Aeromotive Fuel Systems Insight - CHP Insider

Jesse Powell And Brett Clow Of Aeromotive Explain The Science Behind Fuel Delivery And How To Properly Design A Fuel System

Stephen Kim Jun 1, 2009 0 Comment(s)
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Voltage And Flow
Voltage to an electric motor is like fuel pressure to an injector. More pressure in equals more volume out. Higher voltage at the pump terminals increases motor torque, resulting in more revolutions per minute and an increased flow volume for a given pressure. "Our A1000 pump will see a 40 percent increase in volume at 80 psi when voltage is increased from 12 to 13.5 V. This factor is often overlooked and can make or break pump performance, especially at high pressures," Powell explains. The key is to figure out how much a pump flows at a certain voltage. "Although often deleted on drag cars, the presence or lack of a correctly sized and properly functioning alternator is a vital consideration when choosing a fuel pump. Furthermore, proper wiring is no less important than proper plumbing when it comes to extracting the maximum performance from the fuel pump. Poorly executed wiring resists electrical flow to the motor, which manifests as a voltage drop at the fuel pump. In order to prevent this, we include high-quality wiring kits with our pumps along with detailed instructions."

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Feed Lines
After spending the time to select the appropriately sized fuel pump for the application, sizing out the feed line is rather straightforward. According to Aeromotive, the size of the line from the tank to the pump follows one simple rule of thumb: Don't make it smaller than the pump's inlet port size, and don't hesitate to make it bigger if you can. "This is easy to figure out with AN-style pump ports. For example, if the pump inlet port is AN -10, use AN -10 or larger fuel supply line, and make sure the tank outlet or pickup tube is the same size," says Powell. "When dealing with NPT threaded ports, use the line size that is a minimum of 1/8 inch larger than the NPT port thread. For instance, a 3/8-inch NPT port should have 1/2-inch (AN -8) lines. From the fuel pump to the engine, use a line size equal to the fuel pump outlet port for optimum results in low-pressure carbureted systems. This also applies to fuel-injected engines, although you can use a line that is one AN size smaller than the fuel pump outlet port and still get acceptable results."

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Return Lines
A common myth is that sizing the return line is based on the size of the feed line, but it has nothing to do with the size of the feed line. The lower the fuel system pressure, the larger the return line must be. Furthermore, greater fuel pump flow rates require a larger return line. A good rule of thumb is that the return line size must be large enough to handle the flow from the pump while creating at least 3 psi less backpressure than the regulator is set for.

The single biggest mistake we see is reducing the return port in the regulator to accommodate a smaller return line. This results in excessive fuel pressure regardless of how the regulator is adjusted. The solution is to treat the regulator return port exactly as you do the fuel pump inlet port. You can always make the return line bigger than the regulator return port, just like you can make the suction line bigger. This won't have a negative impact on pressure control. On the other hand, a restrictive return line causes excessive pressure, carb flooding, and drivability problems.-Jesse Powell

Venting
Although many people don't give it much thought, the consequences of improper fuel tank ventilation are severe. The tank vent should be equal to or greater than the size of the feed line to the fuel pump. This ensures that the fuel consumed by the engine is replaced with air, which enables fuel to flow smoothly out of the tank. Improper venting can result in a vacuum inside the cell, causing the pump to suck the fuel into a vapor. The results range from vaporlocking the fuel pump to poor drivability to catastrophic engine failure. "A good analogy is to think of the fuel system as a quart of oil," explains Powell. "As you pour the oil into the engine, it chugs in uneven spurts because there isn't enough air entering the bottle to replace the volume of oil that's exiting. If you were to poke a hole in the bottom of the bottle, however, oil would flow out smoothly."

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