Regan Brown
When it comes to fuel lines, the relationship between fuel line size and fuel pressure is an important consideration in the design of a fuel delivery system. The pressure loss attributed to the length of the fuel line can impact the performance of the system. For example, a carbureted fuel pump is more sensitive to fuel line size compared to an EFI fuel pump. The PSI (pounds per square inch) of a fuel line can vary, with some fuel hoses having a working pressure of 150 PSI and a burst pressure of 500 PSI. The fuel line size and PSI will depend on the specific application and requirements of the fuel system.
Characteristics and Values of PSI for Fuel Line
| Characteristics | Values |
|---|---|
| Replacement fuel hosing PSI | 50 |
| Bosch 044 Fuel Pump PSI | 72.5 |
| Minimum required fuel system pressure | 6 PSI |
| Pressure drop tolerance | 2 PSI |
| Fuel line PSI for EFI vehicles | 3 PSI |
| Working pressure range for 5/16" ID Fuel Line Hose SAE 30R7 | 150 PSI |
| Burst pressure for 5/16" ID Fuel Line Hose SAE 30R7 | 500 PSI |
| Temperature range for 5/16" ID Fuel Line Hose SAE 30R7 | -40°F to 257°F (-40°C to 125°C) |
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What You'll Learn
Fuel line size vs pressure drop
When planning a fuel delivery system, it is crucial to understand the relationship between fuel line size and fuel pressure. The length of the fuel line and the fuel flow rate are two factors that influence the amount of pressure loss in a fuel line. Every fuel line undergoes pressure loss depending on its length and the rate of fuel flow.
Friction is the primary source of pressure loss. As fluids travel through pipes and hoses, they experience friction, which reduces the pressure at which the fluid is delivered. The length of the fuel line and the flow rate both increase friction and, consequently, fuel pressure drop.
Fuel lines that feed pressure gauges experience the least amount of loss because the flow rate through the gauge line drops to near-zero flow levels. This is why gauges typically use 3/16" or 1/4" line sizes, as they do not affect performance due to pressure loss.
Now, let's examine how fuel flow rate is affected by fuel pressure drop in a carbureted system. Let's assume we have a carbureted fuel pump with a maximum possible flow rate of 160 GPH at free-flow, but we require a minimum fuel system pressure of 6 PSI. In this case, the maximum flow rate to be considered is 110 GPH.
Comparing -6AN and -8AN supply fuel lines, both 14' long, we can expect a pressure drop of nearly 4 PSID for the -6AN line and nearly 1.5 PSID for the -8AN line at 110 GPH. While a "couple of PSI" may not seem significant, adding just 2 PSI of line drop can reduce the available flow rate from 110 GPH to 80 GPH, which is a substantial decrease in performance. This example highlights the sensitivity of carbureted systems to fuel line size. However, it's important to note that EFI vehicles have different considerations when determining the appropriate fuel line size.
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Fuel line pressure testing
Step 1: Prepare for the Test
Before performing any diagnostic tests, prioritize safety. Wear protective gear, such as safety glasses and gloves, and work in a well-ventilated area. Ensure there are no sources of ignition nearby, such as smoking or sparks, as releasing fuel under pressure can cause fires and injuries.
Step 2: Check the Fuel Pressure
Start by starting the car and letting it idle. Install a fuel pressure gauge and run the pump to obtain a pressure reading. Compare this reading to the manufacturer's specifications. If the pressure is low, address the issue accordingly.
Step 3: Perform a Fuel Volume Test
If the fuel pump is supplying sufficient pressure, the next step is to determine if the correct amount of fuel is being delivered to the fuel injectors. The most accurate method is to use a flowmeter, but if one is not available, a timed fuel delivery test can be performed. Use a glass container to collect a fuel sample for five seconds with the pump running, as fuel can corrode or fog up plastic. Refer to the manufacturer's specifications to check if the pump is delivering the right amount of fuel.
Step 4: Test for Air Leaks and Holes
To test for air leaks or holes in the fuel lines, you can use a vacuum gauge. Hook up the vacuum gauge to one end of the line or filter and plug or pinch off the other end. Additionally, you can submerge a section of the hose in water while plugging off one end and blowing into it.
Understanding Fuel Line Pressure
It is worth noting that the fuel lines feeding pressure gauges experience minimal pressure loss due to the low flow rate through the gauge line. Typically, gauges use 3/16" or 1/4" line sizes to prevent performance issues due to pressure loss. The acceptable fuel system pressure can vary, with some systems requiring a minimum pressure of 6 PSI, while others can handle up to 50 PSI or more.
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Fuel hose pressure ratings
The pressure rating of a fuel hose is an important consideration when planning a fuel delivery system. The wrong choice of fuel hose can lead to performance issues and safety concerns. The pressure rating of a fuel hose is influenced by factors such as the type of fuel, the presence of additives, and the operating pressure of the fuel system.
General-purpose fuel hoses have a working pressure rating of around 50 PSI. They are not suitable for modern fuel blends that contain ethanol, as the rubber can quickly deteriorate and fail. They are also not suitable for in-tank (submersible) applications, as they are only designed to handle fuel on the inside. For submersible applications, a hose that meets or exceeds the SAE 30R10 rating is required.
For modern fuels that contain ethanol, a hose with a fluoroelastomer inner liner, such as the SAE 30R9, is recommended. These hoses have a working pressure rating of 100 PSI and can handle various fuels, including oxidized gasoline, diesel, methanol, and ethanol-blend fuels. They also resist fuel permeation, making them emissions-legal.
The length of the fuel hose also affects its pressure rating. Longer hoses can experience a more significant pressure drop, which can impact the flow rate of the fuel. This is especially true for carbureted systems, which are more sensitive to fuel line size than EFI systems.
It is important to choose a fuel hose that is compatible with the specific fuel and operating pressure of the system. Using an incorrect hose can lead to performance issues, fuel loss, and safety hazards.
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Fuel pump pressure
Gasoline engines require a specific range of fuel pressure for optimal performance. This range can vary depending on the make and model of the vehicle, but for most gasoline cars, fuel pressure falls within the range of 30 to 50 PSI. Some sources state that the fuel pump in a GM system has a standard pressure of 58 PSI in the rail. The Corvette ZR1 runs fuel pressure in the 30s until an increased demand is placed on the system, at which point it can ramp up to 88 PSI in the rail.
Turbocharged engines require higher fuel pressure to accommodate the increased air intake and achieve timely and effective combustion. Supercharged engines also have specific pressure requirements as they create boost pressure and require larger fuel injectors to deliver the required fuel. On the other hand, aspirated engines rely solely on atmospheric pressure to draw air into the combustion chamber.
The size of the fuel line can also affect the pressure drop in the system. For example, a carbureted fuel pump with a maximum flow rate of 160 GPH at free-flow may have a maximum flow rate of only 110 GPH when in actual operation. By adding just 2 PSI of line drop, the available flow rate can be lowered from 110 GPH to 80 GPH. EFI fuel pumps don't experience the same pressure loss issues as carburetor fuel pumps.
Low fuel pressure can be caused by a faulty fuel pump, clogged fuel filter, leaking fuel lines, malfunctioning fuel injectors, or a bad fuel pressure regulator. Symptoms of low fuel pressure include engine stalling, difficulty starting the car, poor engine performance, the check engine light coming on, and increased fuel consumption. Regular maintenance of the vehicle's fuel system, including replacing the fuel filter, inspecting the fuel lines, and ensuring the fuel pump is in good working order, can help prevent low fuel pressure issues.
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Fuel line pressure loss
The most common causes of low fuel pressure are a clogged oil filter or a bad fuel pump. Sometimes the fuel pump slows down or becomes internally damaged, and when this happens, it isn’t able to push enough fuel to the engine, causing low fuel pressure. The length of the fuel line and the flow rate also play a role in fuel pressure loss. The longer the fuel line, the more friction is incurred, and as the flow rate is increased, so is the friction.
Other causes of low fuel pressure include a bad fuel pressure regulator, a stuck fuel injector, a fuel pressure sensor, or a smashed fuel pressure line. The fuel pressure regulator controls the fuel pressure in the fuel rail, and if it is damaged, it can create too low or too high fuel pressure in the rail. If one of the fuel injectors is damaged and stuck open, this can cause a low fuel pressure in the rail.
The most common symptom of low fuel pressure is an unresponsive throttle or a stalling engine. You may also notice signs like difficulty starting the car, a check engine light on the dashboard, misfires, or low performance. It is very important that you have the right fuel pressure so that your engine can achieve the correct air/fuel ratio. As you can see, there are many issues that can result from low fuel pressure.
It is important to understand the relationship between fuel line size and fuel pressure when planning a fuel delivery system. Fuel systems can be incorrectly designed if the pressure loss attributed to the length of the fuel lines isn't taken into account. Excessive pressure drop in the fuel lines feeding a carburetor or EFI system will inhibit their proper function. To deliver fuel at the correct flow rate and pressure, careful consideration of fuel line diameter and length, as well as whether the application is carbureted or EFI, is very important.
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Frequently asked questions
The PSI required for a fuel line depends on the vehicle and the type of fuel pump. For example, the Bosch 044 Fuel Pump can handle 72.5 PSI, while the minimum required fuel system pressure for a carbureted fuel pump is 6 PSI.
Yes, the size of a fuel line can affect the pressure drop in the system. A larger fuel line will result in a lower pressure drop, while a smaller fuel line will result in a higher pressure drop.
The optimal PSI for a fuel line depends on various factors, including the vehicle, the type of fuel pump, and the size of the fuel line. It is important to consider the relationship between fuel line size and fuel pressure when designing a fuel delivery system.
