Question on return line size......

[John McIntire]

I am running a size 8 feed line for my EFI set up. I am using an aeromotive regulator, with 10 inlets and a 6 return line.
I already bout reducers for the regulator to go from 10 to 8, but is a size 6 large enough for a return? I think i remember reading somewhere that you should use the same size line, or maybe even larger for a restriction free return.
Let me know what you guys think!
John

[Fuelie Fan]

Look at any factory setup, the return is usually same or smaller, I don't think I've ever seen it bigger. You pressure regulator's whole purpose for being is to create a gigantic pressure drop (45psi), do you think it matters if your return line has a couple psi drop??

[nitrorocket]

The return line has to flow enough not to create a back pressure on the regulator. This will effect fuel flow and pressure on the rail side. You cannot go to big. Use -8, Same as the feed.

[TUBED]

I spoke with an engine builder today on the very subject. He suggested -8 over the -6 for the same reason nitrorocket stated above.
Gerald

[John McIntire]

thanks guys, I just think its funny how Aeromotive makes the regulator with number 10 feed lines, and a number 6 return. Thats dropping 2 sizes. I'm just going to get an adapter fitting and run 8 line for everything.
John

[Fuelie Fan]

I will repeat that it is not critical. I know because I designed, built and operated a test stand to measure the precise effects of return restrictions on regulated pressure at my last job. Going big doesn't hurt, but it won't really help either. Does anyone have any measured data to refute my findings?

[Blown353]

With a bypass regulator you can only set the regulator as low as the backpressure in the return line.

With a carbureted car, you want the pressure low-- so a low restriction return line is important. If you want 6psi at the carb but your return line has 10psi of backpressure you're hosed, because you'll only be able to dial the regulator down to 10psi.

With the much higher operating pressures of an EFI system a more restrictive return line is OK because of the greater pressure differential across the regulator. Even if the return line has 20psi of backpressure, if you're looking for 45psi at the fuel rails you won't have a problem. That being said I still prefer to have the regulator do the restricting and not the return line.

The -6 return line will not be a problem for you unless you are running a HUGE fuel pump and can outflow that -6 line enough to have backpressure higher than your desired regulator setpoint. What are you anticipating for fuel flow?

[Blown353]

Oops, double post.

[OHCbird]

Protouring72-

Remember that even if you get a -12 adapter for the retun, it still has to flow through the -6 outlet. That will always be your limiting factor. If still in doubt; call Aeromotive- they'll backup all of what FuelieFan has said.

The biggest mistake made on retun lines is that some people route them back into the tank where they aerate the fuel right near the pickup- not good!

[sinned]

The biggest mistake made on retun lines is that some people route them back into the tank where they aerate the fuel right near the pickup- not good!Exactly the same way every factory in tank module is set up...interesting.

Choking off the return too much WILL have an effect on performance, fuel pump life, and fuel rail temperature. I agree that running a -8 inlet and a -6 return won't hurt but to say it makes no difference regardless of size is a bit inaccurate.

Before you get too technical fuelfan you better have the documentation to back up your own claim first.

[Fuelie Fan]

Everyone's a skeptic.

I don't work for that company anymore, so I don't have the spreadsheets. Even if I did, would I go through hastle of hosting our results? Probably not. This site isn't that important. I'm just trying to help by telling you of my direct experience, and if you don't want to believe the one poster here who has ACTUALLY tested it, that's your thing.

But, to be a good sport, I'll try to summarize my results: We tested mainly TBI regulators, both aftermarket and integral GM throttle body regulators. Our setup had negligible restriction to begin with, and an inlet pressure setpoint of 11 psi. Introducing a 1-2 psi of restriction via an adjustable valve actually LOWERED the inlet pressure setpoint by "around" (I don't recall precisely) 1/2 to 3/4 psi. From there, additional return line restriction caused the pressure to gradually creep back to the original setpoint, reaching that point at approximately 9 psi of restriction. Beyond that, the inlet pressure increased linearly, such the inlet pressure = restriction pressure + ~2psi. For those of you who are unaware, 9 psi of restriction is a LOT. You'd only see it if your line was practically kinked.

Have you ever even seen a pump curve before? All a pump cares about is the pressure that it sees. How that pressure gets created is inconsequential.
On a related note, Pumps are most susceptible to damage from adverse INLET conditions. If there is one recommendation i could give, it's that you maintain your inlet like crazy. Keep the clamps tight to prevent air leaks, make sure you don't have any restrictions, and mount the pump as low as possible. Use a prefilter (not too restrictive, 40-100 micron) and service it often.

I'd have to think about the temperature issue, but my knee-jerk response is, if your return line is even remotely correct for size, the temperature input due to line friction is nowhere NEAR the heat input from the engine bay, exhaust, radiated from hot asphalt, etc. Now, if YOU have actual experience showing that you prematurely killed a pump or superheated the gas tank by running too small a return line, please share, otherwise I'm treating it as pure conjecture.

[camcojb]

I have had issues with too small of a return line when using an A1000 pump for a blow-through carb application. But that pump is pretty big and the return line I was trying to use was the 5/16" stock fuel line. I had removed everything in the tank but it just didn't regulate below 11-12 psi. It also had the proer regulator from Aeromotive for that pump in a carbed application. Swapped to a #8 return and the problem disappeared.

It's good to know that you've found it isn't a big deal in EFI applications as I've always run and recommended a 1/2" return for EFI due to the above problem I had with the carb.

Jody

[Fuelie Fan]

That's a pretty big pump, I could see that. We were using something roughly equivalent in flow to a walbro 392, except I think it was a Bosch pump. EVENTUALLY, yes such problems will occur, I'm not saying you can pass 2000hp worth of fuel through vacuum line. I should have addressed that point and did not. However, the same logic should apply to larger systems: Once your supply line is correctly sized, the return should end up the same size or a size smaller and you should not run into problems.

On a similar note: we were running on a dyno using gm throttle bodies, and they had a GARGANTUAN pump, seriously like 2000hp capable. It flowed so much that actual regulator seats (or internal passages?) were too small: No matter what we did (including larger return lines), it wouldn't adjust down. We had to add an ADDITONAL external regulator just to bleed off most of the flow before our regulators would begin to work.

[John McIntire]

I'm sure a company like Aeromotive would not skimp on a device like this. They clain it is to be used in conjunction with their A1000 pump, which is capable of supporting a lot of power. I am using this regulator with a Barry Grant Mighty Sumo pump. I would think that if aeromotive thought a -6 line was sufficient enough, it must work well. As OHCbird has said, The opening is only as big as a -6, no matter if I throw a -6 to -8 adapter on it.

I just have to find a creative way to plumb the return line back into the cell. I don't want to return it back into the other port in the sump, and I dont just want to drop it down from the top and aerate the fuel................

[sinned]

FF, your testing experience is based on a very low flow, low pressure application. I would actually tend to agree with your testing results based on that application. Jumping from a 15psi system that has very little actual pressure fluctuation to a 60psi system with large swings as much as 15psi is a big difference.

My "testing" is not done on a flow bench under very easy to duplicate conditions but in the real world based off of personal experience working in the field hands on for over 15 years.

The heat issue is not a result of line friction but rather heat induction as a result of stagnate fuel sitting to close to the aforementioned exhaust and engine compartment. If the fuel doesn't flow readily it will pick up heat.

[OHCbird]

Like the factory= cheapest way to produce something.

And this isn' true in all cases- there are many return-less sytems out now that use simple internal ports in the pump for regulation (in tank). As far as choking the system, that's a simple matter of using the right line / hose / fittings. My point was that even if he had a perfect -12 running all the way back with minimal restrictions, his limiting factor is still the -6 return port / fitting.

Jody- I can see why you had the prob; the factory 5/16" lines are notorious for crappy flow (either way).

[Fuelie Fan]

Dennis,
your thinking about the flow rate is 100% backwards.

Mass flow rate = density * velocity * area

If the fuel does not vaporize in the return line and remains liquid, density is a constant. (i will address what happens if it doesn't further down, hang with me) So, mass flow rate is proportional to velocity and area.

Fuel flow rate is a function of pressure setpoint and and supply voltage. I will assume that supply voltage is constant. For this conversation, I will also assume that pressure setpoint is constant (which is true for TBI systems and MPFI under constant load such as when you are idling)

Therefore, we can consider for now that fuel flow rate out of the pump to be relatively constant, and since we're considering sitting and idling (which I see as worst case for heat addition), fuel flow into engine will remain constant as well, which means fuel flow through the return line is relatively constant.

so mass flow rate ~ velocity * area = constant

CONCLUSION
If you decrease line size, you've decreased area. This means velocity MUST INCREASE. Fuel is moving faster through smaller lines, not slower, so heat soaking should be less prominent.

VAPORIZING
If your fuel does vaporize in the return line, density goes WAY down. This means the product (velocity * area) must go way up, and since area is fixed, the velocity will increase even more.

NON-CONSTANT FLOW CONDITIONS
Increasing engine load in MPFI systems both increases fuel consumtion and pressure setpoint, both of which contribute to lower return line flow rates, but this is true no matter what the return line size is. So, if you're starting with a smaller return line (higher initial velocity) and then do something to decrease return mass flow rate, you're still better off than if your baseline was large diameter, low velocity return line.

As I said in response to and support of Jody, increasing flow rates requires ALL components to be sized up accordingly. The fuel line from your lawn mower will not be sufficient for your big block. BUT, at no point will the relationship between return line size and supply line size reverse. The same concepts apply.

In fact, MPFI systems with their higher pressures should most likely be LESS sensitive to return line restriction, for the reason I said in my first post: Your pressure regulator is a PURPOSELY PLACED RESTRICTION. With MPFI systems creating between 30 psi and 58 psi of pressure drop depending on the system and operating point, 5ish psi of return line back pressure won't even begin to cause a disturbance.

[EFI69Cam]

Cool discussion.

My question is at what point does a 3/8" feed line become a restriction?

Or how much HP can a 3/8" line feed at 45 psi?

[sinned]

No, I am not thinking of it backwards. I have a very clear understanding of line size vs. flow rate vs. cooling properties of said line; thank you for thinking of me anyways. My argument is not about going with a -8 line instead of -6, it was as a counter point to your original statement that return line size will not make a impact. Running a -2 return will most certainly make a difference in ruining condition and heat dissipation; while on that subject, you are correct that pressure in the line will increase when line diameter drops however pressure is not flow. 1 gallon of fuel will not flow through a -6 line as quickly as it will through -8 line with no other source forcing it through. Even if you use the argument that the pump is the force behind the flow your own argument is that pump output pressure is fixed, let’s say at 30psi in this case. The return line pressure is not going to increase enough to force fuel through faster than it would in a -8 line. Are the effects between the two (-8 and -6) noticeable, hardly? Maybe on a 1000HP TT engine the dyno might pick up a few HP but the ass-o-meter probably wouldn’t pick it up.

[Fuelie Fan]

We're partially misunderstanding each other. Go back and re-read my posts. I've acknowledged that YES, if you keep going smaller, you will EVENTUALLY run into problems. My initial blanket "makes no difference" was in reference to the -6 to -8 discussion.

As far as return line velocity, I'm sorry, but you're still wrong: The return line pressure DOES increase enough to speed up the fluid such that mass flow rate is maintained. That was exactly what I documented in my test: Ever-increasing return line pressures. If a walbro gsl392 is seeing 30 psi at its outlet, it WILL pump 65 gallons/hour. Period. Lets even assume that the engine is off, to simplify the discussion, so that ALL 65 gallons/hour are flowing back to the tank. If you decrease the area of the hose, the fluid HAS to go faster to maintain that flow. What happens is, instead of having 29.5 psi drop across the regulator and 0.5 psi across the return line, you will have gradually more pressure at the beginning of the return line to drive that flow. As you continue to restrict the return via smaller areas, the balance continues to shift, but all the while the TOTAL pressure drop is still 30 psi and flow rate is unchanged. Eventually, the drop across the return line is so great that it overcomes the regulator spring, and at that point you begin to see a shift in the fuel pressure setpoint, meaning the operating point of the fuel pump is finally affected. The pump outlet pressure increases and flow decreases accordingly.

I'm not advocating running a return line so small that 25 of your 30psi pressure drop occurs across the return line. There are other reasons that you would not want to do that. My first argument was and still is: Fuel pressure is NOT incredibly sensitive to return line size, so you do not need to run excessively large lines. My second argument was and still is: your explanation of the fluid "slowing down" in the smaller line, and absorbing more heat as a result, is flawed.

[Fuelie Fan]

Put another way:
in reference to your "with nothing to drive it" statement: As you decrease line size, a greater percentage of the work done by the pump is going towards driving the fluid. That's where the "driving" comes from. The regulator "opens" more, allowing the return line to see more of the pump's "effort". Does that make sense?