Fuel Line Sizing

By: Michael Rauscher

Many people design fuel line sizing based on common misconceptions and customs; As such a fair amount of fuel system installations may be fine for race only applications, but would be unsuited for street applications. This is due to the duty cycle being on average 20%-30% with idle, 15%-20% while cruising, of the engine power and if the fuel system has reserve, these duty cycles would be 5%-10% of the fuel system capacity.

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Fuel Pressure Supply & Return Lines

Fuel pressure supply lines are usually adequate for maximum performance, with a preference of 125%-150% fuel line delivery at 100% engine power. This is to ensure adequate delivery of fuel in the event of an unseen problem (i.e. pump speed/voltage/dirty filter). Fuel pressure lines should be sized to expected capacity and fluid velocity should be kept below 4 FPS to reduce the Reynolds Number which signifies excess turbulence and will result in reduced performance.

With such low volume use of the fuel system capacity, return fuel lines are usually inadequately sized. The purpose of the return line is to return unused fuel delivered to the engine back to the fuel tank. Any pump ran at a constant volume cannot be dead headed. When the fuel is bypassed it experiences a pressure drop and any air dissolved or entrained in the fuel boils out forming air bubbles, as well as any fuel components transforming into a gaseous state. 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 into the fuel tank causing foam to form. 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

Fuel line bypasses are commonly called a regulator which is incorrect. A regulator controls pressure on the outlet regardless of the input pressure on the inlet. A fuel bypasses function is to bleed off a preset pressure to the outlet; the outlet pressure should be atmosphere 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 and 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.

Fuel Line Sizing Calculation

This formula for pressure loss is based on the Colebrook equation using the Darcy-Weisbach friction coefficient.

Average D'Arcy-Weisbach Friction Coefficient for smooth hose, drawn steel tube, aluminum tube with recommended Reynolds numbers: 0.08

Colebrook–White equation


  • f = D'Arcy-Weisbach friction coefficient
  • Re = Reynolds Number
  • dh = hydraulic diameter (m, ft)
  • ε = Roughness height, (m, ft)