EFI Mods to 73 Fuel Tank

[markandson]

I have a 73 fuel tank that I am preparing to use with EFI. It has 3 vent connections. Two have an orifice inside which were connected to the neck vent and then to the charcoal canister and one is just a small tube that used to be connected to the fuel filter vent. I am using the one that used to be connected to the fuel filter vent as my EFI fuel return line. It was a small tube, like 1/4", so I wanted to increase its flow capability. I cut it off flush with the tank and then drilled it out, only to find that it was actually a long tube that runs along to the top of the tank and appears to end at the rear tank wall. The question is, is there anything special about this tube at the end that I can't see, and what if anything should I put inside the tank where I will be connecting the return line?

[opelwasp]

Yes, if you use that tube for the return you will hear the fuel pouring back into the tank. Not so bad if you don't mind the sound of someone urinating into a trash can. I would try to mount a new fitting on the side of the tank at the bottom or install a 90 degree tube that extended to the bottom of the tank.

I can just see it now, every time you get into you GT you get the urge to pee.

[markandson]

Well, what I am doing is welding in a 3/8" NPT coupling, so I can easily put a 90 on it before I weld it in and have it extend to the bottom of the tank. Your comments are exactly the reason that I asked. No trash cans for me .

[namba209]

Either I'm really lucky or the open exhaust on Willit? is so loud I can't hear the fuel returning back into the tank. I'll find out for sure when I get the exhaust finshed and it's a bit quieter. All I'm running for a return line is a #4 (3/16") stainless tube that terminates in the filler neck, below the vent fitting. For those that haven't seen or read about my fuel system, here's a quick and dirty on the fuel return setup. The return line starts as an overflow return at the surge or feeder tank in front of a high pressure fuel pump that dumps fuel at 70 PSI into a "T" from the surge tank return line, then fuel rail pressure is also dumped into the same return line via another "T" and then back to the fuel tank. One single return line with 3 inputs that go back to the tank. I just had to use the "KISS" method.

[tekenaar]

Originally Posted by namba209 Either I'm really lucky or the open exhaust on Willit? is so loud I can't hear the fuel returning back into the tank. I'll find out for sure when I get the exhaust finshed and it's a bit quieter. All I'm running for a return line is a #4 (3/16") stainless tube that terminates in the filler neck, below the vent fitting. For those that haven't seen or read about my fuel system, here's a quick and dirty on the fuel return setup. The return line starts as an overflow return at the surge or feeder tank in front of a high pressure fuel pump that dumps fuel at 70 PSI into a "T" from the surge tank return line, then fuel rail pressure is also dumped into the same return line via another "T" and then back to the fuel tank. One single return line with 3 inputs that go back to the tank. I just had to use the "KISS" method.

KISS may be true but not necessarily the correct solution, allow me expand a little. From what you say here, I take it you did NOT alter the ID of your original '73 tank "anti-perk" return line and are feeding two returns via Tees to it. You are most likely running rich from idle to about 2500RPM as the small size of that line at the tank is causing a restriction to gas flow back into the tank. This acts as a secondary "orifice" pressure regulator because of it.

FI systems require that the return line can handle at least an equal fuel volume (no restrictions) as the pressure line so that this does not take place. This precept applies to all FI systems, stock or modified.

[namba209]

Otto, I used the same sized lines that were on the engine when I got it, almost, I have a #8 (1/2") stainless line from the boost pump to the surge tank. My thoughts were exactly the same as yours, when first looking at the fuel rail. I figured the #6 (3/8") line was the return and the #4 (3/16") was the pressure line. In fact the three manuals I bought for the engine showed that. That's even the way aircraft hydraulic systems are, small lines are pressure, big lines are return. But after getting no pressure to the fuel rail during leak checks, and after pulling the fuel rail to check it out, it was just the opposite, on this engine the larger line is pressure and the smaller is return. Wierd. So far, with the engine on the test stand, no load, it ran flawlessly up to 5000 RPM. To put this in perspective and explain why I did it this way, I have a flow bypass primary or boost pump at the fuel tank, any excess pressure out of that pump is a no flow condition, somewhere less than 5 PSI. The surge tank has a stand pipe in it, so that when the tank is full, it overflows into the return line, at boost pump pressure or less, because there's no restriction back to the tank. The high pressure pump, has a bleed and pressure dump port, that continually bleeds fuel through the pump, keeping it cool, and also maintains 70 PSI in the ouput pressure line. The fuel rail pressure regulator maintains 50 PSI in the fuel rail and dumps excess pressure into the return line also. Because there's nothing other than the "T"'s to slow down the flow back to the tank, if they do cause a restriction, as you suggest, the boost pump will bypass and not flow fuel to the surge tank until the pressure, if any, caused by the "T"'s falls off to lower than bypass pressure. Kinda long winded, but I think you may get the idea. Another point to add, because the pumps are not dependant on engine RPM to function, with the engine off and the fuel system running, I have a steady 50 psi at the fuel rail and listening back at the tank, with the cap off, I can hear the fuel returning at a constant rate. So I can presume the #4 return line is handling the bypassed fuel easily. That was one of my main concerns in building this system, especially after what I considered the norm to be and find out the designers did it backwards.

[markandson]

I had the same concern that Otto is describing which is why I wanted to increase the size of my return line inlet. I would still like to know if anyone knows what is at the other end of the small tube that used to be the return from the fuel filter. RallyBob, are you there? I think you have cut open tanks to put in fuel pickups, so I thought you might know the answer to this question.

[RallyBob]

Originally Posted by markandson I had the same concern that Otto is describing which is why I wanted to increase the size of my return line inlet. I would still like to know if anyone knows what is at the other end of the small tube that used to be the return from the fuel filter. RallyBob, are you there? I think you have cut open tanks to put in fuel pickups, so I thought you might know the answer to this question.

I'm not that familiar with GT gas tanks really.

But basically if you are not making crazy power, then a 5/16" pressure line and 1/4" return line is perfectly acceptable, and in fact this combination of sizes is routinely used by modern EFI-equipped cars. Most retrofit kits for older cars use a return line plumbed into the fuel filler neck to eliminate the dreaded 'sloshing' noises caused by dumping fuel into the main body of the tank. If you choose to return directly to the gas tank, as long as the return line is near the bottom of the tank, the noise will be non-existent unless the tank is nearly empty.

One thing to consider when plumbing your GT for EFI is whether or not you drive aggressively in the corners....unless the tank is appropriately baffled or you have a swirl pot retrofitted like Ron has, you will suffer starvation issues during cornering. In left hand turns you will encounter no fuel pickup when the fuel sloshes to the right hand side of the tank away from the fuel pickup...when that happens there is no pressure in the fuel rail, and the engine stumbles or even dies out.

I know GTZero has (or had) this issue with his EFI equipped GT, and I suspect a few others have run into the same problems.

Bob

[namba209]

Just to amplify what Bob said and to answer a few dozen questions I've had in the last year or so on my fuel system for Willit?, here's a schematic I drew up a while ago to explain how the system should work. So far, so good, but all I've done with the engine in the car, is to run it at idle for 10-15 minutes to make sure my electric fan is turned on and off with the computer. During these runs I had a steady 50 PSI at the fuel rail and a constant flow of return fuel to the tank. So I'm guessing, with no severe throttle applications and both pumps running at max pressure and minimum flow to the engine and max return flow, the 1/4" return line is gonna work, at least for me. I should note, the fuel supply line to the surge tank is 1/2" stainless instead of the 3/8" hose, originally shown in the schematic.

[vikesman101]

OK

I'm always trying to find the easy way out....

On my old 72 GT with EFI I solved the problem by going to the local U-Pull and I found that the pump/sending unit out of a s10 blazer has an almost perfect depth to fit a GT tank. So I cut a square out of the top of the s10 tank that holds the pump/sending unit, took it and my GT tank to the local welder and had them cut the GT tank and weld the new unit in place of the old GT sending unit. Pressure checked for leaks and there were none. Now the vent and return line problems were solved as well as the higher fuel pressure problem needed to run an EFI.

The only problem I found was that the fuel Gage read backwards.....
Easy fix, cut the swing arm off the sending unit, move it to the other side and switch the "swing" from down to up. Worked like a charm. Now I could easily get replacement pumps and install them without to much trouble if needed.

If installed correctly this should be VERY safe as GM puts them in almost every car. Have it professionally welded NO FUMES, wire it properly don't have a SPARK, and connect your lines properly, again, NO FUMES.

Also, no return line noise, and no I could not hear the pump running.

Just my ramblings

Tom

[namba209]

Originally Posted by vikesman101 Have it professionally welded NO FUMES, wire it properly don't have a SPARK, and connect your lines properly, again, NO FUMES. Tom

During my USAF career, I was NCOIC of the Tank Farm, that's the place where the external fuel tanks were stored and prepped for maintenance. Here's the drill for doing any work on those tanks and should be followed in this regard. Drain residual fuel. Purge the tank by running water through the tank for 24 hours. That should take care of any fumes that may have permeated into the steel. Also, if the welder is knowledgeable, he'll have a nitrogen filled bag to weld the gas tank in. HTH.

[vikesman101]

I don't disagree with that at all. By having the professional do the job he incurs the liability. But more to the point, he is a MUCH better welder then I am. After he got it welded we sealed the tank and put air pressure to it then used soapy water to check for air bubbles everywhere, not just his welds, it was perfect on the first try. Also, the tank was empty and dry, but he put about 3 gallons of water in it before the welding. The whole thing worked really well and cost about $50 for the pump/sending unit and the welding, not to bad.

Tom

[baz]

i always have a tank steam cleaned for 3 hours before it goes any where near gas or electric welding and unless the tank is a sieve have always filled it up to just under the area to be welded ,my first day in the workshop when i was an apprentice i was looking at a gas tank that was ballooned out
from just been welded after with some water put in as a heat sink
that will never happen to any tank i weld

[markandson]

My tank has been sitting in open air with all the holes open for about a year. I didn't do anything at all, just picked up the welder and had at it. No problem at all, but there was not even a hint of gas smell, so I figured no problem. I have also looked back at an old thread by RallyBob and found the fuel pickups he used to avoid the cavitation problem that has been mentioned here. I just ordered two to put inside my tank........hopefully problem solved. I just have to cut a hole in the top of my tank to do the install and make a bolt on cover for future maintenance. Still don't have the body back so I have lots and lots of time to do all these small mods. Double edged sword isn't it.

[markandson]

The inside of a GT tank has something in it that makes no sense to me. At the front left corner, spot welded to the top of the tank is a thing that looks like a soup bowl. The rim of the bowl is what is welded to the top of the tank. There does not seem to be any inlet or outlet, it just traps air inside, or so it seems. If you look at the top of a tank, you will be able to see spot welds that form a circle, these are the welds that hold it in place .
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The tube that I cut out that was the original return line from the fuel filter is just a straight piece of tube that was soldered to the top of the tank and ran from rear to front. I had drilled a hole about 3/4 diameter where it used to exit the tank, and the tube was still inside the tank, lined up with the hole. I slid a piece of 1/2" copper pipe into the hole and over the tube and then hit it with a hammer, it sheared off the solder and the short piece of tube was removed through the 5" x 6" hole that I cut in the top of the tank. I will be fabricating a new EFI return line using 3/8" tube, it will have a bend in it to return the fuel low in the tank to avoid noise.
-----------------
There is no large flat area on top of the tank to cut a hole in, so that a flat cover could be used. I had to cut through the rolled in "ribs" that were used to keep the tank rigid. This left me with 4 spots about 1" wide and 1/8" deep that needed to be filled in order to fit a cover. I cut 1" x 1/8" flat stock to fit in the depressions and then welded them all around and ground the whole thing flat and flush with the top of the tank. I have a frame fab'ed from 1 x 1/8 that will go inside the tank, it will have nuts welded to the bottom of it, with a matching cover. I will be plug welding it to the top of the tank, which will hold it in place.

[RallyBob]

Originally Posted by markandson The inside of a GT tank has something in it that makes no sense to me. At the front left corner, spot welded to the top of the tank is a thing that looks like a soup bowl. The rim of the bowl is what is welded to the top of the tank. There does not seem to be any inlet or outlet, it just traps air inside, or so it seems. If you look at the top of a tank, you will be able to see spot welds that form a circle, these are the welds that hold it in place

You mean like this Jeff? Gas tank bowl.

I have no idea why they use them either. My guess is it's an expansion chamber. You'll note in my pic that there are tiny arrows on the bowl itself, those are pointing out the small factory-made holes I found in the bowls.

[namba209]

I've been thinking about that since I first saw your pic of that Bob. I believe you're correct, it appears to be a thermal expansion chamber. Especially with a hole in the bottom and the top outer edge. Fuel would expand by heat and enter to bottom hole forcing air out of the chamber through the top hole. Then as the fuel gets used or cools down and "shrinks", it goes back into the main part of the tank and air is pulled back in through the top hole, negating a vacuum effect.

[markandson]

Yes Bob, just like that! I just added pictures to my post so you can see it installed inside the tank. I didn't want to remove it, some genius must have decided it was necessary.

[markandson]

I guess in theory, if you filled the tank, and both holes got covered up it would trap the air inside, so you can never really have a situation where the bowl filled up with gas all the way to the top and lost it's expansion tank cabability.

[namba209]

Actually Jeff, going back to high school physics, expanding fuel, being a liquid that will not compress, being forced into the bottom or top of the chamber will compress the air inside until it escapes through the hole in the top. That will allow more fuel inside the chamber, which, again will force the air out until a balance in pressure is achieved or the fuel stops expanding. Whew.

[markandson]

True, but what I meant was if you filled the tank all the way to the top and covered both holes. As you are filling the tank there is no way that the bowl would fill up as fast as the pump is filling the tank, so you could trap air inside the bowl once the top hole is covered. It appears that the hole is so close to the top that the gas would have to go up into the filler neck in order for this to happen.

[namba209]

Yep, that's true Jeff. The gas nozzle would shut off automatically before that chamber would fill up. The reason for the chamber, so I was told, is beause the gas in the underground tanks is much cooler, when it comes out of the nozzle, than the ambient temperature, at least other then winter time, that when it does go into thermal expansion, the gas has a place to go, other than dump on the ground via the filler neck. Maybe. Anyway it sounded good when it was explained to me.

[neuropel]

Hmmm...does my Manta fuel tank have the same soup bowl?
I have a couple more questions regarding fuel tank mods:

1). Is there any reason to change out the size of the fittings on a '75 tank from an EFI car? I'll be running twin weber throttle bodies on the 2.5 with the big valve 2.4 head. I don't want fuel starvation issues.

2). If you're modifying a pre-'75 Manta tank (we're talking another car here now), where do you prefer putting the fittings? I notice the sender is in a different location on the '75 factory EFI tanks.

Thanks,
Todd

[namba209]

Todd, mechanical fuel pumps and almost all electric low pressure fuel pumps are of the flow bypass type. That is they will only flow fuel if there is a place for it to go, i,e. the float needle is off the seat in the carb float bowl. Because the EFI system flows at a higher pressure and dumps excessive pressure back to the fuel tank, the fuel line from the tank should be more than sufficient to handle a pair of carbs at lower pressure. HTH.

[markandson]

An EFI pump is a positive displacement pump, it can have an internal relief valve that causes the gas to recirc. inside the pump if the outlet pressure reaches the relief valve setting. The other choice is to not recirculate within the pump but to send the excess flow back to the tank once the relief setting is achieved. The return line size is dependant on the flow capability of the pump, and the amount of fuel that the engine uses, not the pressure. If the return line is too small it will cause the system pressure to become higher and also unstable which will cause flow variations at the injectors.