Running a return fuel line

[Yellow Truck]

I'm planning my fuel system, and I wanted to run a return line using the original 1/4 OD fuel line that came with the F100, and a 6 AN system for the supply. Reading the instructions that came with the Holly 12-802-1 (Holly Blue) fuel pump I found the following:

This port (labeled “RET”) must be connected to the fuel
tank with a fuel line the same size as the supply line (3/8” I.D. is recommended)

This picture is from the Holly documentation:


Anyone have experience running a return line where the supply line was bigger than the return? I was thinking I could do it with two regulators. First regulator has one input, and two outputs. One outlet goes to the carb with pressure set to 9 PSI (or what ever the carb wants) and the second outlet goes to a second regulator set to a lower pressure (ensures the carb always gets what it needs), and its outlet goes to the return line.

I really don't want to run a new line for the return.

[Autoholic]

I normally see return systems where the return line is one size smaller than the supply line. Typically I see 8 AN supply and 6 AN return, in various kits. If you're upgrading your fuel system, I would go larger than OE. It gives you room for fuel demands and you're already spending the same amount. I don't think you'll need a second regulator, the carb outlet should be at a constant pressure. The return line is kind of like a bleed off valve.

[jayb]

The issue you face there is that with a bypass fuel pressure regulator, you need to be able to return nearly all the fuel that gets to the regulator, in case you are just idling and using very little fuel at the carb (or injectors).  A smaller return line will act as a restriction and may cause the pressure to go higher than set in the regulator.  However, you may be able to compensate by setting the fuel pressure a little lower than desired.  If it were me I'd try it with the 1/4" line as a return, and if the pressure bounced around too much, add the larger return line later.

[Autoholic]

If the fuel pressure is bouncing around, that could be due to your regulator though. Not all regulators are designed the same and pressure creep can be a problem.

[cammerfe]

I race ECTA and use a 'spray'/methanol system to justify running in E/F CC class. The methanol set-up is a stand-alone and consists of a five gallon fuel cell in the trunk with a line through an electric pump with filters going to a return-type regulator immediately adjacent to the fuel solenoid in the engine compartment. The line from the cell to the engine compartment is -8 and the return is a -6---both Teflon/SS braid. But I'm only adding a maximum of 200 HP with the system as it's now set up, and I plumbed it with stuff I had on hand. The capacity of the NX system is in excess of 400 HP so before I try to use it to the maximum I'll go up at least one size on the hoses.

As Jay said, the last thing you'd want to do is to have your hose size such that it becomes a restriction.

KS

[Yellow Truck]

Jay, KS, thanks for the info. I was planning on running just a 6 AN fuel line, but a return was suggested by Barry R, and since I was thinking about how to delete the original fuel line (a 1/4 inch OD metal line), it seemed a good idea to re-use it as a return.

The question really is if I ran just a single 6 AN supply line and the regulator that it came with (i.e. no return line), how would that be worse than running a bypass regulator and 1/4 OD return?

I think the answer is to try it and see. If it sucks I'll go with no return and the 6 AN supply.

[Autoholic]

I think the problem you might run into is fuel starvation with 6 AN as your supply line. That however all depends on your fuel needs. How much horsepower are you looking at? What is your displacement?

[Barry_R]

Rear mounted pumps are a very good thing.  With fuel being pressurized in vast majority of the line going forward, the size of that line can go way down.  Using EFI as an example for illustration, its very common for a pretty high powered (600HP capable ) OE style in-line fuel pump from Carter or Walbro to use +/- 1/4" NPT fittings or smaller at over 45 psi - and they run just fine.  A 3/8 or -6 at +/-19psi will serve him just fine for a long, long time.

A lot of cars go from having marginal fuel supply with stock replacement pumps to having dramatic overkill with huge pumps and enormous sized lines.

[Drew Pojedinec]

whats wrong with overkill?   

I ran -8 Fuel line including the fuel pickup just so I'd never have to mess with it again.  No matter my future plans, I doubt I'll ever exceed my current fuel supply.

[Yellow Truck]

Autoholic - I'm building a 445 to go into a 69 F100 4WD. Target is 500 HP, 500 FT/LBS. I'll be running a hydraulic roller cam and (god willing) Barry's heads. It will occasionally see 6,000 rpm since the truck has an NP 435, it is really a 3 speed, so I wanted to be able to wind it out a bit to compensate for the big spreads on the transmission.

Drew, I mostly intend to shred tires, and I went with 6 AN based on the recommendation of a friend who has a 521 he built out of a 460. Originally planned to stay with the stock 1/4" OD line, but had advice that was a little small.

[Drew Pojedinec]

Oh i know man.  I was mostly responding to Barry.
I go by the industrial standard of "find the most suitable size of hose/tubing and go one size larger."

[Autoholic]

With a BSFC of 0.5, you need 1 gallon of fuel to produce 12 HP. So for 500 HP, you need to be able to provide 42 gallons per hour. Whether your current fuel lines will work depends on the pump loss of your fuel pump at your desired fuel pressure up to the regulator. Fuel pressure will drop over the course of your fuel lines as they are a restriction to flow. As long as you can provide 42 gallons per hour at say 20 PSI (at the regulator) you should be fine. This all depends on your fuel pump and how long your fuel system is, any changes in elevation, etc. I could have been entirely wrong about 6 AN meeting your needs.

[Nightmist66]

My two pennies. I'm sure it would be fine to feed it with 6AN, I have 8AN all the way from front to back on mine. I know it's overkill, but the way I look at it I would definately like to see a large pickup to make things easier on the pump and I think the restriction from the 8AN comes into play partly with the regulator(obviously ) but also at the carb inlet. If you have ever looked up inside the inlet of the carb, the hole is very small where it feeds to the needle and seat assy. With that said, I have a healthy electric pump to keep it flowing. I have the Moroso 19lb spring it it as well. One more thing I will add is that it is best to have the regulator after both inlets on the carb and have the return run that way. It is the same thing that manufacturer's have been doing for a long time with efi. The regulator is after all injectors, and that is to balance pressure on all injectors. Some people who are serious  prefer to run two regulators on a carb, but I don't think it is necessary if you just use one after both inlets. JMO

[Yellow Truck]

When you don't know much everything seems complicated. I started looking at the instructions with the Holley Blue pump. I puts out 14psi, and it puts out 110gph unrestricted, 70gph at 9psi. I looked up the QFT SS 830 and it says pressure should be regulated to 6.5psi. Clearly, a regulator is required.

The Holly Blue comes with a basic regulator, one input, two outputs. If you run one carb they recommend blocking one outlet. I have the QFT fuel log, so was originally going to block one side of the regulator and put the regulator on the firewall behind the carb and run a line to the fuel log. I posted one of the diagrams for a bypass system in my original post, here is the other one provided by Holly, and it is the one that is confusing me.


I'm not clear on how Figure 3 works, especially when I looked at the bypass regulator, but I think this is what Nightmist66 is describing. When I looked at the bypass regulator online, the inlet is on one side, outlet on the other, bypass on the bottom. This seems to suggest that the regulator limits pressure on the outlet side to a carb that is downstream, and bleeds off excess pressure to the bypass.

Figure 3 suggests that the regulator would have only one outlet, to the bypass, and it would regulate pressure BEHIND the regulator (meaning on the carb side) by managing the amount of fuel and pressure it passes out to the bypass line. I don't see how the same bypass regulator could do both - to me it seems it would either regulate pressure on one side or the other, the same regulator couldn't be used in Figure 2 and 3 (Figure 2 is the first one I posted). Obviously, the normal (non bypass) regulator is installed inline ahead of the carb.

[Autoholic]

It can do both and this is how. If you had the carb after the regulator, fuel comes in and then keeps going, any excess pressure is sent back to the tank. So theoretically, both sides of the regulator are at the regulated pressure because the bleed off valve prevents the pressure from building up. By controlling the pressure coming in, it is able to ensure the carb out is at that pressure. So if you put the regulator after the carb, it does the same thing. It keeps the pressure coming in constant, any build up is bled off. You can't have 60 psi coming into the regulator, 20 psi going to a carb and the excess 40 psi going to the tank and have it that way as a constant. Once you bleed off pressure, it takes it out of the incoming fuel. So you would have 20 psi coming in, 20 psi going to the carb and 0-1 psi going to thank (or whatever the differential psi is that activates the bleed off valve). In order for pressure to increase, there has to be a reservoir or restriction somewhere. It doesn't just magically happen. The fuel pump has to build up that pressure in the line.  With a truly unrestricted fuel pump, the fuel pressure would be 0. By unrestricted, they mean straight to the carb. Any kind of pressure can only happen in a closed system, be that boost, fuel pressure, air pressure, cylinder pressure, etc.

[Yellow Truck]

Autoholic, thanks. I looked at the QFT documentation and that also made it a bit more clear. They also said it is preferable to have the same size line, but made it clear it didn't have to be the same.

I think I can lay out the system now.

[Autoholic]

Your return line can be smaller than your supply line but I would only go one size smaller. Theoretically, your return line should be able to return all of the fuel but realistically, even at idle the engine will consume a decent amount. Lets say you are idling the engine for awhile, long stop light or something. Your return is 1 size smaller. The regulator tries to send 80% of the fuel back to the tank (just an example, not sure of the percentage). The regulator is set at 7 psi, so regardless of before or after the carb, the incoming fuel is at 7 psi. The return line is a tad too small for the fuel needing to go back, this creates a restriction that remains constant and the regulator is already open. The result is that the return line would be under pressure and the incoming fuel might increase in pressure. By using the same size line, the incoming fuel can never go higher in pressure than your regulator's setting. Even if 100% of the fuel had to go back to the tank, the fuel line size wouldn't create a restriction and so worst case is the return fuel would be at 7 psi, the fuel pump would just be circulating fuel. With different sized lines, the return fuel could be at a higher pressure than the supply side. You could also see a pulsating fuel pressure on the supply side.

[Yellow Truck]

Autoholic, thank you. I'm starting to understand it. I will try with the line as planned, and I'll have a gauge on the fuel log. If I see it too high at idle, I'll have to run a new line, which will suck because it will all be back together and I won't enjoy pulling the tank again. Or I can give up on the return line and just stick in one of the other regulators.

Having said that, we will have a fuel filter in the path of the supply line and none in the return, which will help a little.

[Autoholic]

If you are running 6AN return, I would think that would be ok. You're looking at 7 psi and 40+ gph, I'm not positive but I think that would be ok. You can always ask the company you're purchasing fuel lines from if 6AN is fine with that demand. I've been thinking a long time about this and I can't see the return line ever being under pressure. In order for the return line to build pressure, it either has to pressurize your fuel tank (improbable if not impossible) or it has to run into some sort of restriction and I can't see 40-ish gph resulting in too high of a flow rate. Even under turbulent flow, if there is not a restriction to the flow there is no pressure. Too small of a fuel line can provide that restriction BTW but I think something else would fail before the return line does. By this point there would be a decent amount of back pressure in the system and your carb would most likely act up, idle bad, something due to a large spike in pressure. I think you'd end up running rich if there was a prolonged spike in fuel pressure, resulting in backfires on the overrun being common. Maybe someone else can dive into the issues you might face with a sudden spike in fuel pressure and how it will effect your carb.

[Yellow Truck]

Joe, I am planning on a 6 AN supply line and reusing the original supply line as a return. The original supply line is a 1/4 OD steel line. I don't know the inside diameter.

[Autoholic]

The ID is probably 1/8" or very close to that. I would replace this line before finishing your fuel system. You're running like a 3AN return. For a 6AN system, you want tubing with an ID of 3/8".

[TomP]

I am pretty sure the stock fuel lines are 5/16" not 1/4"

[Yellow Truck]

Tom, that could be true. Took the measurement with a not great set of callipers.

[Nightmist66]

Easy way I've found especially if you don't have a caliper handy, is use an open end  wrench and see which one fits. Works for standard and metric.