Question on loosing fuel pressure going down track.
 

With a quality gauge mounted in front of me on windshield I can watch fuel pressure going down track. What is happening is the pressure is going from 6 lbs to 3.5 lbs. at finish line. I do NOT have a return. I used a good quality pump (dont remember what name brand is in this car) with -8 up to regulator and -8 from reg to carb. My thought is I need a jetted -4 return line at regulator so I can adjust regulator and maintain 6 lbs at finish line. I have plenty of -4 line and would need to only buy fitting that holds jet and my time to run it in car.
Would this help the problem or 3.5 lbs. is enough for a slow car?

Re: Question on loosing fuel pressure going down track.
 

Main thing is to keep the bowl full enough so the jets are covered. What car/carb?

Re: Question on loosing fuel pressure going down track.
 

You may be having a voltage drop issue. The bigger the feed wire, the better, and closer to the battery, The better you will be. Double check your ground connections, to the pump and relay, too. 10 gauge to pump, at minimum, and 8 would be even better.
If you have a good meter, try to read the voltage, thru the relay, it might show you a problem there.

Re: Question on loosing fuel pressure going down track.
 

Either this or your old pump is just worn out! Jim


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Re: Question on loosing fuel pressure going down track.
 

From Areomotive

At the track, from the launch to mid-track, fuel pressure is steady, but then down track, it falls 2 PSI or more, what’s wrong?

The first thing to be blamed is usually the regulator, but in fact this is a strong indication of a fuel supply problem. If for some reason there is insufficient volume available from the fuel pump to feed the engine, fuel line pressure can drop to the carburetor. Even with a static regulator, a significant drop in line pressure affects flow through the regulator, causing regulator pressure to drop. Do not automatically assume the pump is bad or inadequate, inspect and resolve any supply line issues to the pump, ensure the tank is vented and the vent is functioning, and be sure to check the fuel pump wiring, along with the overall electrical system performance. Finally, if you still have problems, contact Aeromotive for a proper flow test you can perform in the field to verify if your fuel pump is performing properly.

Re: Question on loosing fuel pressure going down track.
 

Or maybe you don’t have a problem, Before all this high pressure,high volume systems
I had a max wedge , Carter electric and manual pump had 6 pounds thru the lights at 2 pounds went 127.5 mph that’s 30 years ago, is your car nosing over, ?

Re: Question on loosing fuel pressure going down track.
 

Olds 307 large q jet

Re: Question on loosing fuel pressure going down track.
 

With it being a fairly new combo to me. My opinion is that it is not nosing over as the mph matches the et. Side note is the o2is going from 13.5 to 11 in high gear.

Re: Question on loosing fuel pressure going down track.
 

What brand of pump do you have? A q-jet is notorious for being hard to keep supplied with fuel. When I built my car it had a 780 Holley and I was using a Mallory 140 pump and a 3/8 fuel line from the tank to the regulator and everything worked great. When I switched to a q-jet combination I ended up switching to a Barry Grant 280 pump and a 1/2 inch line from the tank to the regulator to get it to run right. The amount of fuel to a q-jet is critical.

Re: Question on loosing fuel pressure going down track.
 

Take the line off at the carb, run the fuel pump for a full minute and measure the volume it pumps. Do this with the gas cap on and off to see if there's a difference. -Al

Re: Question on loosing fuel pressure going down track.
 

I had the exact same problem become present on my 305, Q Jet Camaro, it was a faulty pressure regulator at the carburetor.

Re: Question on loosing fuel pressure going down track.
 

A friend had this same problem this year and found it to be a very restricted fitting from tank to pump. This was after he changed pump and regulator. It was a 90 degree fitting.

Re: Question on loosing fuel pressure going down track.
 

Is this a stock fuel tank? Not a fuel Cel?
To eliminate a weak (low volume) pump, I would add 5 gallons of fuel. If that stops the pressure drop, it will usually indicate the pump pick up being uncovered due to placement in the tank.

Re: Question on loosing fuel pressure going down track.
 

It is a fuel cell with about 10 gallons in it mostly to make weight. Has rear sump and exit with pump directly behind it. Im pretty sure I 45 degree or straight fitted everything. I did install a -8 inline fuel filter when I built car 35 passes ago.

Re: Question on loosing fuel pressure going down track.
 

It's the G force causing the problem.
;-)

Re: Question on loosing fuel pressure going down track.
 

How quick does it ET? And 60’?

Re: Question on loosing fuel pressure going down track.
 

What side of the pump is the filter. I have never ran a filter in my fuel system

Re: Question on loosing fuel pressure going down track.
 

If you are using a fuel filter, check the Microns.

Re: Question on loosing fuel pressure going down track.
 

1.88 60ft 14.50 ET.

Re: Question on loosing fuel pressure going down track.
 

I have the filter mounted near the passenger door. Pump is directly behind cell

Re: Question on loosing fuel pressure going down track.
 

It is a Summit -8 inline 40 micron filter

Re: Question on loosing fuel pressure going down track.
 

Won’t be G forces.

Re: Question on loosing fuel pressure going down track.
 

Have you checked the pressure before the regulator? Most pumps put out about 15 PSI.
You may have a low pressure pump not designed to use with a regulator or the pump is no good.

Jim Mantle V/SA 6632

Re: Question on loosing fuel pressure going down track.
 

Trust the sensor, there isn't a problem. Its 11:1 in high gear. My cars AFR gets leaner as RPM increases an gets richer at each shift. (completely different carb)

Re: Question on loosing fuel pressure going down track.
 

Jeff did you get a chance to check the volume in a fuel jug for 60 seconds at the front of the car and what volume did you get?

Re: Question on loosing fuel pressure going down track.
 

I just did now im right at a gallon in 60 seconds. That was tested using line right at carburetor

Re: Question on loosing fuel pressure going down track.
 

On a free flow test it should be a gal.in 30 seconds especially with a Q-jet...gsa612

Re: Question on loosing fuel pressure going down track.
 

Should pump a gallon in 30 seconds. I think you just found your problem.

Re: Question on loosing fuel pressure going down track.
 

Its weird that the O2 sensor goes rich , sounds like some work needs to be done to straighten out the fueling of the carb . That being said the carb is using more fuel than can be supplied to it , so the pressure drops . My recommendation is to figure out what's up with the carburetor , but also make sure your fuel system is up to the task . My Q-jet is fed from a BG400 pump through a dash 10 line , and stays right at 6.5 psi throughout the run . Dyno pulls show the A/F ratio varies from 12.8 at 5000rpm to 12.4 at 7400 rpm .

Re: Question on loosing fuel pressure going down track.
 

Agree..I've had cars under 25 sec.

BTW, I was just giving Jeff a hard time. Sorry some of you missed this whole sordid tale, for 5 years ...Check out the build section once in a while ;-)

Re: Question on loosing fuel pressure going down track.
 

Jeff, you don't have foam in the fuel cell do you? Bob Sherwood mentioned the foam yesterday when we were talking about this thread.

Assuming no foam in the tank, start working it back. Take the filter element out and recheck it at the carb. If no change, check it before the regulator. If that's still the same, check it right out of the pump. Make sure the fuel cell vent is open.

Let us know what you find. -Al

Re: Question on loosing fuel pressure going down track.
 

How old are your fuel lines?

Re: Question on loosing fuel pressure going down track.
 

Like other have said a gallon in 60 seconds is not enough volume. At your ET you would be getting less than 1 Qt. of fuel to the carb during your runs. a gallon in 30 seconds is right on the edge for my car.

Re: Question on loosing fuel pressure going down track.
 

No foam in cell. I'm going to bypass filter and retest filling time in bucket. I did notice once after shutting off car the cell acted like it was under a vacuum when I tried taking cap off. I will look at vented line to see if it collapsing or kinked. All lines,fittings,filter,pump,cell were all brand new.

Re: Question on loosing fuel pressure going down track.
 

This seems to be what I'm struggling with. I make small changes to jet,rod,hanger and have not found sweet spot. In the beginning I was way over rich and way over revving on gear change and car went faster. Would think car would pick up allot if afr was correct in 3rd gear

Re: Question on loosing fuel pressure going down track.
 

Did you do the volume test with the engine running? It'll pump more, of course, when the voltage regulator is holding it at 14.7v or whatever it maxes out at.
As mentioned, are your grounds, connections and wire diameter up to snuff?

Re: Question on loosing fuel pressure going down track.
 

Going to verify ground as car was a back east car and frame had some rust. Maybe move ground to rear battery.

Re: Question on loosing fuel pressure going down track.
 

Everyone is focusing on a fuel pump delivery problem , yet the engine goes rich in high gear . That seems to be the real problem .

Re: Question on loosing fuel pressure going down track.
 

Just my 2 cents.....

A lean miss will do that....

Re: Question on loosing fuel pressure going down track.
 

Doesn't a miss make an o2 sensor read lean ?