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Old 02-02-2020, 03:09 PM
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Fuel Pump Question

I'm shopping fuel pumps to feed a 750 Holley and have read they like about 7 psi. I've noticed most pumps advertised give a range like 5 - 9 psi. Is that the range of variability to be expected in the pumps or are they adjustable within that range?

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Old 02-02-2020, 03:23 PM
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That's the range of variability in the pump. It will make a minimum of 5 lbs up to a maximum of 9 lbs.

To get a set pressure would require the use of a regulator.... there is no adjustment to the pump.

When using a regulator, you would only be able to set it to a level equal or less then the pump produces...as in the regulator can't magically take a pump that only makes 5 lbs and turn it into 7 lbs.

Ideally, you want a pump capable of making a higher pressure than you want to regulate down to....but a 5 to 9 lb pump is not a guarantee you'll get anything more than 5 lbs from it....so if you need 7 lbs don't buy a pump with a rating lower than that level.

In all reality, no carb needs more than 5 psi, anything higher is just a crutch trying to cover up for a pump that doesn't have a high enough gallons per hour flow RATE.
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Old 02-02-2020, 05:51 PM
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I had a 1955 Chevrolet with a 455 Pontiac engine with two Holley 660 Center squirter carburetors. The Stewart-Warner electric pump was mounted on the frame just above the fuel tank. It was set at the factory for maximum output pressure of 6 psi.

I had a Stewart-Warner 0-10 PSI fuel pressure gauge mounted outside on the cowl. The fuel pressure indicated 6 PSI at 800 RPM idle speed and dropped to a minimum fuel pressure of 2 PSI at 5,000 RPM. That indicated that the engine required at least 4 PSI fuel pressure at 5,000 RPM. I had 3/8” aluminum fuel line from the tank pick up to the carburetor fuel distribution block on the engine.

When I started the engine, the pump would run about three seconds and quit pumping when it reached 6 PSI and would start pumping intermittently when as the fuel pressure began to drop. The Stewart-Warner fuel pump was self regulating.

The 427 Ford powered Shelby Cobras used the same fuel pump.

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Old 02-02-2020, 05:58 PM
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Depending on what model mechanical fuel pump you have you might as well bite the bullet and buy a regulator as you will thank yourself later on as many of those pumps you don't know what pressure your going to get. The one pump I got only gave me 3 psi which is not enough so I did some homework and found a pump that was only supposed to put out 6 psi but it put out 9 psi and was not supposed to. I now run a holley regulator with a return line and set mine at 5 psi and sometimes according to my gauge it shows over 6 psi when started up but once the engine is stable and warm the fuel pressure shows about 5 psi.

I have seen anymore you can buy three of the same exact pumps and get different pressures with all three of them as quality control is not like it used to be.
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Old 02-02-2020, 07:07 PM
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The trick with mechainacal pumps is running two 3/8" steel lines from a sumped tank or fuel cell to a tee at the mechanical pumps pick up. Those pumps can handle almost anything properly fed..
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Old 02-02-2020, 08:07 PM
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I should have noted in my initial post that I'm looking at electric fuel pumps. To be specific - this is what I'm considering. I'll be running a 3/8" fuel line and would like to just dead head this to a Holley 750 - i.e. no regulator.

https://www.summitracing.com/parts/crt-p4594/overview/
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Old 02-02-2020, 08:15 PM
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You need a regulator even with that pump. When it deadheads it will go over what it's pressure rating is. The regulator will keep that more stable. A return style system is best over a deadhead setup.
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Old 02-03-2020, 12:48 AM
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I ran my holley fuel pump which was the electric one that says no fuel pressure regulator needed and it lasted me for one year working hard being dead head and because of other reasons but I hooked up my fuel pressure regulator just right at the end before it finally took its last breath of fuel and it ran so much better before it decided to die out on me and go bad but it was because of various reasons I do believe do to excessive heat under my hood and also with no return it working hard to keep pushing but nowhere to push it and lets face it quality has went out the door with a lot of things nowadays and my pump worked good but lost pressure fast after a lot of heat and working hard just dead heading and even thought it can be ran like that its known from what I read electric fuel pumps last longer and work better with a return line system and helps keep the pump running cooler over time.

I think mine might still be going if I had done it from the beginning but I like mechanical fuel pumps over electric anyways but that is my choice and with the return line in place it even likes it better then being dead head and it helps with keeping fuel cooler since it keeps a constant flow vs just being there and it helps with getting issues of fuel vapor lock or fuel percolation from heat building up with it sitting there etc. I know its a pain to install a return line as I never ran one for 18 years but after so many troubles almost three years ago I am glad I did and it has helped me a lot at least for my build but results will vary.
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Old 02-03-2020, 01:28 AM
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The problem that goes unstated is volume delivered at what pressure. If your building a grocery getter using a production style ans capacity pump you can expect about 5 to 6 psi with volumes sufficient for idle to 3000 rpm, past that point the pressure falls as does the volume, it just won't feed enough fuel for 5000 rpm and up. This since the carb is usually happy with 5-6psi and the engine isn't strained life goes on without anyone knowing that at 5000 rpm its only delivering 2 psi because the fuel being consumed at a faster rate than the pump's nominal operating volume can provide

If you are building a hot rod the pump still comes rated in a pressure range say 7 to 9 psi. Pumps rated at these pressures can deliver about twice the fuel of a production pump. But since the carb still likes 5 to 6 psi, now you have to deal with excess pressure at idle, plus the pump's large capacity at low speeds puts a heavy pressure drop on the plumbing between tank and pump. This is a major cause of vapor lock as the pressure drop in the line to the pump encourages the fuel to vaprize, which the pump doesn't pump. At the other end of the fuel flow spectrum such a pump can deliver a sufficient quantity of fuel at WOT that the pressure only drops to 5 psi which keeps the float bowl a lot happier.

This brings us to a regulator, since the carb is happier with 5 to 6 psi more than 7, 8, or 9 psi using a regulator to stop the max pressure in a carb friendly range helps level things out in the float bowl and prevents the pump from pulling so hard on the plumbing that it results in vapor lock issues. With a mechanical pump a dead head regulator is sufficient because the pressure differential isn't too great and the pump is not dependent on fuel flow for cooling as an electric is.

Now we all seem to have favorite fuel systems and mine for a hot street rod using a carb is a low pressure, high volume, TBI pump of 15 to 20 psi with a bypassing regulator. This allows a carb friendly 5 to 6 psi and a pump cooling friendly constant flow, and since these are mounted in or near the tank all the plumbing is under pressure so the vapor lock issue goes away.

Bogie
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Old 02-03-2020, 07:27 AM
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The HP this car is making is way more than I'll be utilizing. It's only going to see short bursts of WOT and it won't be going to the track. But it still sounds like I need a higher pressure pump, a regulator and a return line. It's in a '67 Fairlane. Has anyone ever used the tank drain for a return port on the tank? What's the minimum size return line? I'm planning to run a 3/8" feed line.

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Old 02-03-2020, 08:40 AM
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Return line should be the same size as the feed line, due to the fact at idle 99% of the fuel is being returned.

Can't use the drain for a return, return needs to go to the top of the tank, if you try to put it back through the drain it will resist flow, raising the line pressure in the return as the fuel can't freely get into the tank because of the weight of the fuel in the tank above the drain fitting.

If you restrict the return fuels ability to freely return, it can stack up to the linepoint it raises pressure clear back to the regulator, rendering it ineffective.fuel can't freely get into the tank because of the weight of the fuel in the tank above the drain fitting.
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Old 02-03-2020, 04:53 PM
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I think the maximum pressure head that could develop at the drain plug with the tank filled completely would be a little over two tenths of one psi. Putting the regulator and return line up by the carb - where it's higher than the fuel in the tank - should eliminate that resistance all together. If I'm missing something here please point it out. I'm trying to add a return line without having to weld on the tank.

Last edited by Molon Labe; 02-03-2020 at 05:02 PM.
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Old 02-04-2020, 12:41 PM
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9# is too much for the Holley.

My Carter 6900 puts out three time the volume of the stock pump and runs at 7#.
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Old 02-04-2020, 01:03 PM
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Todd,

Are you running a regulator and return line?
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Old 02-04-2020, 01:18 PM
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No, it's mechanical.

I just had mine rebuilt to use all modern fuels after 30 year of service.

It actually cost more to rebuild than a new one, but it can now take all current fuels including ethynol, it hasn't been sitting on some shelf for who knows how long deteriorating, and (best of all) it is cast "Made in the USA" rather than stamped "Made in ROC Korea".

Rock Auto has the best price.

Don't get the Carter 4891 which is the same in all respects, and even cheaper to buy, except that it is rated for 8.5# for use on the "strip", which is too much pressure for the Holley. The 6900 is rated for 6-8#.
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