How big is too big for fuel rail?

[AydinZ71]

I have am using an Aeromotive FPR w/ boost pressure reference. Several other equivalent high-quality manufacturers out there, like "Fuelab". They all run somewhere between $100-200. Typically a 6061 alloy AL body to save a bit of weight. Mine has lasted 20 years now with no problems. It has a soft-metal/hard-metal seat with practically zero need for future maintenance. My fuel filter is upstream, so unlikely to get contaminants on the seat. 

 

If you are planning to use a fuel return line (highly recommended), make sure you get the "right" FPR. 

 

If you want the maintain pressure in your fuel line, do some research on the FRP (reviews) and see how the pressure holds up over time. Also, make sure you have a check-valve at the outlet of your fuel pump, as the pressure can leak-back through the pump depending on the type/manufacturer. 

 

I can't think of a clear downside from using a larger-than-necessary fuel rail besides the obvious size and weight. It will take a fraction of a second longer to pressurize fuel with a large volume, but I doubt you would notice. A fuel rail with a significantly larger ID than your fuel feed line just isn't necessary. 

 

Mechanical engineer here with a working background in process design (I design fueling facilities) so if you need some tips for a rock-solid fuel system design let me know. Extreme demands on fuel systems is not typical unless you are planning for huge torque. For most applications, it comes down to proper placement of component sand focusing on reliability/durability. I see a ton of people overlooking simple fixes like fuel lines touching engine components (boiling gas). 

 

Good luck! 

[NewZed]

1 hour ago, AydinZ71 said:

Mine has lasted 20 years now with no problems

 

Does it hold pressure when the pump is not running?

[AydinZ71]

2 hours ago, NewZed said:

 

Does it hold pressure when the pump is not running?

 

Mine does immediately after the engine is turned off, but does leak down to 5-10 psi overnight. It is bypass type, so it bleeds fuel back in the return line. I'm not certain if this this is caused by my FRP or the lack of check-valve at my pump outlet. It doesn't bother me. As long as pressure drops slowly, I know it is doing its job while the pump is running. Never had any pressure related issues with this regulator, but my Walbro 255 has gone bad once in the last 20 yrs. 

 

I think I read on their website some time ago that they do not claim to meet the 30 min zero p-drop spec. the EPA requires for OEM vehicles. 

 

I also run a surge tank in the engine bay, so my plumbing between the pump and the FPR is only 2' long. A small leak on the FRP seat will cause a quick drop in pressure due to the low volume of fuel in the line. 

Edited September 23, 2021 by AydinZ71

[NewZed]

The issue with the pressure bleeding off is that restarting the engine after a day or two is difficult.  The fuel rail typically empties due to the heat from the engine.  Even if you prime it it's still full of air bubbles.  Besides that, if you're going to prime then you have to add another switch to run the pump.

 

It kind of boils down to how involved a person wants to be in getting their engine started.  An FPR that doesn't leak down will start faster than one that does. 

 

 

 

 

[AydinZ71]

11 hours ago, NewZed said:

The issue with the pressure bleeding off is that restarting the engine after a day or two is difficult.  The fuel rail typically empties due to the heat from the engine.  Even if you prime it it's still full of air bubbles.  Besides that, if you're going to prime then you have to add another switch to run the pump.

 

It kind of boils down to how involved a person wants to be in getting their engine started.  An FPR that doesn't leak down will start faster than one that does. 

 

 

 

 

 

No doubt, if your fuel rail stays pressurized it is ideal. If you are using the OEM key start, I agree with you 100%. Going straight from "off" to "start" in one throw requires your fuel rail to remain at operational pressure. In my case, I have switches and always prime for 1-2 seconds. 

 

I have not experienced the air bubbles concern you have mentioned. When vapor bubbles are created due to fuel boiling from heat, those bubbles collapse/condense back into a liquid under two conditions: 1) temperature drops minutes/hours after engine is off. 2) operating fuel pressure (30-50psi depending on your tune) will likely collapse the vapor bubbles based on the vapor/liquid pressure/temperature graph for gasoline. The pressure component is why it is so much more likely to experience fuel vapor related issues on a carb fuel system (3-5psi) vs. fuel injection. 

 

Air bubbles specifically are a problem. They are unlikely to collapse back into solution even with full fuel pressure. However, getting air bubbles into the fuel rail requires a leak in the system that is allowing ambient air in. If there is a physical leak path for gasoline to come out of containment, there is a path for air to get in. However, air is unlikely to bleed into your fuel rail from the RPR seat as it would require: air downstream of the FPR to be present (should be saturated with gasoline vapor worst case, else liquid gasoline), and ambient air pressure would need to be higher than that in the fuel rail.

 

Here is a sketch of my fuel system on my L28et. 

[AydinZ71]

[mainboyd]

I guess it makes sense why I see highly modified cars have a fuel pump switch to have the pump running constantly before starting the engine. 

[NewZed]

Vapor, gases, "air" that was dissolved in the gasoline (it's a thing) but degassed when the pressure was released...I should have been specific.  But, as someone who had an Aeromotive FPR for a while I know the sound of "non-liquid" being pushed through the rail and back to the tank when priming the system.  The rail empties of liquid and fills with non-liquid, vaporous substance.  The injectors are very hot after an engine runs and just percoate the liquid out until they are dry (I assume).  Then the new fuel has to come back in and refill each injector and pressurize it so that it will squirt the proper amount of liquid out when the injector opens.

 

Or, in short: no leakdown = quick no-prime start; leakdown = prime ,wait, start after extra cranks, and maybe some rough revolutions before things smooth out.

 

It's really just where a person wants to be on the daily driver/race car spectrum.  I got tired of priming and waiting so modified a Bosch regulator, for convenience.

 

Edited September 23, 2021 by NewZed

[AydinZ71]

I agree. I can see why it can be a problem. Il do some research on their competitors (fuelab etc.) and see if they have similar leak down  issues. I did run this as a daily for several years when I had it in high school, but always had the switches. Might be time to try a different FRP on the next EFI engine I install.