We were in this position when we researched on how much the stock fuel lines can flow. With limited information, we decided to do the testing ourselves and share it with the community. As always, if you spot a mistake, please advise us and we will be happy to amend.
Author: 5-0 IGNITE
Original article from our website; https://www.50ignite.com/blog/technical-blog/article-2-pressure-is-sky-high/
A POINTLESS EXERCISE
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.
Figure 2 - Power limit before stock fuel lines can't handle
If you have read this article this far, congratulations! Because we are about to bore you even more!
To understand what was going on, we needed to do some quick measurements and look at fluid dynamics.
It is somewhat valid to some degree to assumed that most Japanese cars that run 5/16” (8mm) outside diameter fuel line (bar the soft lines). Fluid flow and head (pressure) losses can be contributed by major and minor losses in the system;
Major loss - Head losses due to friction within the pipe affected by flow velocity, pipe length, pipe diameter, pipe surface roughness and the viscosity of the fluid being transferred.
Minor loss – Head losses due to friction and flow disturbances within the system affected by components such as couplers, entry condition, bends, valves, filters, diaphragm etc.
To keep things simple, we analysed the major losses in the system only as it will always be the predominant cause of pressure loss. With different fuel system setups differing from car to car and the type and number of fittings used, the minor losses cannot be determined accurately without physical flow testing.
Since we are running the 2 feed pumps in parallel, the theoretical flow rate is just compounded. Remember, pressure is a resistance to flow.
Figure 3 - Walbro 450 LPH performance chart (Walbro 2018)
Analysis of the friction factor for turbulent flow inside a smooth wall pipe using Moody chart;
(S. Beck & R. Collins, University of Sheffield 2018)
From the two intersecting lines above, we can find that the friction factor is approximately 0.048.
The theoretical, calculated value for the pressure drop of 31.5 psi strongly correlates to the 35 psi observed on the vehicle. Bear in mind, that only major losses are calculated, safe to say the remaining 3.5 psi drop can be attributed to minor losses in the system.
So, how much flow can the stock fuel line take without too much pressure drop? Well, it highly depends on what you define as "too much" pressure drop. We set an expectation that the fuel system should not have more than 8 to 10 psi drop between the fuel pump outlet and the regulator. Working backwards with the equations above in the instance of running the same Walbro 450 LPH, we find that the maximum flow rate shall not exceed 0.000066 m³/s (238LPH). This is equivalent to running a single Walbro 450 LPH pump at 95 psi!
Moving forward, the next step is to upgrade the fuel lines to AN -10 size (~12.7mm internal diameter) and increase the boost to 35 psi (however, we expect that the turbo will run out of steam before this). The base fuel pressure will remain at 55 psi with the two existing pumps blazing and the fuel pressure at the regulator, at full boost, should be at 90 psi. At that pressure, the twin Walbro pumps should be able to flow 536 LPH (0.000149 m³/s) with a pressure drop of approximately 1 psi due to major losses. Minor losess within the system should also decrease as all the fittings and liners in the vehicle will be converted to AN -10.
Curtin University. 2015. Fluid Mechanics 433 Course Notes - Lecture 1. Perth.
S. Beck & R. Collins, University of Sheffield. 2018. Moody Chart. 26 07. Accessed 08 29, 2018. https://en.wikipedia.org/wiki/Moody_chart#/media/File:Moody_EN.svg.
Slurry Pipes. 2018. Pipe Roughness. 29 08. Accessed 08 29, 2018. http://slurrypipes.com.au/properties/low-friction/.
Walbro. 2018. Walbro F90000267 Performance Chart. 29 08. Accessed 08 29, 2018. https://walbrofuelpumps.com/walbro-f90000267-fuel-pump-e85.