Flow vs Pressure, an FPR clarification question

[skidd]

Ok.. so.. like many of us.. I've been tinkering cars for a long long time. I've done full rebuilds, tuned EFI systems, built a simple super charger kit for my Subaru... and now I'm on a Mk4.
I like to think I'm pretty dang knowledgeable about cars and the works. So.. someone tell me if my thinking here is off.. or if the people who've answered my question are idiots? I'm hoping for the latter. :)

I sent an email to both QuickFuel tech, and Holley. I asked each of them a question about a particular Fuel Pressure Regulator in each of their lineups. Same question, 2 different regs. The holley was the 12-803bp, and the QuickFuel 30-900. Both are bypass style regulators.

In short I asked the max flow rate capacity of each regulator. I would have assumed this max would be for it's lowest psi setting, but wasn't sure. I told them I had a stock in-tank EFI pump rated at 30gph (110lph) free flow. I wanted to know of either regulator could handle that volume of fuel, and still manage to regulate down to the 5 psi I need for my carburetor setup. Neither of their sites list of any sort of capacity.

I happen to have a Vortech SFMU on my 2000 Subaru, behind a Wallbro 255 pump, raising the fuel pressure at 1:6 as high as 70psi based on the boost I'm pumping in via a M62 supercharger. ( yes, I have supercharged Subaru :) ) So.. again.. I'm pretty confident in my knowledge of fuel and pressure and how it works.. bla bla.. yada yada...

Correct me if I'm wrong (I don't think I am).. but.. pressure is simply a product of the restriction to a flow, given the ability for a pump to push against the restriction.

(note.. the following numbers are made up to simply try and explain my point)

So.. an EFI pump capable of maxing out at 60psi, will make practically zero psi of pressure pumping fuel through a garden hose right back into the tank in a loop... but at the same time can easily produce 30psi up against a proper FPR. So long as the FPR can handle the "bleed" fuel volume to the return, and the return has little-to-no restriction either... the FPR will maintain that steady 30psi. If the FPR can't bleed off enough fuel, the pressure will not be accurate.

Another eg... If the fuel pump will flow 5-gph at 30psi, and the EFI-FPR can handle a max flow of 20gph at 30psi... we're good to go right?

Same thing again... .only with a bypass FPR set to only 5psi. If it has a max capacity of 30GPH at it's lowest pressure (4psi). And the pump will flow 20GPH at 5psi.. so.. again.. the FPR should be able to handle a steady 5psi right?

What is my point?
Ok.. so.. both the QuickFuel guy, and the guy from Holley said the same basic thing.
"Our FPR can't handle the pressure from your pump"???
WTF?? Ok.. sure.. I realize that if the FPR is fed more fuel flow than it can handle, it will itself be an excessive restriction.. and the pressure will rise.. and the 5psi will be impossible. I get that.
But.. am I crazy in thinking they don't know what they are talking about? Claiming only that my pumps pressure is too high?
That doesn't make sense to me.. it's the FPR that will effectively create the pressure... so long as it can handle the bypass capacity?
I even responded to teh QF guy asking for clarification.. and his answer was "The 30-900 Is a by-pass style regulator and that can only by-pass fuel pressure up to 20 PSI when regulating down to 4.5-7".
That made no sense to me since .. once again.. the FPR is the one creating the pressure.

:confused: :confused: :confused:

[UnhipPopano]

So to clarify, neither of the regulators can handle 60 psi?

[NAZ]

skidd, your understanding of the relationship between flow & pressure is correct.

[d_gatorfan]

I went through the same exercise and got the same answers from manufacturers. I think your understanding of pressure and flow is correct. The thing to keep in mind is that the bypass regulator can only regulate down to whatever pressure is present at the return port on the regulator. A large part of the 30 gallons per hour will be returned back through the return line. Depending on your return line size and length, there will be more than 5 psi drop in your return line, so the regulator will see greater than 5 psi at the return port. This will limit the ability of the bypass regulator to regulate down to the 5 psi your are targeting for your carb.

I have read where guys have added a second non-bypass regulator between the bypass regulator and the carb to get the pressure down to good carb pressure.

When i decided to go carb instead of EFI, I ended up getting a low pressure in tank pump and a non-bypass regulator. I capped off my return line and left it in place, just in case i decide to go EFI someday. 10k miles and no problems yet.

Good luck,
Dennis

[mikeinatlanta]

I agree with your understanding, however, I can also see where they may be coming from.

When considering that they have no idea what you will physically do to their product (EG. Trying to use it without bypass), I can see where they have a concern that being subjected to the full pressure potential of your pump may damage the internal components of their regulator. I don't know this to be the case, just trying to think of why they might say that.

[CraigS]

I wonder if the return fitting size on the regulator is maybe a concern for them. If it is too small it could be a restriction in and of itself. So the pressure would rise no matter how the regulator is set. Having experienced a few carb needle valves sticking open over the years, I would get a carb spec pump. Or, depending on your engine, go w/ a mechanical pump.

[skidd]

Thanks guys.. I was pretty confident my thinking was right... It's disappointing that these FPR manufacturers are as un-informed on their own products as they are. I still can't believe how few list the flow capacity of their regulators.. some do.. I think Aeormotive lists it for most, if not all of theirs. I was really wanting to run a simple low-pressure bypass system... I'm thinking now I might use a 1988 F150 low-pressure in-tank pump and a simple dead-head regulator. Living in south texas.. fuel temperature is a concern.. but.. perhaps it won't be an issue.

[Gromit]

I would agree with the 2 regulator setup idea. while the FPR may have the flow capacity to drop a high flow high pressure pump down to the needed pressure for a carb it might not be able to keep a good steady presure at such a low PSI. the regulator is just a valve but the open close might not be smooth enough for a low PSI output.

[CVOBill]

I tried doing this years ago on a Mustang that I converted to a 302 with a carb (was a 4-cylinder car). It did a really good until the diaphragm ruptured and started spraying fuel on the header. Fortunately I smelled the gas fumes and shut the car off before it caught on fire. I agree with your thoughts and it will work, but I think putting a low pressure pump in the tank will be a safer way to go.

Bill

[mikeinatlanta]

IMO: A high pressure pump in a bypass system will result in higher overall fuel temps than a low pressure pump in a non bypass configuration. Most mount the regulator under the hood, but the dead head portion still closest to the engine heat. A high pressure pump moving all that fuel will itself heat the fuel.

Regardless, neither will vapor lock due to the entire system being under pressure. Vapor locks tend to happen when part of the system is suction.

My vote would be low pressure pump with a simple regulator. No need to get too fancy with a carb on the street.

[NAZ]

Mike's right, pressure = heat. Some diesel trucks run heat exchangers on the fuel return to cool the heated fuel. And Mike is also right about vapor lock -- it happens on the suction side where the pressure is below atmospheric pressure not on the pressure side of the pump. Safest & most efficient way to supply a carb is with the lowest pressure pump that will still supply sufficient fuel flow under acceleration. For a mild mannered street car you may get away with 7PSI and probably won't need a regulator. For a high performance car that can pull more than 2-g's off the line you may need more like 18 to 20 PSI to overcome the forces of acceleration. It's not likely you need to be pumping 60 or 70 PSI to a regulator that then reduces the pressure down to the 7 to 9 PSI range to feed a carb.

[mikeinatlanta]

Mike's right, pressure = heat. Some diesel trucks run heat exchangers on the fuel return to cool the heated fuel. And Mike is also right about vapor lock -- it happens on the suction side where the pressure is below atmospheric pressure not on the pressure side of the pump. Safest & most efficient way to supply a carb is with the lowest pressure pump that will still supply sufficient fuel flow under acceleration. For a mild mannered street car you may get away with 7PSI and probably won't need a regulator. For a high performance car that can pull more than 2-g's off the line you may need more like 18 to 20 PSI to overcome the forces of acceleration. It's not likely you need to be pumping 60 or 70 PSI to a regulator that then reduces the pressure down to the 7 to 9 PSI range to feed a carb.

You're still a fairly new guy so you may not know. It's against the rules to agree with me twice in one thread. Tread lightly my friend.:D

[skidd]

lol.. glad NAZ agreed with you twice before I did then. :)
Thanks for the advice guys... double win for me... I'm smarter than the email-robot at the other end of QuickFuel & Holley... and I've been easily convinced to just go with a low-pressure dead-head setup.
I've managed to find about 3 in-tank pumps made for the 1988 Ford F150 (rear tank) that do between 3 and 7 psi. Like you said mike.. might even get away with not needed a reg at all. Worst case.. I add the fpr after the fact if I end up flooding the carb.

[totem]

Aeromotive 13301 FPR has 2 springs, one for carb at 3-20 psi, one for fuel injection at 20-65 psi.

It meets your requirements to not explode at 65 psi and still regulate at 5 psi. It flows 250 GPH!

https://static.summitracing.com/global/images/instructions/aei-13301.pdf