Notes on Fuel Level Sender Calibration

There are some other threads documenting various woes associated with fuel level senders in GT40s, including some Superformance cars. Generally the complaints are that one end or the other (or both) of the range of fuel levels is not represented by the gauge. Several people have switched to solid-state, usually capacitative senders; others to similar looking columnar sensors that contain an internal float. Some have simply calibrated the supplied sender by bending the float arm.

Having read all that I decided to "calibrate" my sender ahead of time rather than waiting until there is gasoline to contend with. I put "calibrate" in quotes because my intent was to fully capture the entire range of fuel level within the range of the gauge, regardless of how accurate those readings would be, although later on I might fiddle some more to get "E" to correspond with some known value relative to where the engine stops.

Since the nominal resistance range for Smiths instruments is 20-240 ohms I set as my objective to have the sender read ~240 with the float resting on the bottom and ~20 with the float as high as it can mechanically move within the tank. This can be done with the supplied sender but, due to the fact that the sender is set down into the tank about 1 inch it requires the float to be able to rise above the plane of the sender mounting. As delivered, my float did neither that nor did it fall all the way to the bottom.

Thus my main problem was to increase the range over which the float operates. To do this requires both straightening and lengthening the float arm. Oddly enough, the last inch or so of arm is actually two pieces of wire overlapping and soldered together. So I "lengthened" mine by simply melting the solder and sliding the two piece so the overlap about a half inch less than before. Then "calibration" is a matter of bending the arm at its one remaining angle until the bottom of the float is 6" below the float mounting surface when it is fully dropped.

Note that at the ends of arm travel the resistance may go to an "open" condition. This does not matter as long as the tank is constraining the float's travel. However, when you are working with it and an ohmmeter the open condition can be a distraction so I adjusted the sender's arm limiters to prevent that. It's also true that at one extreme of motion the internal wiper of the sender can over-travel and drop off a step in the sender, after which it will refuse to move. This can be fixed either by poking carefully through an inspection hole provided, or by bending up three tabs that allow access to the innards of the sender.

Also, the arm engages the variable resistor through a hole in the nylon wiper piece, and the end of that piece pokes out the side of the case and is slotted such that you can hold the arm still and turn the slotted shaft with a small screwdriver in order to make very fine adjustments. This is how I would tweak the units value at a single point, since the arm rod is quite stiff and thus hard to adjust finely by bending.

For those considering replacing the stock sending unit with a columnar float design be forewarned that you are inherently giving up the ability to measure the top inch or so of the tank's capacity, at least 15%, possibly more since the tank is wider at the top than the bottom.

Very helpful. thanks. Two additional thoughts to add on the SPF. It seemed on mine the left tank the lower line that the fuel is delivered from is up a fair amount from the bottom of the tank as opposed to the right tank where it is at the bottom. Given the size of the tank and height I am guessing we are losing a couple of gallons of usable fuel. An additional "finding" I had the rear of the car up on jack stands w/ the tiresabout 6-8 inches off the floor. The tanks were fairly full, however the fuel pump starved for fuel. Just as a heads up if you plan to drive down any steep hills make sure your tank is pretty full.

Al;
Some good research and points you made.
I have seen some pretty shabby setups for fuel tanks, and most of the problems were incorrect installation and/or incorrect or mismatched components. Most of the so called "universal fit" senders don't perform correctly because of resistance mismatch or mechanical constrictions as you mentioned.
A couple of things I have found are that no tank can be filled completely so I always allow for 8 to 10 % air space at the top, and I always mount the sender with the float showing empty when it is off the bottom of the tank at full droop. This gives a little reserve if you will, and it may not show completely full, but overall I was more concerned with the lower readings and not running the tanks dry, although most setups have a stand pipe to prevent sucking to the bottom of the tank.
Also some of the newer senders have provision for a low fuel light which is a nice feature, but may require additional wiring.
Cheers
Phil

With the car set up according to factory specs and the way the suction line on both tanks is located just above the bottom you will find about 3 gallons of unused fuel when you run dry on level ground, now if your going up hill all the time the your OK.

I got rid of the fuel sender unit and went with a capacitance probe like they use in boats and airplanes. Its linear in movement and when you get to E your out. The old float type system stayed at full to half then dropped like a rock after that. I also change all my gauges to speedhut, now you can see when your out at night.

P2164 said:

With the car set up according to factory specs and the way the suction line on both tanks is located just above the bottom you will find about 3 gallons of unused fuel when you run dry on level ground, now if your going up hill all the time the your OK.

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I drive Up Hill...all the time. It's how I drive. Up Hill.

Actually I just got curious about this and since my tanks are out...

The LHS tank pickup is an 8AN fitting welded to the rear face of the tank about as low as is reasonable. The low point of the ID of the fitting is about 0.3" above the bottom surface of the tank and the high point is 0.375" higher (that being the ID of the fitting), or about 0.7" above the bottom surface. In a perfectly still and level situation the pump would start to draw air at that 0.7" point, but I would think that with normal driving up/down/accelerate/brake you would get most of the next 0.4 inches... of the LH tank.

What messes all this up, I think, is the fact that the crossover tube fittings are on the outer side of the tanks, and are 12AN. This means that the low point of their IDs is about 1-1/2" above the bottom surface of the tanks due to the inward slope of the tank bottom. This leaves a volume of fuel in the RHS tank represented by a 7" x 1.5" triangular section, 40" long, that on average will not move from RHS to LHS. If my math is right that's about one US gallon.

And of course it's the RHS tank that gets measured, so the fuel level sender is always telling us, in effect, that we have 1.5 inches more fuel than we really do. In an ideal world the sender would have been in the tank you draw from.

IAE, I'm curious, are people really finding more than a gallon or so left over when the engine quits?

Just FYI, I measured my Smiths fuel level gauge response as follows:

Reading - Ohms
E -170
¼ - 75
½ - 47
¾ - 28
F - 13

I wish we had fuel cells (blatters) this would give you more volume and also eliminate all problem in high G corners. Mike Trusty has a great idea of putting a high pressure pump in each blatter thus eliminating the low pressure pump and swirl tank.

I agree with Jack. My tank 'ran dry' with 3 gallons left. My gauge showed about 1/8 so pretty close to accurate. I try not to run lower than 1/4 to avoid downhill risk.

SwiftDB4 said:

I agree with Jack. My tank 'ran dry' with 3 gallons left. My gauge showed about 1/8 so pretty close to accurate. I try not to run lower than 1/4 to avoid downhill risk.

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i just decided to play both sides of the street and so ordered a Centroid solid state sender which I will put in my LH tank; I'll leave the float-actuated wirewound variable resistor in the RH tank.

But given the layout and dimensions I still don't get how you would run dry with 3 gallons in the tanks. Not arguing, just confused given my knowledge. Can you guys give me some more details? How did you determine it was three gallons? And how was it distributed betweent the two tanks? Is this direct draw from the tank to the carb/efi, or is there a swirl pot or some other intermediate stoage involved?

awatkins said:

i just decided to play both sides of the street and so ordered a Centroid solid state sender which I will put in my LH tank; I'll leave the float-actuated wirewound variable resistor in the RH tank.

But given the layout and dimensions I still don't get how you would run dry with 3 gallons in the tanks. Not arguing, just confused given my knowledge. Can you guys give me some more details? How did you determine it was three gallons? And how was it distributed betweent the two tanks? Is this direct draw from the tank to the carb/efi, or is there a swirl pot or some other intermediate stoage involved?

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That's 1 1/2 gallons per each side. I measured the depth with the sender removed on the right tank. I know the tanks may not be totally symetrical top to bottom, but it's an approximation. I have no swirl pot. I assume because of the hose layout that last amount just doesn't get sucked up.

SwiftDB4 said:

...I measured the depth with the sender removed on the right tank. ....

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Do you happen to remember what that depth measurement was?

awatkins said:

Do you happen to remember what that depth measurement was?

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Sorry, I didn't write it down. I only remember that it was about 1/8 of the total height. Another factor to be considered is rake of the tank. Even slight angling of the tank, especially tilting forward, would leave more gas in the tank. When I ran dry the car was idling in my garage, not driving.

SwiftDB4 said:

...I only remember that it was about 1/8 of the total height.

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Cool. That's consistent with what I expected, since the lowest point of the crossover fitting opening is about that far above the lowest point of the tank.

If one cared enough, one could move the RHS crossover fitting to the inside of the tank and thus capture the wasted fuel on that side. On the LHS, unfortunately, that spot is quite rightly occupied by the pickup fitting, and there is a mounting bracket in between, so it's a more difficult problem, although still solvable with enough metal work (punching through the bracket).

Alan I had both tanks completely dry and empty when I was building my car, had 5 five gallons cans of gas sitting by the car, filled till you could see as the gas at the filler cap then pumped all the fuel back into those cans through a filter set up I made to clean the crap out of the tanks that the factory leaves in them, when the process was completed I was about 3 gallons short.

Fuel blatters like they have in airplanes and boats is what we need. Mike Trusty did the calculations on the area where are tanks are now and by using all the space behind the tanks you will have close to 30 gallons of fuel, if I can remember it was 28 there abouts.

A bladder could be designed and installed in the SPF stainless tank. The original FAV/JW cars had molded rubber bags that you collapsed to install via the front sponson hatch. The bladders held more fuel as the followed the inner contours, up and down over the ribs and went almost the full length of the side. The problem with that style of baldder is the chafing and vibration wear on the outer skin of the bladder. Not a problem in a race car where you might replace it every few races. Not so likely on a street driven car. Do you REALLY want to be sitting in a tub full of fuel after it leaks out? Not this kid!

It might be possible to install a small "sump" at the rear of the stock tanks with a baffle system around it. The sump could not be much more than one or one and half inches deep to be able to still be able to snake the tanks into the sponsons. You could draw the fuel from the sump.

If someone here really needs to get the last gallon out of the tanks, is it because fuel costs are forcing you to do so? I will personally pay for an extra gallon of premium for any Superformance GT40 owner who is in such dire straits!

Mark IV said:

A bladder could be designed and installed in the SPF stainless tank. ....
If someone here really needs to get the last gallon out of the tanks, is it because fuel costs are forcing you to do so? I will personally pay for an extra gallon of premium for any Superformance GT40 owner who is in such dire straits!

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Re bladders and cells: From conversations with Dennis O, that's exactly what he does, although he makes his own aluminum enclosure. I was very interested in re-using the SPF tanks for this but balked at the $many-thousand price of the cells. But my interest in that was only for crash survivability. It's not per se a solution to the "last gallons" problem.

Re the last gallon, LOL, I'm on this purely out of intellectual curiosity and because, like Mt. Everest, it's there. So many people comment/complain about the fuel tanks in general for various reasons (downhill-starvation, fuel smell, inaccurate gauge, leftover gas, etc.), none of these problems really need to go unsolved, and I love a good mechanical design challenge. Hence the fact that one year on, my car's still not running. :shocked:

I'm not worried about the fuel thats trapped in the tanks as much as I am getting hit with metal tanks. Blatters would certainly be safer I would think, when I was in the service our planes could get shot and still make it home with just a small leak. Which leads me to a funny story, in order to change the main tank fuel probe you had to remove a hatch muck like the one thats in our cars on the door seal but much bigger, then you had to put on a mask with a long hose that went outside and crawl in the tank. I remember walking up on a plane that a guy was in the tank and the other guy holding the hose under his arm pit then between his legs. I still laugh about that.

P2164 said:

Blatters would certainly be safer I would think

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I don't know if automotive fuel cells are self-healing they military ones are, so I'll add that to my list of complaints: cannot make it home with bullet holes. :laugh:

Anyway, if it's any comfort you can have fuel cells now; just call up Olthoff and be prepared to pay ~$5,000.

P2164 said:

I'm not worried about the fuel thats trapped in the tanks as much as I am getting hit with metal tanks. Blatters would certainly be safer I would think, when I was in the service our planes could get shot and still make it home with just a small leak. Which leads me to a funny story, in order to change the main tank fuel probe you had to remove a hatch muck like the one thats in our cars on the door seal but much bigger, then you had to put on a mask with a long hose that went outside and crawl in the tank. I remember walking up on a plane that a guy was in the tank and the other guy holding the hose under his arm pit then between his legs. I still laugh about that.

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Like Mike Rowe on "Dirty Jobs" crawling into the wing tanks on a KC135 tanker.......in an oval hatch, too small for a normal human, only to scrape sealant out of a small, enclosed, smelly place!

And I know the "but I have thought of a better/tricker way" thing. Many years ago when I was building my SCCA 2.5 Trans Am Pinto, I was forever delaying completion as I had come up with more sano way/part/idea/concept to do the task at hand. Needless to say, the car only raced AFTER I sold it! But it is now a "vintage" racer and I have been contacted to confirm that the car is the one I built in 1971...and 1972.....and 1973...and 1974.......but then I got married and that problem went away!

If I had a spare $5k, I would buy them, Mike Trusty and I witnessed a horrific accident at Morroso raceway in Florida in 1998, a good friend of ours owns a couple GT40s one being an early Safir car the other a CAV. The Safir car came around a flat corner just before the pits and got the left rear wheel in the gravel and then tried to recover, long story short he crossed the track slid on the grass into a concrete flagging podium. The owner of the car was not driving but had a pro driver at the wheel, it hit the driver side (right hand car) and the fuel tank was empty to offset weight of the driver, thank God it was emtpy if it had been full of fuel it would have exploded I am sure. They had to cut part of the car apart to get him out and he was in ICU for sometime then stayed at my home in the Keys for several weeks just so he could ride in a van back to the north east. He had many pins and wires coming out of his body, anyway I think if fuel had been in that tank there would not be a survivor.

Rick did you say there is an age factor with these type fuel blatters?