative fuels like ethanol or methanol, it is recommended to use one or two larger sizes compared to the gasoline requirement. This is because ethanol requires 30% more fuel, and methanol requires double the amount of fuel than gasoline to generate the same horsepower. The return lines, which are the fuel lines after the regulator, can typically be one size smaller than the feed line.
In summary, when determining the appropriate fuel line sizing, it is crucial to consider the length, diameter, and number of bends in the fuel line. These factors will impact fuel pressure and flow rate, which ultimately affects engine performance. By carefully selecting the right fuel line size, you can ensure optimal fuel delivery and engine performance.
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Fuel supply restriction
The diameter and length of the line must be considered to deliver fuel at the correct pressure and rate. One source of pressure loss is friction. The longer a fuel line is, the more friction there will be. As the flow rate through the line increases, so does the friction. Fuel lines that feed into pressure gauges experience the least amount of pressure loss because the flow of fuel reaches nearly zero.
The maximum fuel flow to the engine at rated speeds and horsepower output is an important factor to consider when understanding fuel supply restriction. The maximum pressure drops that the engine can handle and still work correctly for both the supply and return sides are also key. On the supply side, fuel lines need to be large because the lines are longer. When the lines are longer, there will be more valves, manifolds, fittings, etc. required, so a larger fuel line will result in less fuel supply restriction.
For example, in the case of a 250-450 HP 6-cylinder diesel that uses an ND EP-9 type or Bosch P7100 fuel injection pump, the fuel flow is about 1 GPM. The design of the fuel system dictates the total fuel flow. When comparing fuel injection systems, these are “inline” type injection pumps, and the typical flow is about two to three times the fuel flow in a rotary or distributor type pump diesel engine.
It is important to note that the maximum values or numbers that the engine can tolerate to work properly for both the supply side (suction) and the return side (restriction-pressure of the return line) need to be understood. While the max HP of the engine may not matter as much, the design of the fuel system is crucial in dictating the total fuel flow.
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Fuel pump placement
The fuel pump plays a critical role in delivering fuel from the tank to the engine, and its placement can impact the overall performance and efficiency of the vehicle. For the Detroit Diesel 6-71 engine, the fuel pump is typically located near the front of the vehicle, close to the gasoline tank. This placement ensures easy access for maintenance and repairs, which is essential for such a critical component.
However, fuel pump placement can also impact fuel pressure. As engine RPMs increase, fuel pressure may decrease, and the distance between the pump and the engine can contribute to this issue. A longer fuel line, with more bends and elbows, will create more resistance and result in a greater pressure drop. This is why some vehicle owners opt to relocate their fuel pumps closer to the engine to mitigate this issue.
Additionally, the size of the fuel line itself is a factor. A fuel line with a larger diameter will reduce the pressure drop, but it's important to strike a balance. Running a fuel line with a diameter that's too large can lead to inefficiencies and unnecessary costs. For the Detroit Diesel 6-71 engine, a 3/8" fuel line is commonly used and is typically sufficient for most applications.
In conclusion, fuel pump placement for the Detroit Diesel 6-71 engine should consider both accessibility and fuel pressure. While a standard placement near the front of the vehicle offers convenience, relocating the pump closer to the engine can help optimise fuel delivery and performance, especially in high-horsepower applications.
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Fuel consumption
Firstly, it's worth noting that fuel consumption can vary depending on the specific application, such as in the case of a 40-foot ex-seiner boat with a 200hp-rated 6-71 Detroit Diesel engine, where fuel consumption can be expected to be around 6 gallons per hour when running at 1700 RPM.
When it comes to fuel line sizing, it's a common misconception that increasing engine horsepower requires a larger fuel line. For instance, a nominal 3/8" ID fuel line can adequately support 600 HP with sufficient "pump head," and a bigger pump can be used to achieve higher horsepower without necessarily increasing the line size.
That being said, pump head is crucial in maintaining fuel pressure as engine RPMs increase. If fuel pressure drops, increasing the pump head can be achieved by using a bigger pump, higher pump voltage, or increasing the line size. However, it's important to first check for a dirty fuel filter, as it is often the cause of falling fuel pressure.
At Wide Open Throttle (WOT), a Brake Specific Fuel Consumption (BSFC) of 1/2 pound of fuel per horsepower is considered safe, resulting in A/F ratios of 11:1 or richer. To put this into perspective, with gasoline weighing 5.994 lbs/gallon, we can calculate that approximately 1 gallon of gasoline is needed to support 12 HP.
When calculating fuel consumption and pressure drop, it's important to consider the length and bends in the fuel line. For example, 20 feet of 3/8" steel pipe would result in a pressure loss of 3 psi when using water (with gasoline flowing more freely due to its lower viscosity).
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Fuel line material
Fuel lines can be made from a variety of materials, including metal, rubber, braided stainless or nylon, and PTFE-lined hoses. The most suitable material depends on the specific application and requirements.
Metal fuel lines, typically made from stainless steel, aluminum, or coated steel tube, are known for their durability and compatibility with various fuel types. They are less prone to wear and tear, cracking, or deterioration compared to other materials. However, metal lines can be challenging to fabricate and install, especially in tight or hard-to-reach spaces.
Rubber fuel lines offer greater flexibility, making them easier to install. They are typically connected using standard barbed fittings and fuel line hose clamps. However, rubber lines have a shorter lifespan as they can become hard and brittle over time. Additionally, they may not be compatible with certain fuels or chemicals and should be properly rated for fuel use.
Braided fuel lines, which are rubber hoses with a braided exterior, provide an upgrade in terms of flexibility and connections. They utilize AN fittings, which are superior to traditional hose clamps. However, braided lines can be prone to leaks if the braid becomes damaged, and they may have similar long-term durability issues as plain rubber hoses.
PTFE-lined fuel lines offer excellent resistance to wear, heat, and chemical damage. They effectively block fuel vapors and slow down the degradation process. PTFE-lined hoses are generally flexible but require special fittings and are less flexible than standard braided or rubber hoses.
When choosing a fuel line material, it's important to consider factors such as durability, flexibility, compatibility with fuel types, ease of installation, and long-term maintenance. Additionally, for racing applications, it's crucial to refer to the rules and regulations of the sanctioning body to ensure compliance.
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Frequently asked questions
A 6-71 fuel line is a type of fuel injection system for a 340 engine.
The size of the fuel line depends on the engine's maximum fuel flow rate, which is usually about 1 gallon per minute or 60 gallons per hour for a 250-450 HP 6-cylinder diesel engine.
The size of the fuel line is influenced by the engine's performance requirements, the type of fuel, and the presence of bends or elbows in the pipe.
Yes, some people believe that increasing the horsepower requires a larger fuel line. However, this is not always the case, as a 3/8" ID fuel line can support 600 HP with sufficient "pump head".
