Fuel pressure drops down track with elec. pump.......

Posted By: Dart1031

Fuel pressure drops down track with elec. pump....... - 06/22/10 02:02 AM

I need some opinions on this one......

I noticed the other day that my fuel pressure gauge was down to 3lbs. of pressure from the 1/8 mile on. It's a Mallory pump, not sure which model cause of the way it's mounted in the car I can't see a number on it. It was all ready in the car when I got it. This is a "back up" car and I'm not pulling it apart unless I have to, lol.

The only thing I can think of that's causing this is because the gauge is tapped into the fuel line at the carb, not the regulator. At idle, and up to the 1/8, it's at 7lbs. Does this make sense, or could it be another problem?

Also, the car runs fine and pulls the whole way down the track. It doesn't act like it has low pressure, but the gauge is telling me it does.

Thanks in advance for any help!

Posted By: MoparforLife

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 02:18 AM

What size line. Remember that up to the 1/8 the engine is running freer and not pulling as hard as farther down track so will us less fuel and the pressure & supply will keep up easier that down track where the engine is working harder and using more fuel.

Posted By: BobR

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 02:34 AM

Many causes of this and not nearly enough information to make an educated guess.

Posted By: Bob_Coomer

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 02:52 AM

Quote:

What size line. Remember that up to the 1/8 the engine is running freer and not pulling as hard as farther down track so will us less fuel and the pressure & supply will keep up easier that down track where the engine is working harder and using more fuel.

Hmm
that is a interesting way of looking at it....Im not disagreeing with you, but one would think the car is accelerating less after the 8th mile point, usually gaining only 20-30 mph through the 2nd half of the track, meaning that the consumption of fuel after this point is really not increasing much.....
Next time your at the track, raise the float level about 1/2-3/4 turns out on the bowls, and increase fuel pressure to 8-8 1/2 lbs..see if the car responds with any increase in MPH....If it does, there might be a fuel starvation problem.

Posted By: Dart1031

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 02:53 AM

Sorry, long day..... It's a 3/8" fuel line from the tank to the carb. Mallory pump, Fram canister filter(new element), stock tank, 3/8" pick up, Holley reg., Holley 750, shift at 6,500. Anything else you need to know?

Posted By: Dart1031

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 02:59 AM

Quote:

Next time your at the track, raise the float level about 1/2-3/4 turns out on the bowls, and increase fuel pressure to 8-8 1/2 lbs..see if the car responds with any increase in MPH....If it does, there might be a fuel starvation problem.

Thanks Bob, I'll give that a shot.

Posted By: Ari440

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 03:02 AM

Quote:

Sorry, long day..... It's a 3/8" fuel line from the tank to the carb. Mallory pump, Fram canister filter(new element), stock tank, 3/8" pick up, Holley reg., Holley 750, shift at 6,500. Anything else you need to know?

1/2 fuel line and get rid of the fram fuel filter for a inline screen type fuel filter

Posted By: Cab_Burge

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 06:01 AM

Fuel consumption is much greater in the last half of the 1/4 mile than the first 1/8 mile do to rpm acceleration per second, the first 1/8 you go through two or three gears, the last 1/8 is usually done in high gear only. The Fram canister can be a limiting factor on the supply side, check it to see if it is plug up full of trash or debris

A friend of mine ended up substanially increasing his fuel volume by adding another Fram in parelel from the tank to the pump by adding a Y to the inlet side of the pump from the two filters

If you can swing it use a better in line 40 micron or larger in line race fuel filter instead of one Fram or two Frams

Posted By: Dart1031

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 11:10 AM

Thanks for the idea's guys

Not sure about the 1/2" fuel line right now. I should only be racing this thing for another month till my Valiants done.

Posted By: MR_P_BODY

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 11:24 AM

Basically you could have a couple of issues.... If
the pressure held good before then its a pump MAYBE
going bad(doubt it) or the suction side cant supply
the pump with enough fuel

Posted By: BobR

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 01:16 PM

Is the stock sock in the tank? What model Mallory pump?

Posted By: Thumperdart

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 03:08 PM

Quote:

Fuel consumption is much greater in the last half of the 1/4 mile than the first 1/8 mile do to rpm acceleration per second, the first 1/8 you go through two or three gears, the last 1/8 is usually done in high gear only. The Fram canister can be a limiting factor on the supply side, check it to see if it is plug up full of trash or debris A friend of mine ended up substanially increasing his fuel volume by adding another Fram in parelel from the tank to the pump by adding a Y to the inlet side of the pump from the two filters If you can swing it use a better in line 40 micron or larger in line race fuel filter instead of one Fram or two Frams

That be me and I picked up way more flow w/the two filters but I`d toss those and get something better and less restrictive. Yesturday I changed out my cannister filter and installed an Earls in line screen filter and my gauge read 1/2 lb. more pressure when I fired it up.

Posted By: MoparforLife

Re: Fuel pressure drops down track with elec. pump....... - 06/22/10 04:13 PM

Quote:

Quote:

Fuel consumption is much greater in the last half of the 1/4 mile than the first 1/8 mile do to rpm acceleration per second, the first 1/8 you go through two or three gears, the last 1/8 is usually done in high gear only. The Fram canister can be a limiting factor on the supply side, check it to see if it is plug up full of trash or debris A friend of mine ended up substanially increasing his fuel volume by adding another Fram in parelel from the tank to the pump by adding a Y to the inlet side of the pump from the two filters If you can swing it use a better in line 40 micron or larger in line race fuel filter instead of one Fram or two Frams

That be me and I picked up way more flow w/the two filters but I`d toss those and get something better and less restrictive. Yesturday I changed out my cannister filter and installed an Earls in line screen filter and my gauge read 1/2 lb. more pressure when I fired it up.

Less restrictive will generally mean a more open filter with less filtering. Also remember more pressure doesn't always translate into more volume. In a carburated engine that doesn't require pressure to operate like injectors I prefer larger line size and more volume over higher pressure.

Posted By: Dart1031

Re: Fuel pressure drops down track with elec. pump....... - 06/23/10 12:46 AM

Thanks again for the replies

Mr. P..... This car is fairly new to me, and I just noticed this issue last weekend. It may have happened before and I never noticed it.

BobR.... No idea if the sock is in the tank. I bought the car with this fuel system. I saw the car run 11.70's before I bought it and I "assumed" everything worked OK. The way the pump is mounted I can't read the model number. I'll try and remove it this weekend to verify.

drjay0...... That's how the pump is mounted. I know for a fact the car had 5 gallons of fuel that day. I don't think the pickup would become uncovered with that much fuel, but maybe it did.

Posted By: MR_P_BODY

Re: Fuel pressure drops down track with elec. pump....... - 06/23/10 01:42 AM

I think its time to pull the fuel system to check out
what you have and to correct where its not up to par

Posted By: blownzoom440

Re: Fuel pressure drops down track with elec. pump....... - 06/23/10 01:57 AM

Quote:

I think its time to pull the fuel system to check out
what you have and to correct where its not up to par

maybe even flow test it with a ball valve into a jug.

Posted By: Chassisman

Re: Fuel pressure drops down track with elec. pump....... - 06/23/10 05:03 AM

I know you have a stock tank.....my dads car this past weekend was shutting off down track.....around 1000'...he made 3 passes scratchin his head...tore the whole thing down sat night in the pits....nothing....then I said....you still got foam in this cell?...sure enough the foam had started coming apart and had plugged the outlet just enough that under full power it would run out of gas....
Took out the foam...good as new.....

Maybe there is a wayward piece of something in the tank?.....never know!

Posted By: Dart1031

Re: Fuel pressure drops down track with elec. pump....... - 06/24/10 02:33 AM

All very good points! Unfortunatly I can't look into it until this weekend. Stinkin job gets in the way of my fun, lol.

Posted By: HerboldRacing

Re: Fuel pressure drops down track with elec. pump....... - 06/24/10 04:04 AM

Replace the Fram filter with a Wix.

But really... why is 3 psi a problem? It is when it gets to 0 psi that your motor is consuming fuel faster than your pump can feed it. As long as it is above 0 you have fuel in your bowls. Is the engine missing at the top end?

Posted By: Crizila

Re: Fuel pressure drops down track with elec. pump....... - 06/24/10 01:05 PM

Quote:

Replace the Fram filter with a Wix.

But really... why is 3 psi a problem? It is when it gets to 0 psi that your motor is consuming fuel faster than your pump can feed it. As long as it is above 0 you have fuel in your bowls. Is the engine missing at the top end?

Obviously, you wood like to see the pressure be the same throughout your 1/4 mile trip, but as long as the float bowls remain full, it won't make any difference if you have 4 lbs less in the lights. It does show that your existing system is "boarder line" though.

Posted By: BSB67

Re: Fuel pressure drops down track with elec. pump....... - 06/25/10 01:11 AM

Quote:

Quote:

Replace the Fram filter with a Wix.

But really... why is 3 psi a problem? It is when it gets to 0 psi that your motor is consuming fuel faster than your pump can feed it. As long as it is above 0 you have fuel in your bowls. Is the engine missing at the top end?

Obviously, you wood like to see the pressure be the same throughout your 1/4 mile trip, but as long as the float bowls remain full, it won't make any difference if you have 4 lbs less in the lights. It does show that your existing system is "boarder line" though.

I think that it depends on the rest of the fuel delivery system. My car was way down on power at 3-3.5 psi. I looked at a bunch of other things because I too thought that 3 psi was fine. It was not.

Posted By: Crizila

Re: Fuel pressure drops down track with elec. pump....... - 06/25/10 01:34 AM

Quote:

Quote:

Quote:

Replace the Fram filter with a Wix.

But really... why is 3 psi a problem? It is when it gets to 0 psi that your motor is consuming fuel faster than your pump can feed it. As long as it is above 0 you have fuel in your bowls. Is the engine missing at the top end?

Obviously, you wood like to see the pressure be the same throughout your 1/4 mile trip, but as long as the float bowls remain full, it won't make any difference if you have 4 lbs less in the lights. It does show that your existing system is "boarder line" though.

I think that it depends on the rest of the fuel delivery system. My car was way down on power at 3-3.5 psi. I looked at a bunch of other things because I too thought that 3 psi was fine. It was not.

Hmmmm. Wonder why? Maybe 3psi wasn't enough to keep the bowl fuel levels where they should be?

Posted By: Dart1031

Re: Fuel pressure drops down track with elec. pump....... - 06/25/10 02:14 AM

The element in the filter is a Wix. I replaced it when I got the car. There was no "trash" in the canister from the tank either.

I had a similar set up in another car, stock tank, 3/8" sending unit and fuel line, same regulator, and same carb. Only difference was it had a Holley blue pump. It always had 6.5-7lbs going down the track. I'll flow test the Mallory this weekend.