Flow Limits of 5/16" (8mm) Fuel Line

Author: 5-0 IGNITE  



Figure 1 - The boat on the dyno


Those who pondered on why we stopped at 603whp/27 psi on the project R33 GTR read on….


It wasn’t by choice, we were limited. When we built this car, we had one vision to go by; STREET DRIVEN.


For anyone who desires a go-fast-boosted street car - you will always come across the tough decision of which turbo should you go for and how important the turbo's response is. You are not at the track, you need those down-low torque or at the very least, a strong mid-range for performing street duties and occassional track days. There is no fun in it over-selecting the biggest and (inherently) laggier size turbo you could afford only to be chopped by a Honda until you hit that 6000rpm. We don’t condone street racing but the truth is, any car enthusiast regardless of age or gender will give it a squirt once in a while.

We selected a compromise of power and response on the RB26. We love our Garrett, Turbosmart, 6Boost, Tomei and Haltech products. Looking at very few compressor maps on Garrett GTX Gen 1 turbos (this was 2016 mind you) and drawing lines based on flow and pressure ratio to find the right efficiency on ‘What if’ power level, we settled on the GTX35R with 1.06 twin scroll rear housing. This turbo was chosen after much discussion and thought, we concurred that it won’t be too laggy of a turbo setup and should yield about 650hp at the wheels on the small but mighty 2.6L.

From there onwards, a forged internals build started and supporting mods were chosen; 6Boost twin scroll manifold, Plazmaman intercooler, Tomei cams, Turbosmart fuel regulator, Turbosmart external gate, Turbosmart BOV, Bosch motorsport injectors, 3 Walbro 460lph fuel pumps (1 lift and 2 feed, in parallel configuration) for E85 and so on and so forth.


We had a stop to think, with this much fuel feeding the beast, how much can the stock fuel lines be able to cope? Can it flow enough to match the fuel delivery?


The answer to that question at that time was simply #YOLO (we don’t have Instagram by the way) and #justsenditmate. Information on the internet as to “how much does 5/16” (8mm) fuel line can flow at xx pressure” was sketchy at best and inputs from the forums were a pure guess based on “my dad’s friend’s uncle’s setup” etc. Nothing was solid or backed with numbers, so we thought that it would be a good opportunity to see how much indeed stock fuel lines could flow and answer the question for the world.

During the last tuning session at WTF Auto, Tristan dialled in 30 psi on the Haltech. The dyno session stopped not long after, as fuel pressure dropped the moment we hit full boost. We re-checked the fuel pressure at the FPR and checked the fuel pressure at the fuel pump outlet. The pressure at the FPR was 55psi base, exactly to how it was set, but the pressure at the pump outlet was 90psi. That is a shocking 35psi pressure drop across the hard lines! The over-pressure relieve valve on the Walbro 450 LPH is set at 120psi from factory, so if we are sitting at 90 psi base pressure on the fuel pump outlet and we add 30psi of boost to the system, quick math shows 90+30=120 psi - all adds up to trip the relieve valve. This is why fuel pressure dropped the moment we hit full boost at 30psi.