SQUEEZING MY FUEL SYSTEM, HELP!!

Paul S.

New Member
Apr 16, 2003
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I'm trying to get a bit more fuel out of my fuel system. My procharged 93 cobra made 486rwhp the last time I dyno'd on pump gas w/ac. Since then I did some changes(intake,tuning,throttle body).It started to get lean in the upper rpms with stock rails and 42's with an fmu 255 intank and t-rex inline. I was wondering if changing to billet rails and regulator running -6 line and -6 filter after the intank pump and using the stock return line would help me pickup the extra fuel I need. My combo is complete and don't want to a complete fuel system just trying to use the pumps I have. Any imput would be helpful.
 
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I have a pms on the car and I'm thinking of adding a wideband to monitor the air/fuel before I start it up. Does anyone run the-6 line from the pumps to the rails using the stock return?
 
Well, depending on your actual BSFC (I would estimate around .6), you're fuel requirements for that amount of power is somewhere in the range of 45 lbs/hr per injector at 80%. You're obviously overstressing your injectors which you've said yourself.

Going with a larger supply line will eliminate the possiblility of fuel starvation at the injectors during high RPM and WOT but I don't think that is your main problem. You are simply not dumping enough fuel into the cylinders.

There are two ways to increase fuel flow at the injectors. Bigger injectors or higher fuel pressure. Assuming you're currently running 43.5 psi you'll have to increase to 51 psi to receive adequate fuel (though I would recommend 55 psi). The formula for that is: square root of (new pressure/old pressure)*old flow = new flow.

That being said, you need to be cautious of increasing your pressures too much or you'll actually lock the injectors closed (which is bad). 55 psi is normally not too terrible as long as you've got high quality injectors. I personally have never seen injectors lock up until you exceed 60 psi.
 
I've been considering upgrading my fuel system as well. I also have a 255 in-tank with a T-rex, 42 lb injectors, no FMU, stock rails and lines. I was planning on going to a -6 (3/8) fuel line to get me by for a while. The only thing I was concerned about was how to connect it to the in-tank pump and T-Rex. The stock rails are supposed to be good for 600 or so hp, so I don't think I'll gain much by replacing them. The real bottle neck is in the pumps and the small 5/16 factory feed line. Once you get close to the 500 rwhp mark, you need to start thinking about a complete fuel system. Sumped tank, big lines, external pump, rails, external regulator, etc. I'm looking at the Aeromotive complete kit for down the road. It's a little pricey at $1600, but I like the simplicity of a complete kit.
 
Paul -

Since you are running almost 500 ponies under the hood I would definitely recommend getting new rails. The stock are really only good for an extra 20-30% volume and I think you've well exceeded that value.

Whether or not you need new injectors is a tricky question. Do you know what fuel pressures you are currently running right now? Since injectors are rather spendy, try upping your fuel pressure 5 psi at a time and then go run around the track to see if it made a difference. I would not go above 60 psi because at those pressures you're doing more harm than good. If you don't see favorable results by, say, 50 psi I would go ahead and jump up to the next size injectors.

At this point though, I think you can get by on just some new rails and a slightly higher pressure.

Mike -

I tend to disagree with your stock fuel rail claim. The supply lines themselves can actually be rather small comparatively. As you increase injector size in direct relation to increasing horsepower, the need for a greater fuel reservoir also increases proportionately. The increased injector size effectively decreases the reservoir-to-injector ratio which magnifies pressure fluctuations.

Any pressure fluctuation at the injector is bad as it will deliver less than the intended amount of fuel creating a lean (bad) condition. Anything past a 20-30% increase in injector size should be accompanied by a proportionate increase in rail size.

As for the fuel lines themselves, believe it or not, a 1/4 in. line is sufficient for 500 hp and figure about 3/8 in. for 1000 hp. The overall length of the lines comes into play a little bit but is relatively negligible for most applications. The fuel pickup should normally be about 20% larger than the supply line itself.
 
Vipersix said:
Mike -

I tend to disagree with your stock fuel rail claim. The supply lines themselves can actually be rather small comparatively. As you increase injector size in direct relation to increasing horsepower, the need for a greater fuel reservoir also increases proportionately. The increased injector size effectively decreases the reservoir-to-injector ratio which magnifies pressure fluctuations.

Any pressure fluctuation at the injector is bad as it will deliver less than the intended amount of fuel creating a lean (bad) condition. Anything past a 20-30% increase in injector size should be accompanied by a proportionate increase in rail size.

As for the fuel lines themselves, believe it or not, a 1/4 in. line is sufficient for 500 hp and figure about 3/8 in. for 1000 hp. The overall length of the lines comes into play a little bit but is relatively negligible for most applications. The fuel pickup should normally be about 20% larger than the supply line itself.


I'm not sure I agree with the claim that stock rails are good to 600 hp either, but it's something I've heard time and again on this site as well as several others. If the 255 pump and the T-Rex in conjunction with the stock lines is adequate for folks like Paul and I, could we get by by just adding a set of fuel rails, such as this kit by Aeromotive?
http://www.aeromotiveinc.com/pdetail.php?prod=54
It doesn't seem like it would do any good to feed a 5/8" rail with a stock 5/16" line
 
Mike92GT said:
It doesn't seem like it would do any good to feed a 5/8" rail with a stock 5/16" line

The entire purpose of the fuel rail is to act like a reservoir. The fuel in this "reservoir" is relatively stagnant under most conditions. The idea is that under WOT, the sudden demand for more fuel will be supplied by the extra fuel in the reservoir rather than relying on fuel supplied by a small supply line. (This is why no fuel system worth it's mettle will avoid the use of fuel rails.)

The idea, however, is to properly size your entire fuel system. If your pump(s) is/are adequate, your fuel lines are adequate, your pressures are sufficient, and your injectors are properly sized, you should never run into fuel starvation at the rails.

I have a formula to calculate the fuel flow through a line but I'm currently in Iraq and don't have all of my books with me. I'll keep looking.
 
One of my concerns about going to -6 line was the line from the intank pump to the inline pump. ViperSix said that should be 20% larger than the feed line. Will running -6 hurt the pumps or is their any way to increase that line size by modifying the stock intank pump pickup. I"m very good at fabricating parts and have access to all sorts of tools and machine equiptment. If I could make a mod to it maybe it could help out alot of people in same situation as myself. Its nice to have the stock fuel gauge and I like idea of using the parts I already bought.Aftermarket systems are big bucks and seem like a unncessary expense for my borderline horsepower combo. My theory is if I go with a full on system I'd just want to push that stock block knowing I have all the fuel I want and plenty of blower pullies.Thanks for your knowledge ViperSix and Mike Its very valuable to me and is greatly appreciated.
 
Some aftermarket companies have started to make a fuel pickup tube that replaces the old pump setup. The tube would allow the use of large AN style fuel line. This would also cause the extra cost of running a new external fuel pump. My buddy used one to run his carb setup and it worked well. Search Ebay for mustang fuel pickup and you will find a few on there. I have been debating over this type of setup but will probably go with a sump tank with the complete Aeromotive system. I have tried to see if I could piece together a system cost by cost and it just doesnt seem worth the headache.
 
I think in your particular situation, the stock in-tank pickup would not hamper the overall flow. Two pumps in a series offers the possibility of raising both the flow rate and the maximum pressure potential. One pump alone will always be pumping against the fuel pressure regulator, add the second pump after the first and it will pull on the first pump. This will dramatically reduce the pressure the first pump is asked to make. Realize that the operations of a pump are characterized by the pressures; as pressure goes up, flow comes down.

The second pump is being force-fed from the first rather than being fed by gravity or normal atmospheric pressure. Since the second pump is controlled to a maximum pressure by the regulator and is being force-fed from the first, it does not have to make as much of a pressure change as it does when alone. Therefore, each pump in a series becomes much more potent than when operating alone. Granted the flow rates do not simply add to one another, the flow increase will be about 120% or so of the larger pump's capacity.

Another idea would be to run two pumps in parallel. In this setup, the overall pressure potential does not change but it is much easier to flow a lot of fuel and still maintain a peak pressure near what a single pump can produce at lower flow rates.

In your situation though, I would stick with a series. I highly doubt you'll need to alter the in-tank pickup.

If you are concerned about the flow rates, disconnect the line after the regulator and measure the amount of fuel that comes out in a set period of time. This will let you know for certain whether or not your setup is producing enough fuel at those pressures.
 
Thanks again vipersix for the info just made my parts order and hopefully get started on it this weekend. I'm lucky to have a brother in law with an extra lift in witch my car has occupied for the past few weeks minus motor and trans. It's a real nice thing to have access to when working underneath your car. Thats why I'm addressing the fuel system now while I'm waiting for the machine work to be finished. I've built and raced cars since I was a kid and I think this is the first time my car will be done before the weather gets nice. I've also went full circle street car to full race car back to street car. The only thing I really miss the transbrake! Forget the trailer and crewcab and moochie friends emptying the cooler and eating all the food before you get a chance and leaving before the car is loaded. I kept my license just incase I get the chance to get in the seat of a friends car. Don't worry I'll make sure I have a cold beer for him after the cars in the trailer! Who knows those 10 inch tire cars are fun or maybe a turbo I haven't had one of those yet! OH! NO! HERE I GO AGAIN!! HELP ME!! MAYBE I SHOULD CHECK INTO G.A.(gearheads anonymous).
 
Hey Paul, what did you end up buying? I also have my engine and trans out right now, so I'm thinking about doing my upgrades now. I was thinking about doing the Aeromotive fuel rail kit, and feeding it with a -6 line from my two existing pumps.
 
Mike I have a complete parts list I'll post as soon as I get home this evening. I already had the aeromotive fuel rails bought a regulator,-6 filter,push lock lines and earls fittings. It was about 450.00$. lwill break down the parts list and tell you how I'ii run it asap. Thanks for your help Mike.
 
here it goes fuel system:
Attach -06an to -08 male reducer unions with o-rings on -08an side of fittings to rails plug drivers side front inlet install on motor. Then from T-rex outlet use a hose clamp over barbed fitting with -06 twist-lok line to fuel filter using a -06an hose end fitting to -06an fuel filter -06an hose end fitting twist-lok line to regulator with a -06 hose end.-06an male union with o-ring to inlet side of regulator -06 male unions with 0-ring on each side of regulator outlets.-06 hose ends on each line going to fuel rails. Measure line from front side of reg. to front side of passenger side rail install -06an 90 degree fitting on line end.Run rear regulater outlet -06an line from regulator to drivers side rail install -06an 90 degree fitting attach to underside middle inlet of rail. Attach a -06an 90 degree fitting to each line going from the rear rail fittings to the stock return line location put -06an hose ends on each line.Use a aeromotive ford pressure line fitting with two -08an female to -06an male coupling wit nuts. Attach return hose's to wit nuts and attach to stock return.Turn on pumps check for leaks.

parts list:
1 aeromotive 14101 rails
1 aeromotive 13109 regulator
1 aeromotive 15102 ford pressure line fitting
1 earl's 23106 -06an inline fuel filter
30 ft. earl's -06 twist-lok line
4 earl's -06an 90 degree twist-lok fittings
4 earl's -06an male to -08an male reducer unions
3 earl's -06an male unions
2 earl's -08an female to -06an male coupling wit nuts
7 earl's -06an straight twist-lok hose ends
4 earl's -08 o-rings
3 earl's -06 o-rings
cost around 475.00 excluding rails
 
Sounds good I might have done the same thing if I didn't have the rails already. The push lock lines are nice and the kit comes with braided lines in which I'm tired of dealing with. The blue push lock lines look good with the teal paint on my car and they install very easy and it's a little different look.