Fuel Tank Starvation

[Walt Fricke]

Long ago I realized that if I got my stock SC gas tank down below about 1/4 tank on the track, the engine would cut out on long right hand corners of appreciable G forces. Last time out, it appeared that this was happening at about a half tank. No problem at a DE to keep sort of topped up. But at a race, when you are overweight to start with, that's not the way to go if you can help it.

It has always first presented itself on right handers. Maybe it would show up on lefts, but fortunately, at least racing, I haven't had to go that much longer to the checker.

The pickup in these tanks is on the centerline of the car, not offset to one side or the other, so I am baffled by the right hand bias. And a friend, who measures everything just like the pros do, says he hits this point at about the 3 gallon mark. I was more like at five or six gallons, and now 11.

Subsequent to some of this behavior I had the tank out and apart (as much as one can), and found no gunk, no plastic bags, no obstructions on the plastic screen at the pickup point (as I recall, above the large bung on the center bottom of the tank). And the gauge seems accurate enough when I decide to fill to the brim from fuel jugs. If it shows half, I can't get much more than two 5gal jugs in, at 3/4ths I can get 5 gallons in, and so on.

Does this happen on any Porsches other than the early 911s through 3.2?

Short of a fuel cell with trap doors or multiple pickups, has anyone figured out a simple (and especially don't have to drain the cell) method of sucking the cell farther down before centrifugal forces slosh the fuel away from the pickup?

In my fuel celled GT car, I have a pickup in each rear corner, each with its own pump, and the lines T after that. It is so efficient that I get right about zero warning that I have, once again, messed up and run out of fuel by sucking the tank dry of all but the fumes residing in the foam.

[winders]

My Fuel Safe fuel cell has a surge tank in it that seems to work very well. I have run around Sears Point with a gallon in the tank and had no problems with the engine cutting out. I have a single Bosch 044 fuel pump external to the tank. The fuel cell came from the factory with the surge tank installed so I have no idea where it is located. But, based on where the pickup line is, it is probably just right of and just forward of center.

[Walt Fricke]

Usual place for one of these internal surge tanks is rear center, with maybe three of the one way ports - one to each side for catching and holding fuel for lateral Gs, and one facing forward to help fill it on acceleration. Braking doesn't seem to phase any kind of tank, as the car is not apt to need much fuel until acceleration pushes some toward the back at low levels.

Fuel cells are easy - you can open them up and arrange things inside pretty much at will.

My favorite fuel cell story involves an East Coast engine and car builder. Wife's hot rod 911 just wouldn't run right on the track. If the cell was full to overflowing, it would run fine. Below that, fuel starvation. He finally opened the cell up, and found the manufacturer's helpers had laid the pickup hoses on top of the foam. Egad. And not long ago there was a problem of some cell maker or parts seller using a hose material for inside the cell which was incompatible with gasoline, or at least high test gasoline, or maybe affected by the alcohol in pump premium in many sates. Left blue bits clogging fuel pump filter screens.

However, I don't really want to install a fuel cell in this car, so I am wondering what I could do to lessen the problem without exotic measures like cutting the tank open and doing stuff.

Maybe nothing.

[winders]

Install an external surge tank.

[juanbenae]

walt, I did this to a stock SC tank to avoid the cell route. post 155, http://forums.pelicanparts.com/california-southwest-us-az-nv/443725-plunge-8.html

ive done nothing in the way of baffling, but the ability to dip the tank is nice n accurate. just before I pulled the tank my gauge was reading low with the warning light on and I still pumped 7 gallons out prior to removal. after reinstall I ran it till the first sign of starvation, dipped the tank with a paint stir stick and that be came "empty". I then added 5 then 5 more gallons dipping and noting location each time. transferred that to the hood underside in sharpie and carry the paint stick on board. I also feel that this took the weakest link of the stock fueling system, the rubber hose between the tank & the filler neck out of the mix. this may not help on the long righthanders, but it will help to put NO MORE fuel than needed, as called out by the in dash gauge. poor mans cell

[JohnJL]

I blew up a supercharged motor when it detonated itself running lean in the corners.

That $6,000 lesson was learned the first time. I put in an external sump system which I hope guarantees that wont happen again.

[Walt Fricke]

311 - let me see if I figured out what you did. Looks like you welded on a collar to take a cap where the fuel level gauge went. And you just use a stick to see what you have?

That's an accurate method. I do it with my 15 gallon fuel cell. It is rectangular, and 10" high. So each inch is 10% of 15 gallons, or a gallon and a half. A guy gets frazzled at the track, so I made a table so I don't have to figure what 3 or 8 x 1.5 is, though none of that is hard. A friend gave me a square yardstick. I shaved two sides so the gasoline soaks in, making it easy to read from the markings on the other two sides.

I once quizzed a Champ Car mechanic as to why he also used a stick in practice, with all the data and telemetry, and so on. He said it was never wrong, and in practice you aren't in a hurry.

I'm guessing you cut off where the stock filler hose attaches, and have rigged something else up there? Why? Is that where you installed a submersible pump? Whose wires stick out the vent fitting?

I have thought about an external surge tank. The one I have on my track only car with the fuel cell has one - two pumps fill it, and it drains into the high pressure pump. It takes the return from the regulator at the engine, and also connects back to the fuel cell, where any excess gets dumped.

Am I correct that I'd need more than one pump? Because the fuel would have to rise to get into the surge tank, and the high pressure pump would draw from that. Trying to feed the surge with the regular CIS pump wouldn't work - when the pump sucked air, the tank farther down the line wouldn't send fuel at the right pressure to the engine, would it?

Maybe the sequence could be tank, surge tank, pump, line to engine? With the return line going to the top of the surge tank, and another line from the tank top going to the regular return fitting on the tank?

I suppose I could calculate how bit a surge tank would need to be to supply X seconds of gasoline at the flow rates the CIS needs.

[juanbenae]

Originally Posted by Walt Fricke

311 - let me see if I figured out what you did. Looks like you welded on a collar to take a cap where the fuel level gauge went. And you just use a stick to see what you have?


i'm guessing you cut off where the stock filler hose attaches, and have rigged something else up there? Why? Is that where you installed a submersible pump? Whose wires stick out the vent fitting?

yeah, got a weld on filler/cap combo deal that I welded to the gauge sender local. I used sender local as the old filler spout was too close to the fuse panel to introduce fuel there. what you're seeing in the photo is just a piece of steel welded to a plug piece that I welded the original filler neck closed with. the lil tab was a temp measure to hold the plug in place while I tacked it. that was removed after it was in place. the wires in breather hole were just to keep junk from entering the tank during the weld & grinding work. they were laying near by and I just shoved then in there.

much like your yard stick the paint stirrer is absorbent and very good at declaring the fuel level even on the brightest of days.

some notes regarding welding on the tank. I did not fill it with water for the welding. I emptied it of fuel and ran my shop vac discharge side to the filler neck and ran it for hours,,, & hours pushing air through it. when it no longer smelled of gas I took it way out front and "flashed" it with a propane torch to confirm id moved the remaining fumes out of it. no boom, or flash, so a welding I a-went..


t

[JohnJL]

Originally Posted by Walt Fricke

That's an accurate method. I do it with my 15 gallon fuel cell. It is rectangular, and 10" high. So each inch is 10% of 15 gallons, or a gallon and a half. A guy gets frazzled at the track, so I made a table so I don't have to figure what 3 or 8 x 1.5 is, though none of that is hard. A friend gave me a square yardstick. I shaved two sides so the gasoline soaks in, making it easy to read from the markings on the other two sides.



I have thought about an external surge tank. The one I have on my track only car with the fuel cell has one - two pumps fill it, and it drains into the high pressure pump. It takes the return from the regulator at the engine, and also connects back to the fuel cell, where any excess gets dumped.

Am I correct that I'd need more than one pump? Because the fuel would have to rise to get into the surge tank, and the high pressure pump would draw from that. Trying to feed the surge with the regular CIS pump wouldn't work - when the pump sucked air, the tank farther down the line wouldn't send fuel at the right pressure to the engine, would it?

Maybe the sequence could be tank, surge tank, pump, line to engine? With the return line going to the top of the surge tank, and another line from the tank top going to the regular return fitting on the tank?

I suppose I could calculate how bit a surge tank would need to be to supply X seconds of gasoline at the flow rates the CIS needs.

I am not an authority on external surge systems, but this is what I did;

fuel cell bottom fitting - filter - A1000 fuel pump - bottom of surge tank - bottom of surge tank to 2nd a1000 fuel pump - fuel filter - injector rails - regulator - surge tank - top of surge tank overflow to fuel cell.

[Dave at Pelican Parts]

The CIS does not have any special ramifications as far as how much fuel the car uses. The pump delivers more pressure and volume than a carbureted motor can use, true, but most of that goes back into the tank. The engine only uses what the engine uses, which is going to be pretty close to the same amount for similar motors even when running different fuel systems.

...How much that is, though, I don't know...

--DD

[Walt Fricke]

The 2.7 CIS Bosch troubleshooting manual says you divert the return line from the fuel distributor into a measuring glass. The spec is 750 cubic centimeters in 30 seconds. That would be 0.4 gallons or 1.5 liters per minute. This is after a 5 bar spring has been forced open enough so that fuel is returned, so maybe this is not the free rate often used in advertising pump characteristics. Don't know (references are in trailer) what this is for the 3.0.

[J Richard]

I chased a fuel starvation issue in a stock 944 tank for a year and a half, half tank or better sustained right hand turns, engine cut out. replacing pump, lines, vent lines, filter, etc..nothing. The in tank screen was missing, I thought it might be the pump getting debris in it, even though I took the tank out twice. turns out putting the screen back in solved the problem. You know how when you drain your bathtub the vortex at the drain can draw air all they way from the surface through 8" of water? well the fuel pump was doing the same thing. with half a tank it only had to suck through 3" of fuel, the path of least resistance is to the surface of the tank to suck air. The screen was enough to diffuse the suction to keep it from drawing through the surface. I confirmed this by putting the fuel inlet into a can of fuel and running the pump. I think this is actually a commonly overlooked problem...

[Walt Fricke]

Interesting observation - opens up possibilities, like just putting a flat plate over the top of the screen?

[Walt Fricke]

Well, I drained the tank, pulled the fuel strainer from the bottom, pulled the level sensor tube out, and looked around, including using my borescope. Then I did what I should have - did a search of the Pelican fora. It seems that all the cars up to the 964s have pretty much the same tank setup (maybe the early 2.0s are different). There are differences in the upper part of the tank (full size spare vs inflatable). And the steel crossmember cars had the fuel lines coming out at the center rear of the tank, while the later aluminum crossmember cars have the tank outlet on the left side, and the return line on the right.

But they all have a swirl chamber inside in the center toward the rear. The strainer goes up inside it and seals off the top of the chamber. Gasoline gets into this via the return line, and via an opening under where the return line enters. This then travels in a circular pattern until it empties into the middle, where it goes through the strainer, drops down, and goes through slots into the line to the fuel pump.

Here is a picture scarfed from Pelican:


This is an earlier tank with the lines in the middle.

Here is the strainer, which also closes the hole in the top of the swirl pot.



The swirl pot is orange, and looks a lot like a distributor cap - it is often referred to as being Bakelite, which I think is also what the dizzy cap is. The swirl pot has a clip-on top cover, and you can see where the strainer's top can fill the hole in this cover.




Here is where the return line, and the fuel from the tank enter the swirl pot:



Someone - maybe Grady, who contributed to several of these past discussions - said the swirl chamber was intended to de-aerate the fuel. I guess the return fuel might have entrained air, but my race car fuel cell, which has a return line, just dumps that into an external surge tank, and from there back into the top of the tank. Perhaps I don't know what the symptoms of excessively aerated gasoline are, but my race motors have worked fine as far as fuel delivery goes. But Porsche must have had some good engineering ideas behind this rather complicated (for what it does) system.

What did I conclude from this exploration/education? Well, I am dubious about the vortex theory, though I suppose it might hold water - er, gasoline. Wouldn't be easy, short of cutting open the tank (and one post showed a guy making a nice large access port on top of his tank - cool thing to have), to get in there to fix some kind of lid over the area where fuel enters the swirl system.

And I hit on a rather more prosaic cause for my car cutting out on track corners with an indicated half tank of fuel. The gauge was wrong. I don't think it necessarily is always wrong, but I think racing may cause its internal up and down part to hang up some and not go down as it should. In any event, the half tank showing when I finally sputtered in to finish the race was not a half tank - I pumped about 2.75 gallons out before futzing around. Stuck the pump down through the sender hole and cranked backward into a jug.

What to do about this? I took a -6 bulkhead fitting, drilled a suitable hole in what I hoped was a suitable place, installed it, and bought a 1/4" dowel. Voila. Next comes putting fuel back in a gallon at a time and marking the stick.




If I can discipline myself to check fuel after each run, write things down, estimate fuel useage, and always add enough to have five gallons in the tank when I am done, I should be able to avoid the indignity of the engine cutting out on corners.

A second pump and an external surge tank would be better (so I could run a lighter fuel load, as my car is somewhat over the minimum weight as it is), but more complicated. And a real racer would know his fuel consumption for each track, wouldn't he?

[wildcat077]

It's always important to know how much fuel you have at the end of a race ... especially a 90 minute Enduro when the ever vigilant PCA scrutineer gets you disqualified because you are 12 lbs underweight on the scales after 90 minutes ... right Walter

Had to throw that in ... lol

Cheers
Phil

[Walt Fricke]

Well, I wish I was at the point where, with an empty tank (though with an SC you won't finish on that) you just make weight. I'm about 80 lbs away from that. I'm reluctant to remove the stock bumpers (because of the protection they provide), or to bulge out my door bars and remove the window glass and mechanism. So I have to concentrate on me.