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Discussion Starter · #1 ·
​I am in the process of finishing my build for my birthday 06 WRX and looking to ask the question in regards to fuel line configuration.

I will have a 550 whp capable turbo running e85, wanting to know for such a setup should I run the fuel lines in series or parallel?

From my own reading it seems like a debatable topic as to which is right or wrong, possibly. But that is why I am trying to gather more information and also ask the community who is very knowledgeable so I can make sure that I make an informed decision.

Thanks
 

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What does the person tuning your car want?
​Tuner mentioned they prefer parallel, the mechanic and shop think series is good for up until you get to 600whp and above.

So kind of battle between the two so I was just trying to decide and figure ouy myself and what not.

Also do you know if you do decide to go parallel after the fact if you already finished the build (Not really an issue if during build), but if you end up puting the T splitter to split the fuel lines heading to each fuel rail. Does it matter if it is not equi-distance from each fuel rail and equi-distance from the fuel rail to FPR? What happens if you put the T splitter so it is closer to one rail than another, etc does it matter much in the end game or is it still good just because it is in parallel?

Also someone local suggested that reverse series, I have not heard of that and was wondering if that would make sense?



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One rail is fed then the others vs both fed at the same time off of a split.

I would do exactly what your tuner is asking for. He is familiar with how that setup performs and has certain expectations when making the tune.

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​Tuner mentioned they prefer parallel, the mechanic and shop think series is good for up until you get to 600whp and above.

So kind of battle between the two so I was just trying to decide and figure ouy myself and what not.

Also do you know if you do decide to go parallel after the fact if you already finished the build (Not really an issue if during build), but if you end up puting the T splitter to split the fuel lines heading to each fuel rail. Does it matter if it is not equi-distance from each fuel rail and equi-distance from the fuel rail to FPR? What happens if you put the T splitter so it is closer to one rail than another, etc does it matter much in the end game or is it still good just because it is in parallel?

Also someone local suggested that reverse series, I have not heard of that and was wondering if that would make sense?

Arguments can be made for going either way.. Dom from Maxwell Power has a good write up on the potential advantages of series. He says in his experience series results in less misfires than parallel. His assumption is that in parallel one side will end up being hotter than the other side, unbalancing the density of the fuel. His argument is pretty good and well thought out. After asking my tuner about it he basically said that's hogwash and in the real world parallel is proven to work better (he also said Dom has a lot of crazy ideas, some of which don't really pan out). My tuner really knows his stuff, so pretty much anything he says I will stick to 100%.


Personally, if your tuner is someone highly reputable in the community I would go with what they prefer. That's who is ultimately going to be determining how the car runs after all. Plus it sounds like the shop isn't saying there's anything wrong with parallel. If anything their statement would lead me to believe they also think parallel is better (just not absolutely necessary).

Either way, I wouldn't change things up after the tune. If you tune it in series I would just leave it like that unless you're going to get it retuned after switching to parallel. I would make the lines equidistant if possible. One line longer than the other means that more flow will be diverted to the shorter side.

I would not consider running it reverse series from how Subaru set it up. The OEM series setup feeds the injectors in reverse firing order (which is 1-3-2-4) and if you choose to stay series I would definitely stick with that. It just makes sense because the pressure drop from #1 isn't going to effect anything since it will be downstream of all the other injectors. Then #3 fires and it is now the injector furthest downstream, and so on. By the time #4 fires it's far enough from #1 that it won't effect it. Reversing the flow would mean that the pulse from each injector will be meeting the next injector in the series right as it's firing.. doesn't sound like a good thing to me.




Please explain what you mean by series or // with regard to fuel lines. I know what it means electrically but I am not sure I know what you mean here.

Series means that there is one fuel line coming from the filter (tank).. it feeds into one rail, through those two injectors, crosses over the top of the block into the other rail, through that, and to the FPR then return line. Parallel would be a splitter immediately before the rails, two lines, one goes to each rail, then the two outputs from the rails T back into a single return before hitting the FPR (or potentially even better, into two separate ports on the pressure regulator, then a single return line exiting the FPR and back to the tank).
 

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OK, I see now how we are defining series vs // here. Mainframe I don't think you actually mean "through those two injectors" as that would mean it gets squirted. But you mean in goes through a tube/rail 'past' those two injectors directly above them, right.

So now I can say that series or // makes no difference. No difference because of type of system. The difference only comes with respect to length of fuel travel to each injector. Ideally we would want these flow lengths to all be the same. But if they cannot be the same, minimizing the longer travel lines is best (be it series or //.)
 

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OK, I see now how we are defining series vs // here. Mainframe I don't think you actually mean "through those two injectors" as that would mean it gets squirted. But you mean in goes through a tube/rail 'past' those two injectors directly above them, right.

So now I can say that series or // makes no difference. No difference because of type of system. The difference only comes with respect to length of fuel travel to each injector. Ideally we would want these flow lengths to all be the same. But if they cannot be the same, minimizing the longer travel lines is best (be it series or //.)

Well yeah.. through the rails and out the other side (and through the injectors into the engine, but I see what you're saying).


The flow lengths cannot be the same, but parallel would mean much more equal length.. one and two would be equal, as would three and four, with the two pairs being only a couple inches apart. In series number four will be probably about two feet away from number one, and one and three and two and four will still be a couple inches apart. But I feel like you're right, it shouldn't make a huge difference either way with how the system is set up.. pressure from the pump to the FPR will be equal. Although there are still pulsations when each injector fires.. hence the stock dampener (some rails even have dampeners on the rail itself as well). At one point I had asked Lance Lucas (an engineer at Cobb) about running PTFE (Teflon) fuel lines in the engine bay to prevent ethanol vapor from leaking. He recommended not to run PTFE between the filter and the FPR because it behaves as a hard line in the sense that it has no expansion/contraction properties. He said the nitrile fuel hoses act as a dampener in the system themselves helping to even out the pulsations.

Some people even choose to run a dead head fuel system, where the ends of the rails are capped off, pressure is applied to the rails, but the FPR is also before the rails. I guess it works, but doesn't seem like a good idea to me because I would think the fuel will have more time to heat up sitting in the rails instead of continuously flowing through and cycling back through the tank.
 
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