Fuel Pressure

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rdb
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Fuel Pressure

Post by rdb »

I have a sbc 383 with mechanical fuel pump. The fuel pressure at idle bounces between 6-8PSI, when wide open it goes down a little below 2PSI. Is this normal? Or do I need to look for a problem?
Forgot to mention Holley 650 Double Pumper.

josh_2562
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Re: Fuel Pressure

Post by josh_2562 »

What size fuel line? How much power is it making? how long are you holding it wot before it drops?

rdb
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Re: Fuel Pressure

Post by rdb »

3/8" fuel line, got 350 HP forget what the torque was but it was up there. Once I get past 3200RPM the fuel pressure starts to fall from 6PSI to 4PSI and around 4000rpm wide open on the trailer it drops to 2PSI.
Yesterday I bypassed the electric pump, put new filter on, checked pick up tube, checked vent. Everything inspected between tank and carburetor. Only thing that was not replaced or bypassed was the mechanical fuel pump. Thinking about replacing fuel pump and seeing if the pressure changes. If the pressure does not change after that I guess this is normal for the fuel pressure to drop off at wide open throttle.

I googled fuel pressure for Holley carb and it said you should have 5PSI at wide open throttle. Does any one know if this is correct? Or what the fuel pressure should be at wide open throttle?

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Re: Fuel Pressure

Post by josh_2562 »

I'm no pro but check your float level. What kind of fuel pump you running?

josh_2562
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Re: Fuel Pressure

Post by josh_2562 »

Fuel pressure shouldn't drop that much at wot. Your engine will run lean and mess stuff up pretty quick

rdb
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Re: Fuel Pressure

Post by rdb »

I adjusted the float level using the Holley instructions. Running a stock fuel pump from O'Reilly for a 350.

rdb
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Re: Fuel Pressure

Post by rdb »

Sent email to Holley and this is what they said:

That is normal. When at WOT its using fuel so quickly, that it doesn’t build up a lot of pressure. If the car still runs well at WOT, then I’d say that you don’t have anything to worry about.

SWAMPHUNTER45
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Re: Fuel Pressure

Post by SWAMPHUNTER45 »

Not normal with my engine.

Sounds like the pump is inadequate, could be an issue mounting or feeding it. We mount inlet portion at or below bottom of tank.
Do you know the part number of the pump? If it is that little mini pump it is not large enough to feed a V8.

My Mallory Comp140 is set at 6.25 pounds and at WFO with a 800cfm carb feeding 542 cubic inches and a nitrous plate drops to 5psi.

One Eyed Gator
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Re: Fuel Pressure

Post by One Eyed Gator »

What pump you using as the lift pump? I have seen the cheaper universal pump with 1/4" fitting cause a similar problem.

rdb
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Re: Fuel Pressure

Post by rdb »

The lift pump is a 3/8 fitting precision pump. I bypassed the lift pump and went straight to the mechanical pump and had the same results.

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glades cat
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Re: Fuel Pressure

Post by glades cat »

Equally important is the flow rate of your pump. A 350 hp engine needs 175 lb/hr of gasoline. That's about 3 GPM. If your pump doesn't supply it, ( most stock pumps are about 115 gph ) I would look at getting a high volume pump with 3/8 fittings, electric or mechanical, that meets your flow requirement that does not exceed 7 psi…or regulate it down…with a quality pressure regulator. Fuel delivery is about volume and uses pressure to overcome restrictions and friction loss.
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rdb
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Re: Fuel Pressure

Post by rdb »

So I ended up buying a Holley fuel pump to get more gph to fix the fuel pressure problem and received it Tuesday night. I installed it yesterday with the hose fitting snug and of course they leaked. Long story short I tightened the fittings 3 or 4 times trying to get them to stop dripping and the housing cracks at the out fitting. I was wth I didn't have much pressure on the fitting when I was tightening it. I called Holley today and waited on hold for 30 minutes before I finally hung up and just emailed them what happened. Hopefully they will replace it.

Anyway was wondering what type of mechanical fuel pumps you guys use.


I prefer mechanical because I ride in saltwater and worry that electrical pump would be a lot more likely to fail due electrical wiring and salt corrosion.

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90chevy396
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Re: Fuel Pressure

Post by 90chevy396 »

I use the stock fuel pump on a 455 buick turning 3100 wot and never drop below 5 psi with 3/8 s/s line run throughout with just rubber at the connections between components. watch your suction line when at wot and see if it is vapor locking when at wot. could be the issue. when your pump is trying to pull more fuel than the line can flow or the rubber hose is soft it will collapse when demanding large volumes of fuel. I suggest running stainless steel lines on the suction side of the pump on all applications to prevent this issue.
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90chevy396
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Re: Fuel Pressure

Post by 90chevy396 »

especially when at wot the suction side of the pump is creating exactly that vacuum!! put hard line on the suction side always. had a buddy just the other week that when he stood on it running dry for more than 2 minutes the engine would fall on its face due to vapor lock in the suction side of his fuel system and he only had 3 foot of rubber line installed and since installing s/s he hasn't had the issue since
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SWAMPHUNTER45
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Re: Fuel Pressure

Post by SWAMPHUNTER45 »

Use ARP thread sealant it is a Teflon paste, you can use red RTV but it is an "unapproved" use. It only takes a little on the threads, then let it dry 24 hours to keep a leak from happening.

You have to be sure you don't get it where it can be ingested into the line.

The old plumbers Teflon tape always caused me grief but since using red RTV no issues.

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Re: Fuel Pressure

Post by SWAMPHUNTER45 »

The Mallory Comp110 and metal lines are the way to go!

We just use 3/8 brake lines, Blu bends um up and we splice with rubber fuel line and double clamp, sort of sounds similar to what 396 is doing with s/s.

Cheap and works great

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