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ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

Hey, EFI guys, what are you running on your fuel delivery system: a fuel return as with the stock setup, or are you running returnless? Can't you just eliminate the return line to the fuel tank? All of the "endwrench" rails I've seen are setup with a port on both ends, so I'm assuming these have all been setup with a return to the fuel tank? I'm trying to ascertain why a return-based setup would be required if you're running EFI.
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morghen · Joined: 21 Jan 2005 Posts: 9095 Location: Romania

Umm....because the pressure in the fuel rail would never be constant...it depends on how much you "bleed" into the engine so you cant calculate the amount of fuel or pressure the fuel pump must pump....and even if you could...you'd have to open loop regulate the pump.
The pump always puts out the same amount of flow and the FP regulator bleeds the excess into the tank.

IE: the pump puts out a max flow matching the need of the engine at 6000RPM...if the pump would output the same flow at idle the pressure in the fuel rail would increase and blow up or at least make the idle mixture very rich.
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Rich H · Posted: Tue Dec 09, 2008 1:10 am Post subject:

Please please please run a return setup.

Problem is that without a return regulated rail you have fuel sitting in the rail all the time and it gets hotter and hotter and hotter as it never gets cooled by fuel flowing through it all the time. With a return you get a cool flow all the time, without you only flow the bit the injectors use.

You will still need a fuel pressure regulator somewhere, you have to regulate the fuel pressure in the rail somehow.

run a return

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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

Got it. I was planning to run a return-based setup, but somebody asked me why and I didn't know how to answer.

So how do the returnless systems work? This is just for my own personal edification.
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erstwhile owner of just about every 924 variant ever made

Rich H · Posted: Tue Dec 09, 2008 1:30 am Post subject:

The regulator works in the feed line and bleeds pressure off it there, achieves the same effect but you don't get the through flow at low throttle.

At higher throttle the injectors grab enough fuel to create enough flow to cool the rail.
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

Min · Posted: Tue Dec 09, 2008 2:04 am Post subject:

Rich H · Posted: Tue Dec 09, 2008 2:36 am Post subject:

Raceboy · Joined: 01 Mar 2004 Posts: 2327 Location: Estonia, Europe

Returnless fuel system relies on PWM-ed fuel pump but that is pretty much out of the question for do-it-yourselfer car enthusiast.
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Smoothie · Posted: Tue Dec 09, 2008 5:25 am Post subject:

"Pulse Width Modulation" - to answer an imaginary next question.
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ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

My DTA Fast unit supports PWM...
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erstwhile owner of just about every 924 variant ever made

Martijnus · Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

But they're not E85 safe

I have to replace them with teflon-lined braided stainless, which ain't exactly cheap.
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erstwhile owner of just about every 924 variant ever made

bass gt · Posted: Tue Dec 09, 2008 6:35 am Post subject:

Dan,

seriously, run a return'd system. The fuel pressure reg sits on the retun end of the fuel rail, and keeps the rail pressure constant. Even small voltage changes will affect the pump, which will wreck the fuelling. It also makes the regs job of raising the fuel pressure in line with boost a lot easier.
Finally, air bubbles aren't trapped in the rail, and that can lead to serious issues

Steve
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Min · Posted: Tue Dec 09, 2008 6:40 am Post subject:

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

yes yes, will do...again, just asking the question because I didn't know how to answer the question that was asked of me...he specializes in high output Rustangs, and apparently the fuel systems they're installing are returnless. He thought it was unnecessary, but I didn't know how to explain why it should be return-based as opposed to not...
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erstwhile owner of just about every 924 variant ever made