Return line to surge tank, or cell?
 

I know nearly everyone returns their fuel to the surge tank, but to me this is counter-intuitive. That is the hottest fuel, and much of it goes straight back to the engine. Other than not scavenging the last the drop of fuel, why not return to the cell instead? Thanks, Carl

If your high-pressure pump outflows the lift pump, your surge tank will always be empty. Also, if the lift pump inlet is uncovered for some time, the high-pressure pump would quickly drain the surge tank. Better to return the fuel to the surge tank. And have an overflow line from the surge tank that returns the fuel to the main tank (replacing hot fuel overflowed back to the main tank with cooler fuel provided by the lift pump).

-Max

O'K, Don't you think it would be nearly impossible for a 255lph pump @~5psi to get outflowed by an Aeromotive A1000 @ ~75psi?

Seperate question. If I am running parralel -6 lines out of a Y block, can I use -8 for the main line (main external pump to rails), or do I need a -10? Also, Can I use a -6 return, or do I need to go up to a -8.

Third question, I am using one port on the FPR(aeromotive) for a FP gauge. Can I feed both rails back into a single port on the FPR, or should I use one for each rail, and then tee off one line for the gauge?

so, are -6 sufficient for the feeder pump, or do I need -8.
I ask these questions due t having everything to run the set-up as follows.
Bosch in tank(911 turbo pump)/-6 to surge, -6 back to cell
-8 out of surge to -10 to Big SX filter
-(?) to front, Y block to (2)-6 to rails
Y block(or two seperate lines) to FPR all -6
-6 return to surge.



I can do -10 to the rails, and -8 return with seperate -6 to the FPR. I can also do all -10 surge/pump/filter/front to rails/ -6 to Y block-6 to FPR--8 return to surge. Max what whould you do?

You could run your return fuel through a cooler before it goes back into the surge tank.

No, I don't think it's impossible. I think the surge tank would be sucked dry right away at idle, and still might get sucked dry under boost. The A1000 flows ~376L/hr at 45psi. My intank BNR-32 pump flows 250L/hr at 75 psi and 13.2v, and I would assume the A1000 flows more. I am sure the lift pump you are using would flow more than 255L/hr since the pressure would be lower than what was used for that rating, but I am almost certain it wouldn't keep up with the A1000 at idle (~45 psi).

I understand the concern about hot fuel, but I think if you setup the surge tank properly, it will get a continuous supply of cool(er) fuel from the lift pump to keep the temps in check. Routing the return line back to the main tank just seems like a recipe for certain starvation of your high-pressure pump to me.

-Max

Good enough for me Max, I plan on monitoring fuel temps anyway, fuel coolers are cheap enough, they just strike me as vulnerable to damage. Any thoughts on the myriad of line size variations?

I am not sure. I understand some of the issues and know how to calculate flow areas of the hoses and stuff, but I really don't have much experience in choosing plumbing for very high HP fuel systems. I'll give you my best guesses here, but realize that they are just guesses. I'd be more inclined to trust someone else's experiences than my speculation. Hopefully someone else will share their experiences.

Here are the cross-sectional areas for different line sizes (IDs from my Pegasus catalog for Goodridge or Aeroquip lines):
size, ID, flow area
-6, 0.34", 0.09"^2
-8, 0.44", 0.15"^2
-10, 0.56", 0.25"^2

So, two -6 hoses are a little bigger than one -8, and are a good bit smaller than one -10. My guess is that feeding the y-block with a -8 hose would be fine, but it probably depends on how much total flow you need. I have heard that using very large (-10) supply lines can itself cause some problems due to the voluminous (and heavy) column of fuel in the line when you accelerate. I'd try the -8 supply line.

One issue that you can run into if the return line is restrictive is that you'll get elevated fuel rail pressures when the injectors aren't flowing much (like at idle). I would be inclined to use a -8 return line to match the supply line for this reason.

Generally, that should work fine. But you might run into the "restrictive return line" problem again (and it might not show up on the gauge in this case, since the gauge is after where the lines from the rails are joined). Does your FPR have a dedicated 1/8NPT gauge port? If so, I would use that and run separate lines from the rails back to the FPR. That might help to isolate each rail from the other rail's pulsations a little better, too (again, just a guess). If there is no dedicated gauge port, I would get a regulator that does have one.

-6 are probably fine for the feeder pump and surge overflow. I would run -10 from the surge tank through the filter and to the pump. -8 from the pump to splitter, -6 to rails, -6 from rails to separate ports on the FPR, and -8 return to the surge tank. But again, these are just semi-educated guesses on my part. I'd try to find someone that has plumbed a lot of high-HP EFI fuel systems (and stayed involved to see how well they worked) first if I could find such a person, though. Maybe a domestic shop would be of some help -- V8s often have two fuel rails, too. Or an application engineer at Aeromotive or something like that.

-Max

Your final thought is where I really thought I would start. I have a couple different FPRs, I'll see if one has a fourth port. Thanks for your thoughts, Carl