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Old 09-12-2011, 02:47 PM   #1
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Default fuel level/consumption

I'm probably the last one to figure this out, but the stock fuel level sensor is pretty useless when you're out on the track. I have it wired into my racepak iq3 and it goes from ~30% of a tank to ~6% around turns and on a second gear pull on the front straight. I set the warning light to be at 10% and it flashes all the time. I saw a similar issue on my 220 whp turbo miata, too, but it was filtered a bit by the stock gauge. On that car it would read 1/4 tank lower until the car was stopped.

Is the right way to accurately estimate available fuel done by sampling injector duty cycle and computing fuel consumed?

Maybe I can do something with the iq3 to try to calculate an average value over a period of time and filter out the dips. Thoughts?
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Old 09-12-2011, 02:49 PM   #2
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The rental has the same issue. My solution was to turn the warning off and just check the gauge before each session while the car is sitting on level ground.

It's a voltage signal so someone might be able to design a circuit to average the last ~10sec of readings or something, though.
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Old 09-12-2011, 03:41 PM   #3
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Originally Posted by orion4096 View Post
Is the right way to accurately estimate available fuel done by sampling injector duty cycle and computing fuel consumed?
Duty cycle would maybe work well enough at high loads after calibration. But for the whole span you probably need a calibration curve (I guess 90% duty does not means exactly 10 times the fuel at 9%).
Add to that fuel pressure dips as high flows (unless you have everything 100% perfect).

Endurance racing is one area where it can be of high importance to know if you have .5 or 1 gallon left. Not to mention predicted duration on the same tank of gas (step back a bit and save one splash&dash).
But I have no solution, just been thinking about it a bit (issue have come up on some other forums a number of times).
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Old 09-16-2011, 10:12 PM   #4
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Endurance racing is one area where it can be of high importance to know if you have .5 or 1 gallon left.
I could never enduo my miata as is... I ALWAYS have issues with picking up vapor in tight left hand turns when I get to half a tank - so says my oem gauge, and filling 5gal to full in paddock. So 10% fuel left will stand me on track after I stall out. I've debated on getting a new fuel cell all together - have any of you had issues like that (not to change the topic)?
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Old 09-16-2011, 10:43 PM   #5
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I've run mine down to the 4% mark on my iq3 (as measured in the paddock), but the level sensor doesn't start changing its output resistance until somewhere between 1-2 gallons so I still had ~2 gallons in the tank. No issues.
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Old 09-17-2011, 02:18 AM   #6
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I've had a student in one of the track rental miatas get a stumble in t1, we babied it the 2.9mi around the track into the paddock where it promptly shut off and would not restart. Sure enough, he put close to 11 gal into the tank.

I have personally run mine down to under 1 gal on track and not had any fuel starvation issues.
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Old 09-17-2011, 04:47 PM   #7
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Then I have problems... I might have heard once that there are baffles inside the tanks - is this true for a '91? Man, if I think I have issues now, wait unitl I'm on E85 at 20psi...
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Old 09-17-2011, 07:01 PM   #8
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My previous FD used to have this kind issue around sever corners where you were pulling high G's. Even with a full tank.
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Old 09-18-2011, 01:03 PM   #9
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Then I have problems... I might have heard once that there are baffles inside the tanks - is this true for a '91? Man, if I think I have issues now, wait unitl I'm on E85 at 20psi...
Yes. We had issues in Savington's black race car in the past and I believe all that we did to fix it was swap the in-tank filter (on the end of the fuel pump) from one that came with a Walbro install kit back to the OEM filter.
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Old 09-18-2011, 02:19 PM   #10
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Hmm I can get hard out-of-fuel cut when I WOT in 1st when I have about 2/3 gallons in the tank. Kind of sucks.
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Old 09-18-2011, 03:39 PM   #11
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Originally Posted by crashnscar View Post
Yes. We had issues in Savington's black race car in the past and I believe all that we did to fix it was swap the in-tank filter (on the end of the fuel pump) from one that came with a Walbro install kit back to the OEM filter.
That sure would be a LOT easier to do that than swap tanks... worth a try anyway. Thanks Nick -
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Old 09-18-2011, 04:32 PM   #12
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One item of note- some of the aftermarket in-tank fuel filters/sock fittings need to be trimmed down or the filter intake makes contact with the filter which = only a small amount of filter surface area is utilized.
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Old 09-18-2011, 05:35 PM   #13
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Time to add a anti reverse container. Good news is its cheap to make, bad news is you need a 2nd pump or a stocker in tank and use your aftermarket inline to boost psi.

Blue comes from the intank pump and its job is to keep the tank over filled. Red goes out to the tank as a air escape valve (small 16th inch hose) Yes some fuel will flow out if the container is full.

green goes to next fuel pump.

On turns the angled plates will block reverse flow so the secondary pump will get uninterrupted fuel supply. The size of the container depends on how long the turns at ur track are and how long the intank pump is not submerged to continue filling the container.

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Old 09-18-2011, 05:41 PM   #14
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Oh my ******* gosh, **** that. Stupid fuel system is going to cost more than my turbo setup.
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Old 09-18-2011, 05:48 PM   #15
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I can make that tank in under 30 bucks worth of parts if i had a welder.
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Old 09-19-2011, 01:05 AM   #16
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Yeah you dont even need the baffles, just use a 12inch length of 3inch stainless exhaust pipe, one feed at the top, one return at the top, one to the secondary fuel pump at the bottom, even at the side.

Dann
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Old 09-19-2011, 01:17 AM   #17
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Great, I am running two pumps in tank. Why not add a third.
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Old 09-19-2011, 01:33 AM   #18
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Why do you need to run something like that? I have no issues running below a 1/4 tank. I do as Sav says, check it before I go out. Anything less than 3/8 tank and I get fuel, since by the end of the session it'll be below 1/4.

Isn't that why he turned his car into a TT? The miata's tank is just not good for enduros when running beyond stock power. It's marginal at best from the factory.
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Old 09-19-2011, 03:30 AM   #19
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Im running an atmo na6 and at a track I drive if im much below half full going off the guage I get a big starvation around a particular left which goes...

Almost out of 4th gear, down to 2nd as you turn in, and then its feather 2nd for 50m as it tightens, and thats where it surges.

Its a fish-hook corner.

Dann
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Old 09-19-2011, 04:24 AM   #20
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Quote:
Originally Posted by nitrodann View Post
Im running an atmo na6 and at a track I drive if im much below half full going off the guage I get a big starvation around a particular left which goes...

Almost out of 4th gear, down to 2nd as you turn in, and then its feather 2nd for 50m as it tightens, and thats where it surges.

Its a fish-hook corner.

Dann
Then your car is broken and you should fix it.

I can run the black car to the point of fuel starve bogging in 5th gear, take it back to the pits, and put 10+ gallons of fuel in it. Spec Miatas can literally run until they sputter, then you'll unhook the fuel line and get OUNCES out of the tank before the pump runs dry on flat ground. Do you guys really think that you're pulling so many more G's than a Spec Miata that you're causing fuel starvation at 1/4 or 1/3 tank?

The suggestion that a Miata needs a surge tank, anti-reversion doohickey, a surge whrrgrbl, or anything other than a completely stock fuel hangar and a properly oriented in-tank fuel sock is absolutely ridiculous. If you've dicked with your fuel setup, and your **** bogs or starves with the needle above the E mark, then you screwed something up.

The orientation of the in-tank fuel sock that clips onto the pump is absolutely critical. When I first did my 255HP install, I used their shitty little flat one, turned it backwards, and had fuel starve with 4+ gallons left in the tank. I turned the pump back around, put the stock sock on it, and I could run until there was <1 gallon in the tank.
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