Fuel Line Sizing Calculator

The reason line sizing matters so much is hidden in the geometry of pipe flow. For a given fuel mass flow, the pressure drop along a hose scales roughly with length divided by diameter to the fifth power. That fifth-power term means diameter overwhelms every other variable: shortening a run helps a little, but stepping up one -AN size transforms the result. Going from -6 (about 0.34 inch bore) to -8 (about 0.44 inch) is only a 27 percent increase in diameter, yet because of the exponent it roughly triples the flow the line can pass at the same pressure drop. This is why a tank-to-rail run that seems hopeless at -6 becomes comfortable at -8, and why over-thinking length while under-sizing diameter is the classic plumbing mistake. The two practical targets to hold are a supply pressure drop under about 3 psi at peak demand and a fuel velocity under roughly 5 ft/s; higher velocity invites cavitation at the pump inlet and noisy, pressure-unstable delivery.

Sizing is not simply about chasing the biggest hose, though. An oversized line carries more fuel volume, which adds weight and slows pressure recovery at throttle tip-in because the regulator has more dead column to act against; it also costs more in fittings and can complicate routing. The goal is the smallest line that clears both the pressure and velocity limits with a little margin. Remember that fittings and tight bends each add an equivalent length of straight hose, so a run with six aggressive 90-degree turns behaves like a much longer pipe than the tape measure suggests. Account for those in your count rather than assuming a clean straight shot. E85 flows about 30 percent more mass for the same power, so an alcohol build should generally step up one size over the gasoline pick. Once the supply line passes, confirm the pump can actually deliver that flow at operating pressure with the fuel pump sizing calculator.