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Fuel system architecture question (mechanical pump / filter)

General engine tech -- Drag Racing to Circle Track

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Ohio Rob
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Fuel system architecture question (mechanical pump / filter)

Post by Ohio Rob » Mon Jun 29, 2020 4:34 pm

Hello all.
I’m currently sorting out a fuel supply issue on my offshore boat and I need some ideas / advice.

The boat is a naturally aspirated, BBC with a Baker 6 vane fuel pump. I have an Aeromotive bypass regulator and the pump puts out 15PSI. I have the regulator Set to bypass all but 6 PSI.

I’m using a FST fuel filter with -12 ORB ports. The fuel tank is in the belly of the boat and has a 1/2” fuel pick up tube (no sock) and a 3/8” NPT to -10 AN adapter. -10 AN line to the filter inlet and -10 line to the Baker pump. The output of the pump is -8 and it feeds both bowls with a -8 “T” on the rear bowl and feeding the bypass regulator.

The filter is roughly 6” above the top of the tank.

My issue is that as my fuel level in the tank drops, my fuel PSI becomes unstable. It has dropped to 3 PSI, leading me to idle the boat back to the dock. I thought maybe I had a ruptured diaphragm on the regulator and it was bypassing most of the fuel. I’ve disassembled it and it’s fine. I disassembled the maker pump and it’s fine. I’ve removed the comp cams bronze tip pump pushrod and it shows perfect witness marks. I’ve checked all the lines for blockage and checked the pickup tube in the tank for a fault. All is well there.

I’m wondering if having the filter higher than the tank could be causing my issue. My thought is that, when full the pump has less vertical rise to siphon the fuel into the filter, and as the fuel level drops, I’m asking more of the pump on the vacuum side.

My other thought is that I should have the filter after the pump and before the carb. I called and talked to the rep at FST filters today and he says that the filter will flow in excess of 300 gallons an hour. It’s only 3 microns, but has a bunch of surface area. He thinks my issue is the location of the filter being 6” above the tank.

I’m wondering if I would be better off with the filter mounted on the front of the cylinder head, above the pump and near the carb feed. I would have a horizontal run from the tank to the pump, a 10” piece of -10 up to the filter and the -8 feeding the carb.

Any suggestions or thoughts would be greatly appreciated.

Rob

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Re: Fuel system architecture question (mechanical pump / filter)

Post by Ohio Rob » Mon Jun 29, 2020 4:39 pm

This is the filter
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Re: Fuel system architecture question (mechanical pump / filter)

Post by Racing68 » Mon Jun 29, 2020 5:40 pm

I always put filters on the pressure side of the pump. Had a customer put one on the suction side and promptly burned a hole threw the side of the head on a blown alcohol sbc once. Changed location and no problems since.

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Re: Fuel system architecture question (mechanical pump / filter)

Post by rebelrouser » Mon Jun 29, 2020 10:04 pm

I use a NAPA 4003 fuel filter cartridge. It's billet aluminum, has nice big pipe fittings so their is no restrictions when installing AN fittings, and it is rated at 12 gallons per minute of flow, don't remember the micron rating. Replacement filters are $13.00 and can be bought at any parts store. I install mine at the fuel tank before the electric pump. I am a big believer of doing the simple test of timing how long it takes your fuel system to fill a gallon container. Typically, at wide open throttle, full power, an engine requires 0.5 lbs. of fuel per horsepower every hour. A gallon of gasoline weighs approximately 6 lbs. Therefore an engine rated at 350 horsepower will require about 175 pounds
(29 gallons) of fuel every hour. (350HP x .5 lbs = 175 lbs of fuel
So using a stop watch to see how long it takes, and do the simple math. Don't know how many race cars I have done this test on, and they flunked. Many times the issues were the restrictions when installing fittings, the internal orifices of the fittings are not the same as the fuel lines.

My son is a mechanical engineer, and he worked in hydraulics several years ago. His company had some software for calculating flow and pressure in hydraulic systems. So just to do it, we plugged all the dimensions of my fuel system into the program, and used the specific gravity of the fuel I was running and he did some calculations. Seems the needles and seats of my two AFB carbs were the main restriction of the system. I converted my AFB's to dual fuel line intakes and went with .120 needles and seats, and while it did not really make the car go faster, it made it really consistent, and less prone to changes in performance as the weather changed.

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Re: Fuel system architecture question (mechanical pump / filter)

Post by rp930 » Tue Jun 30, 2020 9:02 am

I think 3 micron is way too restrictive on the suction side.

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Re: Fuel system architecture question (mechanical pump / filter)

Post by Ohio Rob » Tue Jun 30, 2020 9:26 am

rp930 wrote:
Tue Jun 30, 2020 9:02 am
I think 3 micron is way too restrictive on the suction side.
That’s the same filter they use on all the high end Merc racing boats and several desert race trucks (Baja 1000) according to the FST rep. The filter will flow over 300 gallons an hour due to the large surface area.

This is according to FST.

I thought the same thing and that was one of my first questions to him.

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Re: Fuel system architecture question (mechanical pump / filter)

Post by dannobee » Tue Jun 30, 2020 10:32 am

You could always tee a vacuum gauge on the inlet side of the pump to see if there's a restriction in either the plumbing or the filter element. The pump itself should be able to pull at least 15" Hg vacuum. When running, any vacuum means a restriction.

If it runs good with a full tank, what changes? Is there baffling in the tank and when it gets low does the fuel slosh around everywhere? And where do you have the fuel return line plumbed?

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Re: Fuel system architecture question (mechanical pump / filter)

Post by Ohio Rob » Tue Jun 30, 2020 10:41 am

dannobee wrote:
Tue Jun 30, 2020 10:32 am
You could always tee a vacuum gauge on the inlet side of the pump to see if there's a restriction in either the plumbing or the filter element. The pump itself should be able to pull at least 15" Hg vacuum. When running, any vacuum means a restriction.

If it runs good with a full tank, what changes? Is there baffling in the tank and when it gets low does the fuel slosh around everywhere? And where do you have the fuel return line plumbed?
The return line “T’s” into the fill line (1.75” fill hose) roughly 4 feet above where the fuel fill hose enters the tank.

The tank is a Florida marine tank with a factory pick up tube. I’m assuming there’s baffling as the tank is a marine unit. I’ll call them an ask.

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Re: Fuel system architecture question (mechanical pump / filter)

Post by rp930 » Tue Jun 30, 2020 10:52 am

Ohio Rob wrote:
Tue Jun 30, 2020 9:26 am
rp930 wrote:
Tue Jun 30, 2020 9:02 am
I think 3 micron is way too restrictive on the suction side.
That’s the same filter they use on all the high end Merc racing boats and several desert race trucks (Baja 1000) according to the FST rep. The filter will flow over 300 gallons an hour due to the large surface area.

This is according to FST.

I thought the same thing and that was one of my first questions to him.
On the suction side? I believe it is meant to be installed on the pressure side.

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Re: Fuel system architecture question (mechanical pump / filter)

Post by Ohio Rob » Tue Jun 30, 2020 11:00 am

rp930 wrote:
Tue Jun 30, 2020 10:52 am
Ohio Rob wrote:
Tue Jun 30, 2020 9:26 am
rp930 wrote:
Tue Jun 30, 2020 9:02 am
I think 3 micron is way too restrictive on the suction side.
That’s the same filter they use on all the high end Merc racing boats and several desert race trucks (Baja 1000) according to the FST rep. The filter will flow over 300 gallons an hour due to the large surface area.

This is according to FST.

I thought the same thing and that was one of my first questions to him.
On the suction side? I believe it is meant to be installed on the pressure side.
According to the rep at FST, it really doesn’t matter. He says the differential pressure across the filter is minuscule (he gave me the number, but it escapes me)

My main question to the filter company was if I should have it on the pressure side of the pump.

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Re: Fuel system architecture question (mechanical pump / filter)

Post by rp930 » Tue Jun 30, 2020 11:13 am


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Re: Fuel system architecture question (mechanical pump / filter)

Post by Ohio Rob » Tue Jun 30, 2020 11:43 am

Thanks RP930.
I have the stuff to move the filter to the pressure side of the pump. That’s what I plan on doing even though the filter rep says it won’t matter.

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Re: Fuel system architecture question (mechanical pump / filter)

Post by rp930 » Tue Jun 30, 2020 11:45 am

Ohio Rob wrote:
Tue Jun 30, 2020 11:43 am
Thanks RP930.
I have the stuff to move the filter to the pressure side of the pump. That’s what I plan on doing even though the filter rep says it won’t matter.
I think you will be better off. Good luck. Love a fast boat.

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Re: Fuel system architecture question (mechanical pump / filter)

Post by dannobee » Tue Jun 30, 2020 11:59 am

We ran fuel filters on the suction side with mechanical pumps for decades in all of nascar. And big rigs with mechanical fuel pumps have what, a 4 ft vertical pull from the bottom of the tanks to the engine mounted fuel filters? And with 500 sq in of filtering media, I doubt it's the problem.

But in case the filter is plugged, a vacuum test will reveal it.

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Re: Fuel system architecture question (mechanical pump / filter)

Post by MadBill » Tue Jun 30, 2020 10:05 pm

Ohio,
If your pick up is routed through the top of the tank, a leak at say half tank height would start sucking air as the fuel level dropped.

Long shot possibility: if the problem develops only after the tank empties from a long run it could be a blocked tank vent.

2x re 3 microns on the inlet side.
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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