electric fuel pump prime problem, Mr Fixit or others?

snake farmer · #1 (**permalink**) 04-14-2003, 07:17 PM

My Fuel Safe cell has a top of cell fuel outlet rather than the bottom outlet seen on many cells. After asking both Fuel Safe tech people and Barry Grant tech people to help me set up fuel system that would support a 750 HP engine and a designated fuel pump for a 250HP NOS pro fogger system, they advised of 2 fuel pumps and fuel safe advised a -10 pickup. this was fine because BG people said I absolutely needed a -10 supply line from the cell to the pump. I spent over a thousand dollars on pumps/filters and fittings. Now I discover and have since been told by BG that none of their pumps will "pull fuel",but rather they have to push fuel and their pumps must be mounted lower that the cell, calling for a bottom pickup. (None of their written instructions tell you this.) I discovered this when the pump would not prime. After pressurizing the cell with air I get fuel to the cargs but when I shut it off the car the fuel keeps running back to cell as the pump is mounted about 12 inches above the cell. Then the need to reprime the pump. If those tech people had listened to me about the application (Cobra) they would have know that the cell sits very low in the floor of the trunk,almost impossiple to mount a elect pump lower than the cell (cell is about 6 inches off the pavement. I ask anyone if I can put a fuel check ball valve in the line between the pickup and the pump, thus holding fuel from running back to the cell and unpriming the pump. I've been told that I may need pressure equivilant the weight of the fuel (several gals) to keep the pump primed. With my check valve plan I would have fuel to the pump but with no real pressure behind it. Anybody have a pump for the pump?

Signed, desperate in Des Moines.

mr0077 · #2 (**permalink**) 04-14-2003, 08:43 PM

Snake Farmer, your supply line comes out of the top of the fuel cell, but the pickup end of it must be at/near the bottom of the cell, right? If so, then if you mount the pumps low, at the same elevation as the bottom of the fuel cell, they should stay primed until the fuel level drops below the pump inlets. A check valve might work if it has the flow rate you'll need. Could you have an in-tank pump to prime the primary pumps? Never heard of something like that, but...

coyled · #3 (**permalink**) 04-15-2003, 07:53 AM

Snake farmer, if you have a csx cobra and a fuel cell from fuel safe, I would bet that it is exactly the same as my hi-tech fuel safe cell. I had a similar problem but my cobra is fuel injected. Originally the pick up was on top but went to the back of the cell or rear of the cobra. The rear of the cell is higher then the front of the cell as it mounts in a cobra. It made sense for fuel safe to put the pick up there because fuel goes back there on acceleration, except for in my cobra, the rear of the cell is higher.. I sent it back to fuel safe twice and hear is what they did. They moved the pick up location(still on top) but to the front of the cell. They also gated the area so it would retain fuel. Lastly, they installed an in take fuel pump to make sure that the pick up area would not run dry and thus loose prime. The set up is perfect now. But, I chased this one around for months, died around corneres, would loose prime, only worked well on a full tak of fuel. Finally, solved the problem. You may have to gate the pick up area in the tank, make sure its in front and have an in tank pump to make sure, at least on start up, that your external pump is primed. good luck, scott.

Cobra20646 · #4 (**permalink**) 04-16-2003, 06:27 AM

I doubt that one intank pump will keep two external pumps supplied with enough fuel in this case. If it could, why the need for all of the external ones?

coyled · #5 (**permalink**) 04-16-2003, 08:23 AM

Jim, an in tank pump will not supply enough fuel for 2 big external pumps. But, it should help to keep the external pumps primed. Once these big pumps loose their prime, they are tough to get re-primed. Scott

Cobra20646 · #6 (**permalink**) 04-16-2003, 10:38 AM

Scott,

I understand the principal of keeping the "big" pumps primed, but won't the intank pump be a 'large' restriction for the amount of fuel that the two external pumps are capable of flowing? This is assuming that the two external pumps are fed directly and only from the intank pump. If this is the case, and the intank pump can flow (GPM) as much as the two external pumps together, then is there really a need for the extra 2 pumps? Now if you're cobbling something that will allow the two external pumps to draw fuel directly from the fuel cell, while suppling prime from another source via a "T" arrangement before the pumps, then that might work.... emphasis on "might"

- Jim -

Richard Hudgins · #7 (**permalink**) 04-16-2003, 12:12 PM

Snake Farmer,

I have never understood the requirement for massive fuel pumps on auto engines.

You say that you have a 750 hp engine. Let's assume that you are running a little rich at a brake specific fuel consumption of .5 lbs per hour per HP.

Therefore, at peak power of 750 you have (750hpX.5lbs per hour)=375 lbs per hour of fuel usage.

Assuming a gallon of gasoline weighs 6.5 lbs. (Actually varies between 5.8 and 6.5) You will need to flow 57.69 gallons per hour to the engine to support this HP.

Note: You only need this much flow at the 750 power output level. Meaning wide open throttle at at peak output. If you are driving down the road at 60 mph, you are probably only using 75 hp. Therefore you only need 5.76 gallons per hour. (this is also a pretty bad fuel mileage of 10.41 mpg.)

It would seem to me that you could certainly just use a in-tank pump as many of these units are 75gph and above.

I know that everyone is thinking that the above is BS. But lets look at Winston Cup cars. They have about the same HP as is mentioned above. They use mechanical pumps in the range of 90>120 GPM. Maybe they know what they are doing.

Lets just drop the BS from the folks who wish to sell expensive bits and look at the reality of fuel requirements for a given HP range.

RallySnake · #8 (**permalink**) 04-16-2003, 12:40 PM

Snake Farmer,

The simple solution is to use a fuel cell with a bottom outlet and mount your fuel pumps on the frame rail behind the right rear wheel. You should be able to get the bottom of the pumps at the same level as the bottom of the cell. This is the way I set up mine (except with a steel gas tank from a Bronco, not a fuel cell) and it's been running great for 16 years. Once in a while, one of the pumps gets too much trash in it and quits. Then, I turn on the spare.

The only drawback to this setup is, if the line is broken in an accident, all the fuel will pour out. To prevent that, I used 3/8th inch steel brake line for the whole system with short pieces of rubber fuel line to connect the parts. I just bought an Oberg fuel filter on eBay to run the gas through before the pumps to prevent clogging them up. It's surprising how much trash comes out of the filling station tanks!

I am using a Carter fuel pump with a standard universal pump as a backup. I used to worry about them being exposed to water splash and flying rocks from the nearby wheel and thought about making a little shield to protect them. But I never got around to it and they don't seem to mind. On these rallies, we run some pretty rough roads, too.

Paul

khall · #9 (**permalink**) 04-17-2003, 10:19 AM

Snake Farmer,

I experienced the same problem using a Carter electric pump mounted in the trunk of my SPF. I solved the problem by installing a Holley pump on the frame rail. The Holley has the pump head on the bottom of the motor which placed the fuel intake level with the bottom of the tank. I suspect that the Carter, which has the pump above the motor, would have worked just as well as the Holley if installed on the frame rail. Electric pumps just do not suck very well. You need to get the pump head lower.
Fuel pressure problems are compounded this time of year by the high volitility of winter pump gas. The fuel pump lowers pressure at the fuel inlet side which allows the fuel to percolate (boil) at ambient temps. The problem is so bad here in Denver (5400 ft) this time of year that we have to mix 110 octane race fuel 1:3 with pump premium to prevent vapor lock. Last week, when temps here reached 70 deg, I was forced to fill up on the way home from an afternoon on the chasis dyno. I only made it a few miles beyond the Texaco station before the motor quit. I poked my head under the rear of the cobra to see fuel boiling in the clear fuel filter which is 3" behind the pump inlet. On the dyno we actually had fuel guysering out of the bowl vents 6" when we turned the pump on with a hot engine. Geeze what a sight!

scottj · #10 (**permalink**) 04-17-2003, 10:49 AM

Richard,

I don't think your lesson it's BS. I use the same mechanical pumps on my circle track engines and they make near 800 hp. Fuel supply/pressure has never been a concern for me with these pumps.

Scott

Steve R · #11 (**permalink**) 04-17-2003, 03:51 PM

Snake,
You might want to ask if a check valve would be appropriate. It seems to me it could keep the pumps primed.
Richard Hudgins,
Did you note that Snake is planning on supporting a pro shot nitrous system? As I understand it, they require a lot of fuel flow when in use.
Steven

snake farmer · #12 (**permalink**) 04-17-2003, 04:58 PM

Thanks for all the feedback. All points well taken. I agree with the fact that many of the companies seem to hype the need for higher flowing pumps. Way more than what really is needed. But here is my problem, partially restated. I have a designated pump for a pro-fogger port NOS. I also have a smaller pump for the engine itself. I have hugh money in all this setup and plumbing, return lines, regulators and all. The systems won't stay primed and I am trying to MEND the system without putting everything on the shelve and spending more $. Rather than rip out the system and patch the holes etc, I plan to do 1 or maybe 2 intank pumps to help prime the others. I really have no faith in the check ball thing working. Someone pointed out that it is not just enough to have fuel to the external pumps but one needs to have a little pressure there too. They are gravity fed pumps and they rely on the weight of the fuel against the pump to aid priming. Ford used the pump in the tank to push fuel to an external pump mounted on the frame in some of its trucks. I might be wrong but I bet they explored the check valve as a means to keep fuel from running back to the tank and un-priming the pump but found it didn't work as well. I think I will put a performance pump in the tank and see how it does. I will post the outcome. Thanks again, as always, good feedback.-Ray

Richard Hudgins · #13 (**permalink**) 04-17-2003, 07:24 PM

Hi Steve R.

Actually, I really do not know anything about Nitrous systems.

But from the Holley Web site, the following:

"System Requirements
To supply the carburetor and the Pro Shot Fogger System use a fuel pump designed to operate between 5 and 10 psi with a flow rate of 0.1 gallons per hour per horsepower at 6 psi. Please note that most aftermarket pumps are rated under free flowing conditions. At 6 psi their flow rates may be greatly reduced. Check the manufacturer's specifications carefully or have the pump checked before you use it. "

0.1 gallons per hour per horsepower equates to .065 brake specific fuel consumption. Therefore, looking at Snake farmers 750hp motor with a 250 hp notrous add in you have 1000hp (Scary, at best!!!!) Therefore, 1000X.65=650 lbs per hour at peak output. Therefore a fuel flow requirement of 100 gallons per hour.

Still not a big flow rate. There are submersible pumps that will give you this number.

Snakefarmer,

I understand your wish to keep your current components and not throw away the dollars spent.

A ball or flapper check valve will not fix your problem as these are not a positive seal and you will lose prime. I would also be a bit concerned about just moving your pumps lower to reduce the head distance. I have never had good luck with that method.

Therefore, the only recommendation that I can make is to install a small pickup tank above the level of the existing pumps. Such as the Nitrous enrichment tank from JAZ (click on fuel cells. I hate frames pages!!)or one of their other like products.

This could be feed by a in tank pump of proper size.

Sorry, I know this means more money, fabrication, and bleeding knuckles, but I know no other way to fix the problem with your existing fuel cell pickup system that would be reliable.

Steve R · #14 (**permalink**) 04-17-2003, 08:11 PM

Richard,
Thanks for that info. My nitrous knowlege is also pretty limited. I would do a little homework and then give the Holley tech support people a call.

aumoore · #15 (**permalink**) 04-17-2003, 10:37 PM

All you need is to mount the pump head at or below the bottom of the fuel cell. I had the same problem with my Holley pump untill I actually read the directions which state the pump should be mounted at or below the bottom of the tank.

Also do not make a tall loop our of the top of the tank with the fuel line. turn it back down as soon as you can so the fuel does not have to travel too far up hill before it goes back down to the pump.

If you hold the fuel line below the bottom of the tank fuel will actually flow on its own after it gets started. Just like a redneck fuel card(rubber hose in a fuel tank with a redneck sucking for the initial prime)