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Old 01-06-2016, 01:49 AM   #21
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Even if the small return line affects fuel pressure by a little and you are running an adjustable fpr and a megasquirt your just going to tune it to the available fuel. Should not make a difference.
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Old 01-06-2016, 02:32 AM   #22
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Even if the small return line affects fuel pressure by a little and you are running an adjustable fpr and a megasquirt your just going to tune it to the available fuel. Should not make a difference.


It would be inconsistent and annoying. Just like the returnless system.
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Old 01-06-2016, 02:46 AM   #23
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Why? It would consistently add the same resistance and lets hypothetically say it adds 10psi to the fuel, you would never know it but you would set the fuel pressure to 40psi or what ever you want as long as it was above 10psi. It would probably only affect it at all at idle.
I understand its just wrong but sometimes wrong works.
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Old 01-06-2016, 03:17 AM   #24
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Why? It would consistently add the same resistance and lets hypothetically say it adds 10psi to the fuel, you would never know it but you would set the fuel pressure to 40psi or what ever you want as long as it was above 10psi. It would probably only affect it at all at idle.
I understand its just wrong but sometimes wrong works.
The problem is that it's not consistent. You're getting whatever the pump can deliver, and that varies with voltage, temperature, amount of fuel in the tank, and probably a few other things. So you turn the headlights on, system voltage drops by half a volt, and idle fuel pressure goes down by 5 psi (making up these numbers, but you see the point)

--Ian
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Old 01-06-2016, 03:18 AM   #25
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The problem is that it's not consistent. You're getting whatever the pump can deliver, and that varies with voltage, temperature, amount of fuel in the tank, and probably a few other things. So you turn the headlights on, system voltage drops by half a volt, and idle fuel pressure goes down by 5 psi (making up these numbers, but you see the point)

--Ian
Exactly.
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Old 01-06-2016, 03:23 AM   #26
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Would that voltage issue not happen anyway with any return system? A smaller than ideal line would just act as a regulator, its its even a restriction. Regulators are static (except boost reference but not at idle).
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Old 01-06-2016, 09:30 AM   #27
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Schuyler you're gonna log fuel pressure right? You can do it for like <$30

The fuel rail outlet is small, i think its a 1/8npt adapter, not to happy about that, but its the only adapter for our rail.
I can yeah. I could use whatever I want, I have a spare -10AN port on my AFPR lol.
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Old 01-06-2016, 12:35 PM   #28
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But if you use an intake referenced fuel pressure regulator (i.E. a regulator that keeps the fuel pressure at a certain level ABOVE intake), it would set the Fuel Pressure at the setpoint (IM + FP). The only time when the backpressure from the return line would affect the outcome is when the backpressure is higher than or close to the setpoint.
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Old 01-06-2016, 12:40 PM   #29
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No, backpressure from the return line should not exist at all, and he's saying if some were to happen, it would cause all sorts of wacky behavior in the fuel system. He's right
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Old 01-06-2016, 01:13 PM   #30
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I can yeah. I could use whatever I want, I have a spare -10AN port on my AFPR lol.
Get a 10an plug with a 1/8npt hole. Buy an ebay pressure sender and wire it into the ms3
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Old 01-06-2016, 01:42 PM   #31
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Get a 10an plug with a 1/8npt hole. Buy an ebay pressure sender and wire it into the ms3
Yeah that'd be the plan. I'll order this, I can drill/tap the plug. The premade 1/8npt ones are like 4 times the price.

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Old 01-06-2016, 01:44 PM   #32
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Is that a linear sender? I use some 0-5v sensors I got I can link you. I paid like 8$ for mine. But drilling and tapping is fine too.
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Old 01-06-2016, 01:45 PM   #33
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Is that a linear sender? I use some 0-5v sensors I got I can link you. I paid like 8$ for mine. But drilling and tapping is fine too.
Should be linear according to the description, yeah. Always down for a cheaper sender though.
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Old 01-06-2016, 01:46 PM   #34
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Originally Posted by IcantDo55 View Post
Would that voltage issue not happen anyway with any return system? A smaller than ideal line would just act as a regulator, its its even a restriction. Regulators are static (except boost reference but not at idle).
In a properly functioning fuel system, the pump is sized to deliver more flow/pressure than is required for the engine to operate in all conditions. Yes, in any system the output of the pump will vary with the voltage, temperature, etc, but that's what the regulator is for. It uses a spring and a diaphragm to measure the inlet pressure and drive a variable restriction in the outlet such that the inlet pressure matches the set point. If things are operating inside the proper range, then when the outlet of the pump drops a bit due to voltage drop, the regulator tightens up the outlet restriction so that the pressure in the rail remains the same.

The variable restriction in the regulator has a mechanical limit on how far it can open and if you hit the limit then the regulator stops being able to adjust the pressure. At that point, any variance in the flow output of the pump will be directly reflected as a change in the pressure in the rail. A return line that is too small for the desired fuel flow will cause pressure to back up into the regulator and put you into this state.

--Ian
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Old 01-06-2016, 01:47 PM   #35
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Old 01-07-2016, 01:04 PM   #36
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Quote:
Originally Posted by IcantDo55 View Post
Why? It would consistently add the same resistance and lets hypothetically say it adds 10psi to the fuel, you would never know it but you would set the fuel pressure to 40psi or what ever you want as long as it was above 10psi. It would probably only affect it at all at idle.
I understand its just wrong but sometimes wrong works.
It would work as you described and be ok at idle too, so long as the regulator can open up enough to bypass enough fuel to hit the target PSI at idle. It depends if your regulator can make an adjustment larger than the pressure drop on the return line. My guess is a 5-10 PSI pressure drop on the return line wouldn't be a problem at all on an aftermarket regulator, not sure on the OEM one.
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Old 01-07-2016, 01:56 PM   #37
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+1 ^ What Pat said!
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Old 01-07-2016, 02:43 PM   #38
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It would work as you described and be ok at idle too, so long as the regulator can open up enough to bypass enough fuel to hit the target PSI at idle. It depends if your regulator can make an adjustment larger than the pressure drop on the return line. My guess is a 5-10 PSI pressure drop on the return line wouldn't be a problem at all on an aftermarket regulator, not sure on the OEM one.
Someone that sees the light. Agreed.
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Old 01-07-2016, 02:45 PM   #39
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I did some testing yesterday on my new fuel pump setup, a Walbro 450 and a Walbro 255. I have 6AN return line with aftermarket regulator. At idle, I can turn on the 255, then turn on the 450. Fuel pressure stays within 1PSI of target regardless of which pump, or both, are on. Flowing 700 LPH, it's safe to say there's a few PSI pressure drop in that line. Regulator takes care of it.
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Old 01-07-2016, 02:49 PM   #40
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Someone that sees the light. Agreed.
No, that was a guess. There is no light until its actually tested, or math is done.

Pat has a 6an return line. We are discussing a 4an return line. That is over 50% smaller.
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