Fuel Line Sizing

Many people base fuel line sizing based on common misconceptions and customs and as such a fair amount of fuel installations may be fine for a race only application they are unsuited for street applications due to the street duty cycle being on average 20%-30% with idle, city cruise of 15%-20% of the engine power and if the fuel system has reserve these duty cycles would be 5%-10% of the fuel system capacity.

Fuel pressure supply lines are usually adequate for maximum performance, with a preference of 125%-150% fuel line delivery of 100% power to ensure adequate fuel in the event of an unforeseen problem with fuel delivery, i.e. pump speed/voltage/dirt. Fuel pressure lines should be sized to expected capacity and fluid velocity below 4 fps to reduce the Reynolds Number which signifies excess turbulence hence reduced performance.

With such low volume use of the fuel system capacity return fuel lines area usually inadequately sized. The purpose of the return line is for the unused fuel delivered to the engine to be bypassed back to the tank as all constant volume pumps cannot be dead headed. When this fuel is bypassed it experiences a pressure drop and any air dissolved in the fuel or entrained in the fuel boils out forming air bubbles as well as any fuel components vaporizing during this pressure drop. Ideally we want this activity to occur in the fuel line and not in the tank. When the pressure drop in the line is not sufficiently close to ambient the gasses do not condense back into the fuel and the air does not form complete bubbles in the line to escape the fuel in the tank. What does occur with an insufficiently sized return line is that when the bypassed fuel returns to the tank there has not been a sufficient pressure drop in the line and the remaining pressure is discharged in the tank and with it air and gasses discharging forming foam. Return line velocity should preferably be less than pressure line fluid velocity to accomplish the reduction of foam and fuel rail pressure rise.

Fuel line bypasses are commonly called a regulator which is incorrect. A regulator controls pressure on the outlet of the regulator, regardless of the input pressure to the regulator. A fuel bypass function is to bleed off a preset pressure to the outlet; the outlet pressure should be atmospheric and is determined by any restriction downstream. The bypass outlet pressure is ideally ambient atmospheric although return line sizing if too small will increase outlet pressure and added rising fuel pressure line/rail pressures. Another problem with fuel bypasses is the under sizing of the bypass orifice resulting in low flow causing low fuel consumption pressures higher than high fuel consumption pressures causing erratic fuel rail pressures. A larger bypass would be required or a series of bypasses to bypass the total fuel not used at low fuel consumption conditions.

Return line example - If the engine requires 600 lb/hr of fuel at maximum power and is used in low speed/power applications then the return line and bypass needs to return 90% of the fuel with ideally no pressure loss. That would mean a return line sizing equal or larger than the pressure line.

The fuel line sizing calculator is used to determine pressure losses and velocities of fuel by incorporating viscosity, Reynolds Number, D’Arcy-Weisbach Friction Coefficient, Colebrook Equation for friction for the final output of pressure loss in psi. Head pressure is also used and requires the input of the fuel rail elevation above the fuel pump to establish pump pressure to overcome gravity. This calculation can also be used to establish suction head which is the distance from the fuel level to the pump inlet and it can be used for pump performance.

The output of the fuel line sizing calculator is that with the known pressure losses one can consult fuel pump performance charts to properly design the fuel system to engine rated power.