pettrix

On a 1972 Olds 442 with a performance 455. Is it possible to pump too much fuel, too quickly, from the gas tank causing a pressure drop in the fuel line?

I am wondering if the OEM gas tank vapor lines are too small to handle the fuel draw from the tank on a performance 455 build. This then causes a negative vacuum inside the gas tank, causing a fuel pressure drop under hard acceleration.

OLDSter Ralph

"Under hard acceleration with an OEM gas tank" was more likely to be fuel sloshing away from the fuel tank pick up when running a very few gallons of gasoline. Some of the newer gas tanks being made use baffles to minimize that now. "G forces" were also exerted on the fuel column being pumped from the tank. Electric fuel pumps are mounted near the gas tank to "push" the "fuel column" forward.
......Just my two cents worth.

VC455

The vapor line has adequate size to keep the tank from being in vacuum.

If you are having a specific fuel delivery problem, tell us about the problem so we can help you sort it out.

pettrix

Even on a performance 455 with an electric fuel pump (mounted outside of tank with a 1/2" line to carb)?

I've been noticing a fuel pressure bounce under hard acceleration (goes to 4-5 psi) and then comes back to 6.5 psi under normal throttle.

My concern is that the vapor line is inadequately sized, causing the pressure drop/vacuum issue.

OLDSter Ralph

What happens if the return line is capped off ?

VC455

Let's say you make 500 hp with 0.500 bsfc. That would require 40.5 gallons per hour from the pump (and from the return line as you intimate).

That volume of fuel is equivalent to 2.6 cubic inches per second. You can blow that much air through a straw without any effort.

It's more likely that the external pump is the problem. External pumps are very sensitive to pressure drop on their inlet side and will drop volume and pressure under load. That's one reason OEMs put the pump inside the tank--there's less inlet line length to degrade performance.

You can try an experiment to check this out. With a low volume in the tank, leave the gas cap loose and check for fuel pressure drop. The return line will be out of the equation.

pettrix

I will let the gas cap "breathe" by leaving it partially open. That should tell me if a vacuum issues is causing the fuel pressure drop.

So you are saying that the factory vapor line going from the gas tank to the front charcoal canister is large enough to handle the fuel level drops in a high performance 455?

Were the gas tank setups the same on 350 vs 455 on a 1972?

VC455

Yes. Your experiment with the loose gas cap will determine if this is indeed correct.

Yes.

VC455

Thinking about this some more, there's another possibility. If you have a fuel pressure regulator and are measuring pressure after the regulator, some regulators droop at high flow rate.

To check this out you would need to measure pressure before the regulator to see if it goes below 6.5 psi under high load.

Sugar Bear

Is the symptom new to the car or it has been there since you have owned it? Did the symptom start after replacing fuel line components or the symptom is why the parts were changed?

Had this problem in a Z28. Did your method of a stand alone independent tank and it ran fine. Problem was in tank but due to a clogged sock vs. a collapsing one.

Not that you want to fill the tank to test this in case you have to drop it; but, sock problems often showed up during low fuel level in the tank.

Good luck! Persist and you will conquer it.
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gs72

4 to 5 psi is plenty to feed the engine. The drop in pressure means there is an increase in flow. 6.5 at ldle is with the needle an seat mostly closed. The drop you are seeing is because the needle is off the seat under hard accel. If it was dropping to near zero then I would be worried. Is the engine losing power when the pressure drop occurs?