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Understanding base fuel pressure and aftermarket pumps.

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I am still pretty new to the world of tuning and I could use some insight as to why my base fuel pressure is set the way that it is from the factory, and the ramifications of changing base fuel pressure when moving to an aftermarket ECU. As for why I want this info, I am realizing that most aftermarket fuel pump PRV's open below or very near to my base fuel pressure, and the addition of a boost referenced FPR seems like it may be a cause for a future issue when it exceeds the PRV pressure. I would also like to know if changing the base fuel pressure to a lower value may be an answer and what cascade effect this may have on the rest of the system or implications for tuning.

I have a 2014 Hyundai Genesis Coupe 2.0T that has a Theta II 4 cylinder engine similar to the 4B11 and/or 4B11T found in later Mitsubishi Lancer Ralliarts and Evo 10s and a Haltech Pro Platinum PnP for the BKTheta (The 2013-2014 models have partial support from Haltech due to minor changes).

It is an MPI engine with a returnless fuel system and has a base fuel pressure of 85 PSI (gathered from the shop manual and measuring at the end of the rail). After researching and information gathering from this site and others, I am starting to wonder why the base fuel pressure is so high where most MPI engines like this one use between 43-58 PSI normally. I know that GDI tends to have much higher pressure because the injector is in the cylinder and the pressure differential is higher, but mine are most definitely on the intake manifold. To add to this, the model years 09-12 use the exact same Theta II engine, but with a different turbo and have a base fuel pressure of 58 PSI. I imagine the larger and more efficient twin scroll TD04HL in my model have something to do with the large increase to base fuel pressure.

I have recently purchased a Radium FST-R (for reference http://www.radiumauto.com/FST-R-Fuel-Surge-Tank-with-Integrated-FPR-P348.aspx) for use as a surge tank, but also to allow for a boost referenced FPR to be added to the returnless system so that I can use the boost reference setting while tuning. The stock system has a saddle style tank with a syphon mechanisim on one side that makes conversion to a full return system difficult. When deciding on a pump for the FST-R, I noticed that most popular aftermarket in-tank options have a PRV set at 80-100 PSI which does not leave much headroom when referencing boost (I am currently at 21 PSI).

Do I have the correct idea of how the PRV operates and limits the fuel system pressure? Will most of these pumps really only reach 80-100 PSI before they start to vent pressure and I won't be able to increase beyond that point?

If this is the case, I will have to continue to look for a better pump for my application or would it be possible to lower the base fuel pressure safely? If I do lower the base fuel pressure, what else do I have to account for or what issues may this cause? I know that the injector scaling data will have to be correct in any case and that flow and pressure are diametrically opposed, but if there is something outside of this that I am missing please let me know.

Sorry if the question is not clear or seems incomplete, but I am still figuring out how base fuel pressure is decided, how it effects the system overall, and what to do in corner cases like mine. I know this is a fundamental concept, but I haven't been able to put all of the pieces together yet. I appreciate anyone that takes the time to read all of this, and hopefully someone can give me a hand understanding these concepts.

In simple terms, it is fixed at that pressure, ~5.7bar or so, because it is of the non-return (AKA dead head) type and it isn't possible to increase the pressure and ESPECIALLY lower the line pressure easily to match changes in the manifold pressure. With a return the regulator will simply adjust the bleed offback to the fuel tank but the single line has no return line to bleed it off.

The regulators, themselves, are also designed differently and cannot be swapped over.

For relatively mild tuning, you should be OK, but if you're looking at a significant increase, I'd suggest installing a return line to the tank, aand converting to the return type - there are a number of advantages and few disadvantages other than a bit of time and money for the line... If you're serious, you'd probably be replacing the pump and regulator, anyway, and they should be cheaper then the dead-head types.

An option may be to fit a return type pressure regulator, referenced to manifold, into the single type fuel line somewhere and run that back tot he tank - it will work up to the maximum pressure the pump can supply. It isn't as effective as a true return, but the engine manifold/rail may make things awkward and force that on you.

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