Help me get my mind around this...accurately.

Posted By: StrokerPost

Help me get my mind around this...accurately. - 01/24/09 04:25 AM

Ok, this is a fuel pressure question pertaining to WOT only. I'll use my car as the subject. I run a Mallory 140 pump fed by 10AN line from the cell to the pump, then 8AN all the way forward to the reg, then dual 6AN lines, one to each bowl and the press gage is reading press to the rear bowl, just off the reg. I set the press at 7.5 psi. At WOT, in high gear the cowl mounted gage will always be flicking between 6.5 & 7 psi, so obviously, its not maintaining press at WOT. My engine builder is always on me about correcting that to keep the press steady at whatever I have it set at, said I'm probably leaving something on the table ET wise. I've always maintained that that little bit of fluxuation isnt enough to be hurting anything, after all, the car is a deadly consistant, multi winning car (not to be read as bragging) so why waste $$$ to have a steady gage. This "fluxuation" has me puzzled now though, and I'm wondering, in a race environment, at WOT, how big a deal is it to rely on the low press area in the intake to PULL fuel through the jets vs adequate press PUSHING fuel to the jets? Obviously the car will run fine with fluxuating gage, but are you REALLY giving up any REAL performance by NOT having full press at WOT?

I've always kinda thought that the engine will pull what it needs so long as the bowls arent being sucked dry, and with only about 3/4 of a pound loss in press I dont see that happening, but, I'm willing to listen and explore that possibility. I'm not looking for ideas on a new pump or why the press is fluxuating, but is this an issue or not, with some explanation. Thanks for any and all input, I want to put this to rest in my head once and for all.

Ken.

Posted By: prochargedhemi

Re: Help me get my mind around this...accurately. - 01/24/09 04:29 AM

maybe the carb is being sucked dry and the needle and seats aren't big enough to allow enough fuel in to compensate

Posted By: dthemi

Re: Help me get my mind around this...accurately. - 01/24/09 04:45 AM

Doesn't sound like anything to worry about to me. I think it's flickering because the pump is sufficient for the job. The seat opens, the pressure drops, it closes and it spikes back up. Dead head regs do that. I've got a Magnafuel 500 with 12 to 10 to 4 -6 lines and it flickers around too, even with air bleeders back to the tank, and it does it idling or at wot, just faster at wot. If it were steady and dropping I'd worry about that, because it'll only be steady while the seats are open or completely shut. Oil filled gauges or isolated gauges look steady because of the snubbing effect they have. A dry gauge close to the carb, or on the reg reads totally different than the others.

Posted By: TS3303

Re: Help me get my mind around this...accurately. - 01/24/09 05:08 AM

you are seeing the needle opening and closing. You will also never "push" fuel through the jets, thats not how a carburetor works. The bowls are not pressurized, they are filled to a level set by the float. If the needle was stuck open and the bowls were allowed to completely fill under pressure it would come out the vent, boosters, etc.

Posted By: StrokerPost

Re: Help me get my mind around this...accurately. - 01/24/09 05:30 AM

Quote:

you are seeing the needle opening and closing. You will also never "push" fuel through the jets, thats not how a carburetor works. The bowls are not pressurized, they are filled to a level set by the float.

Yep, thats why I said "to" the jets instead of through the jets. I was just referring to the pumps ability to keep the bowl full all the time.

Well you and DT seem to be viewing it as I have been, not a big deal. Thanks for your thoughts guys. Any others care to weigh in? I am FAR from being an expert on this stuff and I know there are a bunch of savy guys here.

Posted By: Thumperdart

Re: Help me get my mind around this...accurately. - 01/24/09 06:22 AM

Mine does that cruising sometimes and I just contributed it to vibrations cos there`s no way I`m taxing my fuel pump cruising.

Posted By: Cab_Burge

Re: Help me get my mind around this...accurately. - 01/24/09 06:34 AM

Someone made a comment recently(in the last year, if I remember correctly) that they wanted the fuel pressure set at or under 5.5 to 6.5 lbs to reduce the possibility of aerating the fuel from the tank to the carbs

I had my fuel pressure set at the regulator with the engine running at a idle at 9.5 lbs, I reduced it to 6.5 lbs and didn't see any noticeable change at the track or on the street

My car is not real fast (10:34 best ET in the 1/4 mile) so my results may not carry any weight, your results may vary

I did find out later using a O2 wide ban that the car didn't have a good enough fuel system to allow me to fatten up the carbs big enough to slow the car down until I switch pumps from a Magnafuel 275 to a Magnafuel 300 pump,whch did slow the car down with no other changes made to the car except the pump switch

My message is if you can slow the car down in 1/4 MPH by fattening the mixture up enough until it slows down your fuel delivery system is probally okay, no matter what the fuel pressure gauges reads during the run, if you can't fatten it up enough to slow the cars MPH down you probally have a fuel delivery problem. I did not say fuel pressure problem

Air in the line can compress showing pressure when it is not all fuel in the line flowing

Posted By: Quicktree

Re: Help me get my mind around this...accurately. - 01/24/09 07:24 AM

Quote:

Doesn't sound like anything to worry about to me. I think it's flickering because the pump is sufficient for the job. The seat opens, the pressure drops, it closes and it spikes back up. Dead head regs do that. I've got a Magnafuel 500 with 12 to 10 to 4 -6 lines and it flickers around too, even with air bleeders back to the tank, and it does it idling or at wot, just faster at wot. If it were steady and dropping I'd worry about that, because it'll only be steady while the seats are open or completely shut. Oil filled gauges or isolated gauges look steady because of the snubbing effect they have. A dry gauge close to the carb, or on the reg reads totally different than the others.

with DT and TS

Posted By: StrokerPost

Re: Help me get my mind around this...accurately. - 01/24/09 09:43 AM

Quote:

I run a Mallory 140 pump fed by 10AN line from the cell to the pump, then 8AN all the way forward to the reg, then dual 6AN lines, one to each bowl and the press gage is reading press to the rear bowl, just off the reg.

While we're discussing it, does anyone here see a problem with the way I've got it plumbed? (above and attach) Pic is just to show where I'm picking up the press reading. The line goes to a cowl mounted gage.

Attached picture 4969500-regmount.jpg

Posted By: smokinwoody

Re: Help me get my mind around this...accurately. - 01/24/09 12:47 PM

Quote:

Quote:

I run a Mallory 140 pump fed by 10AN line from the cell to the pump, then 8AN all the way forward to the reg, then dual 6AN lines, one to each bowl and the press gage is reading press to the rear bowl, just off the reg.

While we're discussing it, does anyone here see a problem with the way I've got it plumbed? (above and attach) Pic is just to show where I'm picking up the press reading. The line goes to a cowl mounted gage.

I see no problem with what you have...mines almost a carbon copy and keeps fuel pressure rock stable and fuel delivery is no problem..

Posted By: cgall

Re: Help me get my mind around this...accurately. - 01/24/09 01:36 PM

Ken, mine does it too, so I relocated the guage under the hood where I can't see it. That'll get it out of your head!

Posted By: 440Jim

Re: Help me get my mind around this...accurately. - 01/24/09 02:22 PM

The gauge variation is not a problem, IMO.
As long as it isn't going leaner the farther down the track you go, I wouldn't worry about it.

Posted By: MR_P_BODY

Re: Help me get my mind around this...accurately. - 01/24/09 02:23 PM

If you want to get rid of the needle flicker put a
orfice in the gauge. See if it has a screw in orfice,
if not you can put one at the inlet... use about a
.020 orfice max... this will slow the gauge down
but still be correct.... remember your dealing with
air here, its not liquid and air compresses

Posted By: MR_P_BODY

Re: Help me get my mind around this...accurately. - 01/24/09 02:50 PM

How big a deal is it to rely on the low press area in the intake to PULL fuel through the jets vs adequate press PUSHING fuel to the jets?

Also once the fuel enters the bowl its no longer
under pressure(the bowls have a vent) so atmospheric
pressure is at work from this point on. The pressure
pushes, the neg pressure in the intake(cylinders)
creates the place for the charge to go.

Posted By: Dunnuck Racing

Re: Help me get my mind around this...accurately. - 01/24/09 03:29 PM

In my opinion,pressure is not nearly as important as volume.It would not matter if the pressure dropped to 1 p.s.i.,so long as the bowls stay full.The reason the pressure is set where we do is to overcome the restrictions to get the fuel into the bowl.

Posted By: MR_P_BODY

Re: Help me get my mind around this...accurately. - 01/24/09 03:45 PM

Quote:

In my opinion,pressure is not nearly as important as volume.It would not matter if the pressure dropped to 1 p.s.i.,so long as the bowls stay full.The reason the pressure is set where we do is to overcome the restrictions to get the fuel into the bowl.

Very true on the volume. The reason we use 7.5 or
so(based on the gauge accuracy)is because if you go
much higher it will blow the needle off of the seat.
Also the thing with pressure is... its a by-product
of resriction, the greater the restriction the greater
the pressure untill you reach the max of the supply
pressure. So that 7.5 psi is what the needle and seat
are holding

Posted By: Bad340fish

Re: Help me get my mind around this...accurately. - 01/24/09 03:48 PM

Quote:

In my opinion,pressure is not nearly as important as volume.It would not matter if the pressure dropped to 1 p.s.i.,so long as the bowls stay full.The reason the pressure is set where we do is to overcome the restrictions to get the fuel into the bowl.

Thats the way I understand it as well. If there is pressure the bowls are full and the needle is closing.

One of my biggest let downs was a fuel system upgrade. I put a guage on my car and it only had 2psi at WOT. I could roll down the throttle on the highway and the fuel pressure guage would go quickly down to 2psi.

I put an electric pump and new line all the way to the carb and headed to the track. I had rock solid fuel pressure at 7psi. I ran the exact same ET and MPH as well.

Also, is it possible for some of these mondo fuel pumps to keep pressure in the line even with the line open? If possible that could cause a false reading.

Posted By: MR_P_BODY

Re: Help me get my mind around this...accurately. - 01/24/09 04:06 PM

Also, is it possible for some of these mondo fuel pumps to keep pressure in the line even with the line open? If possible that could cause a false reading.

The reading you should be getting(low side) is on
the other side of the diaphram that the high pressure
is (or the pressure check valve)

Posted By: 70blackfish

Re: Help me get my mind around this...accurately. - 01/24/09 05:15 PM

I wouldnt worry about pressure dropping, as long as you have pressure. at least 4-5 psi

Posted By: dthemi

Re: Help me get my mind around this...accurately. - 01/24/09 06:20 PM

About volume and maintaining pressure, I can pull a -6 line off, put it in a jug and my pressure will stay where it's set as if no fuel were flowing. If your pressure is dropping going down the track you don't have enough pump because it can't keep up with the needles and seats being wide open for long. If the pressure is dropping the fuel level in the bowls is going down. It may not be enough, or long enough to run them dry, but it is going down. Splitting hairs on it means that with the bowl level going down you can be sucking some air into the mains with all the vibration, bouncing of the car, and the fuel spraying into the bowl, and the weight of fuel over the jet. I've seen cars slow down when they put enough pump on the car to keep the pressure because the car got enough fuel on the other end to go fat. What I'm saying is if a car with too small a fuel system get's tuned well it'll have too large a main jet to keep it from going lean on the other end. when you give it the pump it needs it'll go fat. Agreed most pumps are more than enough for the job, but that's so much better to tune with than the alternative, one less variable in your tuning equation.

Posted By: moparmanjames

Re: Help me get my mind around this...accurately. - 01/24/09 07:28 PM

Quote:

Someone made a comment recently(in the last year, if I remember correctly) that they wanted the fuel pressure set at or under 5.5 to 6.5 lbs to reduce the possibility of aerating the fuel from the tank to the carbs I had my fuel pressure set at the regulator with the engine running at a idle at 9.5 lbs, I reduced it to 6.5 lbs and didn't see any noticeable change at the track or on the street My car is not real fast (10:34 best ET in the 1/4 mile) so my results may not carry any weight, your results may vary I did find out later using a O2 wide ban that the car didn't have a good enough fuel system to allow me to fatten up the carbs big enough to slow the car down until I switch pumps from a Magnafuel 275 to a Magnafuel 300 pump,whch did slow the car down with no other changes made to the car except the pump switch My message is if you can slow the car down in 1/4 MPH by fattening the mixture up enough until it slows down your fuel delivery system is probally okay, no matter what the fuel pressure gauges reads during the run, if you can't fatten it up enough to slow the cars MPH down you probally have a fuel delivery problem. I did not say fuel pressure problem Air in the line can compress showing pressure when it is not all fuel in the line flowing

I believe what you are referring to at the pump is "cavitation", which forms bubbles in the media by going from a liquid to a gas state due to a pressure drop. This happens when the vanes or rotors of a pump form a low pressure area on the suction side. This usually occurs if the pump speed is too high (which does not apply here since the speed is not variable) or the inlet is not large enough and a low pressure area results. These bubbles will quickly collapse when they enter a region where the pressure is sufficient to become a liquid again.
Another cause is heat and vapor lock, which is what you were referring to with a vapor bubble in the line.
What has always concerned me with these fuel pumps is that most are "deadheaded", meaning that we have a high volume pump that is pumping fuel to nowhere while the needles are closed. This builds heat in the pump and produces cavitation fro the heat buildup.
I have been contemplating some type of pressure bybass flowvalve back to the tank to avoid this type of cavitation and also avoiding a warmer pocket of fuel reaching the carburetor and affecting consistency.

Posted By: Anonymous

Re: Help me get my mind around this...accurately. - 01/25/09 06:20 PM

The way I look at it is, if the gauge is moving around, then the float bowl probably isn't staying full............and if the float bowl isn't full, that means that the engine would have to suck harder to get the fuel up the main well........It's like trying to drink a pop with a ten foot long straw......You'd have to work harder to get it up that far............so, in my opinion, so I can sleep better at night, I'd want it to be rock steady..........

Have any of you guys seen where a guy will raise the float level on a motor that's on a dyno to fatten the fuel curve? I've seen guys pick up power by just changing the float level.......

Will it make your car any faster? Who knows......it depends on whether your motor needs the extra fuel or not.........