How important is 1psi of fuel pressure?

[Syleng1]

I been trying to fine tune a Holley Street warrior carb for my engine. Something caught my eye where Holley asks for 5-7psi in fuel pressure.
I popped out my fuel filter (between the pump and the carb) to put in an inline gauge to see the pressure. It’s at 8psi.
Rev it up it’s 8psi, idle 8psi under load 8psi.No issues with flooding. Just wondering if going thru the hassle with a regulator is worth the 1-2psi drop to meet the Holley spec?

 

[eekvonzipper]

I'm pretty sure that's a recommendation to not Overpower your Needle-Valve and nothing more...
My little gauge bounced between 7-9psi with a Musclecar Series Pump So, I put in a Reg for 6-7psi.

 

[jimjimjimmy]

yes 8psi is way to much for a holley i find 5 - 5 1/2 best

 

[TT5.9mag]

If it’s not overpowering the needle and seat it’s fine, as long as you know that fuel pressure has a tuning effect. And if you get everything set up perfectly and then change the pressure, the tune up in the carb will likely have to change.

 

[GTX JOHN]

I run 7 to 8 PSI in my Holleys and as long as Needles and seats are good
I never had an issue. I run .110 needles and seats and the do not overflow.

 

[4mulas]

If it wasn’t important Holley would say5-8psi is fine….. but they don’t

 

[RustyRatRod]

I've seen Holleys stand 10PSI. They can usually stand more fuel pressure than say a Carter or Edelbrock.

 

[Bewy]

If you are going to run 8 psi, you should probably lower the float level slightly [ 0.030" ]. Raising fuel pressure increases the fuel level.

 

[Syleng1]

If you are going to run 8 psi, you should probably lower the float level slightly [ 0.030" ]. Raising fuel pressure increases the fuel level.

So exactly how does fuel pressure control float level? Especially on a running engine?

 

[TrailBeast]

So exactly how does fuel pressure control float level? Especially on a running engine?

Higher pressure will cause the needle to close ever so slightly later.

But not anywhere near enough to make any difference in the tune.

 

[Mattax]

You'll know its a problem when you lift off the throttle cruising on the interstate.
For racing in particular there are advantages to running lower pressures during flow (your measurement is the no - flow pressure).
Lower pressure can mean more flow into the bowl and less aeration. This means more consistant fuel level and delivery.

 

[Mattax]

Lower pressure can mean more flow into the bowl and less aeration. This means more consistant fuel level and delivery.

Flow test result

 

[toolmanmike]

You may not have a problem with 8# but why push your luck?

 

[Vertex_Jeff]

What is the psi when the car has been running and is at operating temp? Any change? Reason I ask is that I have my Aeromotive regulator set at 6psi, and the gauge will read that at start up. But, when I get the car up to temp after driving a while, it will get down to around 4.5 psi. I'm not really sure why this happens, I have an in-tank Walboro fuel pump that is rated for 380/400lpm, and seems to operate fine. My regulator is a return- style, and I have the lines sized per instructions.

I have wondered if there is some physics or thermodynamics reason for fuel heating up and interacting with the regulator diaphragm differently than when cool? I might adjust my regulator when the system is hot and see what that does.

Maybe something for you to experiment with as well.

 

[toolmanmike]

What is the psi when the car has been running and is at operating temp? Any change? Reason I ask is that I have my Aeromotive regulator set at 6psi, and the gauge will read that at start up. But, when I get the car up to temp after driving a while, it will get down to around 4.5 psi. I'm not really sure why this happens, I have an in-tank Walboro fuel pump that is rated for 380/400lpm, and seems to operate fine. My regulator is a return- style, and I have the lines sized per instructions.

I have wondered if there is some physics or thermodynamics reason for fuel heating up and interacting with the regulator diaphragm differently than when cool? I might adjust my regulator when the system is hot and see what that does.

Maybe something for you to experiment with as well.

That's very interesting. As long as you're not over pressure while running, all is good.

 

[Vertex_Jeff]

That's very interesting. As long as you're not over pressure while running, all is good.

Yeah, I have not noticed any fuel starvation at WOT with this condition, so I kinda just shake my head when I look at the fuel gauge.

 

[Syleng1]

You may not have a problem with 8# but why push your luck?

Because I am trying to keep my engine bay as period correct as possible. Adding a regulator that does not look correct for a late 60’s street vibe does not fit how I want my car to look. Obviously Mike if it was 10psi… I would most definitely need to add one. I’m trying to get a general consensus if it is worth the energy if I am having no troubles with needle and seat leakage or Fuel troubles in general. At 1psi- it could even be a gauge out of calibration. I’m going to try a new gauge and see where it sits.

 

[toolmanmike]

Because I am trying to keep my engine bay as period correct as possible. Adding a regulator that does not look correct for a late 60’s street vibe does not fit how I want my car to look. Obviously Mike if it was 10psi… I would most definitely need to add one. I’m trying to get a general consensus if it is worth the energy if I am having no troubles with needle and seat leakage or Fuel troubles in general. At 1psi- it could even be a gauge out of calibration. I’m going to try a new gauge and see where it sits.

It seems that you answered your own question. Period correct look wins it.

 

[Syleng1]

What is the psi when the car has been running and is at operating temp? Any change? Reason I ask is that I have my Aeromotive regulator set at 6psi, and the gauge will read that at start up. But, when I get the car up to temp after driving a while, it will get down to around 4.5 psi. I'm not really sure why this happens, I have an in-tank Walboro fuel pump that is rated for 380/400lpm, and seems to operate fine. My regulator is a return- style, and I have the lines sized per instructions.

I have wondered if there is some physics or thermodynamics reason for fuel heating up and interacting with the regulator diaphragm differently than when cool? I might adjust my regulator when the system is hot and see what that does.

Maybe something for you to experiment with as well.

Nope- just 8# every check. Idle, cold hot. Warm start, under load (umbilical hose up to windshield. Cruise- hard brake. #8psi rock steady.
Also- Carter competive fuel pump and 3/8” lines front to back. Even fuel filter is 3/8 barrel.

 

[Vertex_Jeff]

Nope- just 8# every check. Idle, cold hot. Warm start, under load (umbilical hose up to windshield. Cruise- hard brake. #8psi rock steady.
Also- Carter competive fuel pump and 3/8” lines front to back. Even fuel filter is 3/8 barrel.

Since you are trying to keep a stock look, I would probably let it ride if it is running good.

 

[j par]

It's only one pound to much if it's one pound to much...

 

[Mattax]

Yeah, I have not noticed any fuel starvation at WOT with this condition, so I kinda just shake my head when I look at the fuel gauge

Scroll back through all of the pump-pressure-flow posts in the thread I linked above. Its the same type of setup you have.
Duane (PRH) started this off a couple pages earlier, I diagramed it and showed how flow changes with pressure at the outlet (including a Walbro pump chart ). Cudafever did the real work, setting up the experiment and testing it.

 

[Mattax]

In my experience the "Competition" pumps had higher cutoff pressures than the 'street pumps'.
By that I mean the pressure the pump will not go above even when output flow is blocked.

Mechanical Fuel Delivery for Mopar LA Engines

I have had issues with both too much pressure (pump) and too little pressure (pinhole and/or too close to exhaust in feed line by axle).
Too much pressure will cause flooding with closed inlet valve. Too little shows up as running low of fuel in the bowl approaching the stripe. (pressure gage goes to zero shortly after launch).

Good explanation of the mechanical pump operation here (bonus link to rebuilding a Carter pump for Studebaker supercharger applications

)

 

[Newbomb Turk]

1 psi won’t make a huge difference but dropping it 3 ponds down to 5 where it should will.

I get you want period correct and don’t want a regulator (I wouldn’t do a dead head regulator so I’d say a return regulator but that’s even more expensive and more work) then leave it at 8 psi and tune for it.

 

[Bewy]

Below is a chart that Carter provided for fuel pressure v needle & seat size. It also works for Edel AFB/AVS cabs because Edel copied the Carter fuel delivery system without making any changes.