Show me your fuel cell

[JohnJL]

Scott, thanks for the surge tank pointer, I was shocked though at a $300+ price tag for those little rubber boxes and fittings!

My rectangular cell has the fittings facing forward, with an aeromotive pump mounted underneath. IT's always the same corner when I find out it's time to come in for fuel...hairpin left-hander, roll on the gas and it sputters...

[JohnJL]

I saw a few references to external surge tanks, would anyone care to elaborate?

[JP911]

A separate tank, approximately a gallon (or less) is fed from the main tank/cell by a separate pump, and then the main pump is feeding the engine from the smaller tank. The small tank is sort of like a reserve supply that is constantly being replenished. Search for "fuel swirl pot" using google and you'll see some diagrams and approaches.

[JP911]

Here's a visual reference:

[Mahler9th]

Here is a picture of my installation as it existed until 2011:





Hiding behind the carbon fiber "covers" are the dual fuel pumps... one for low pressure to fill the surge, and the other for high pressure to drive the EFI.

Here is a picture of the pumps pre-CF and external surge tank:



More info to follow.

[Mahler9th]

The surge tank was made by Mat or one of his contacts. I have seen this type of surge tank in many of his customers' cars. And I think he runs one.

[Mahler9th]

Here is a description of my system that I wrote some time ago for one of our PRC racers:

Originally the second fuel pump was in the back of the car in the stock location, and fuel traveled back and forth in the stock lines. I decided to move pump #2 to the front, and replace the stock lines with -6 stainless braided lines. Also to add a fuel pressure sender and a pressure regulator, and convert to a stock 993 filter set-up.

The braided lines run through the heater ducts on the driver's side.

There are two fuel pumps. Both are mounted in the front of the car. Custom plastic bracket. You can also buy dual fuel pump brackets. I had some CF tubing laying around, so I covered the pumps with that. Form and not function.

There is also a surge tank in the front of the car. And a 914-style FP regulator in the return line.

At the rear of the car, there is a sender block and pressure sender, and a filter (993 style).

All of the fittings /hose ends are Aeroquip or equivalent AN style. The metric to AN adapters used and the sender block are mostly Earl's or Aeroquip, with the exception of a few like the hose end on the return line which connects to the fuel rail, which came from JWE or BAT.

The basic routing is out of the cell, to the first pump, to the top of the surge tank. Then from the bottom of the surge tank, to the second fuel pump. Then to the back of the car.

At the back of the car, the fuel first goes through the sender block, then through the filter, and on to the fuel rail.

The return goes from the fuel rail, back to the top of the surge tank. At the very top of the surge, a line carries returned fuel through a 914-style FP regulator, then back to the cell.

Plus there is a vent line from the top of the cell that arcs up and goes into a plastic "catch box" (which is a part from some Porsche or another) then vents to the ground underneath the car. This idea comes from Mat.

[Mahler9th]

Since those pics, I moved pump #2 into pump #1 position (low pressure), and then installed a new Bosch 044 as pump #2. So modest changes in lines and fittings were required.

My HO 3.8 liter engine likely has different fuel requirements from your 3.2.

[Mahler9th]

When I helped install a nearly identical cell in one of our racer's cars a few years ago ( a 69 911), we used alu angle to make a bed. We had to notch out a rectangular opening in the sheet metal, so we used a sawzall and grinder. Then threaded steel rods and aluminum tubing to held down the cell. Completely a bolt in deal... no welding. Was not very difficult.

[Mahler9th]

If you look carefully you can also see the Harley battery in the smuggler's box next to the surge.

[JP911]

Mike-
I looked through the NASA CCRs and couldn't find any mention of surge tank/swirl pot rules. Since the it isn't a stock tank or FIA fuel cell I was worried that it wouldn't pass inspection. Ever get any flack during an annual inspection?

[Mahler9th]

You are correct. The NASA CCR is silent regarding external surge tanks.

When I race under NASA sanction in the Porsche Racing Challenge Series with the Porsche Racing Club, special rules apply. The Series rules can be downloaded on the PRC or NASA websites. The Series' rules work in conjunction with the CCR and the PCA Club Racing rules. The downloadable Series rules document highlights key differences from the CCR, just like several other NASA series. In this case, the Series rules also refer to the PCA rules in a number of key areas.

In the case of fuel cells the Series rules allow either the CCR OR the PCA rules to be in effect. Both of those rule sets are silent about surge tanks. In addition, the PCA rules, unlike the CCR, do not require mandatory changes of the cell bladder.

My cell and the way it is maintained meet the PCA rules.

In any case, I suspect that tech inspectors would not have an issue with a small external surge tank. Think of it as part of the fuel lines as opposed to a repository where a lot of fuel is stored.

[Mahler9th]

I always encourage Series participants to be thoroughly familiar with the rules and to have their annual tech inspections done at shops that also know the Series rules as well as the CCR and PCA rules to which they refer.

[Mahler9th]

A couple of additional considerations that may have not been discussed.

1. As you can see in the pictures, my Fuel Safe cell has an alu outer case. Weight matters in my application, though it is hard to say how much. In a different application, like Spec 911, it may matter less. So a somewhat less expensive steel outer case may be less of an issue. I do not know the weight difference, but I am sure that Fuel Safe knows. And experienced folks like Mat may have an opinion. The PRC number 24 car converted to an alu cell case only about 5 or so years ago.

2. If a person did not want to change lines and all of that, there would be a minimum number of new fittings/connections to deal with the cell outlet and return. These might be just rubber hoses and hose clamps. I cannot remember what was in my car when I bought it, but it was set up this way. I ran the car with that set up for a few seasons before I changed out the factory lines and moved pump #2, adding the 993 filter, fuel pressure sender and so on. It was fine the way it was. But I needed a winter project and I did not want to build a ship in a bottle.

[JP911]

Thanks for the thoughtful response, Mike. I have a similar setup that I installed this past winter to deal with my thirsty fuel injected engine. I was getting starvation in high-G turns (exit of the carousel at SP, T2 at Thill) with 1/2 tank of fuel (stock tank). The surge tank setup has cured it.

[petevb]

Originally Posted by winders

For a race car, I see no advantage to the Porsche-specific fitted fuel cell and a clear financial disadvantage.

I don't agree with the first part of that. And I do think both Dave and Mike are making perfectly valid points.

You can take one look at the two options, fit vs generic, and calculate that for a given amount of fuel the fit tank solution has both a lower CG and a lower polar moment. Both of those are good things. Without measuring you could eyeball maybe a 1-2" difference in the height of the tank CG (both because it sits lower, some might argue too low, and because it's wider an squatter at the base rather than taller and narrower (relatively), so what fuel there is will average a lower height in the tank.

Now if you fill it up, moving ~5% of the car's weight up by an inch or two will raise the car's CG, no doubt about that. Of course from behind the wheel, will you even be able to tell that the CG is ~.1" higher?

He're where trade-offs need to be made, and I personally like to use numbers to guide these. In saving weight, for example, you can easily set a $100 per pound cutoff. Mike's titanium belt hardware might need some very deep pockets and a $500 per pound cutoff to "make sense", but of course on many pro cars built to even higher standards that's "cheap" weight.

Setting a similar cutoff for CG height, polar moment, etc gets trickier, but it can be done. Aesthetics are harder to put a price on...

In any case, those that are arguing it's worthwhile can be just as right as those arguing it's not depending on what metrics they are using.

What I don't agree with, however, is the idea that it's not worth anything. Even if you can't feel the change, the difference between a street car and a professional race car like an RSR (or an F1 car, for that matter) is a series of small decisions, any one of which by itself doesn't amount to much but the sum of which make all the difference. How far you want to take it is up to you...

[winders]

Peter,

First of all, we are not talking about a professional race car or race team here. Club racers don't typically have the kind of money a professional race team has budgeted for their cars.

Let's look at the weight numbers more practically. You basically have around 3 gallons (~18 lbs) of fuel capacity about 2" lower. Let's make it an even 20 lbs. If you were to fill up both fuel cells to 15 gallons, it would mean that 20lbs is sitting 9" higher in the non-fitted fuel cell. How much difference does moving that 20 lbs down 9" actually make in cg or real life?

If you only put 7.5 gallons in the fuel cells, that 20 lbs is sitting only 4.5" higher. Now much difference does it make? Would anyone here notice if I bolted 20 lbs of lead any place I chose into their 2350 lbs race car?? Anyone? Would it make a difference in your lap times? Now think about this: if I moved an existing 20 lbs down 9", would you notice that in any way, shape, or form?

As fuel is used the impact becomes less and less.

This isn't 20 lbs we get to remove. It's 20 lbs we get move down a whole 9". For a normal sprint race, that is going to be more like 6" since you would start with about 10 gallons of fuel. Maybe less. And, during the race, we don't even get to keep that whole advantage as fuel is burned off. If the race cars burns off 4" of fuel during the race, the average is what, 4"? 20 lbs moved down 4"? Big whoop.

If there were a competitive advantage to using the fitted cell, I am confident that the folks at JWE would be using the fitted cell. The budgets for those cars certainly allow for the fitted cells.

Let me modify my statement:

For a race car, I see no practical advantage to the Porsche-specific fitted fuel cell and a clear financial disadvantage.

Again if you have thousands of extra dollars to burn, by all means get the fitted cell.

Scott

[petevb]

Originally Posted by winders

First of all, we are not talking about a professional race car or race team here. Club racers don't typically have the kind of money a professional race team has budgeted for their cars.

This is a bang for the buck argument, which I made clear I think is totally valid. However absolute performance is down to the laws of physics, and they are the same for club racers and pros alike.

Originally Posted by winders

Would anyone here notice if I bolted 20 lbs of lead any place I chose into their 2350 lbs race car?? Anyone?

You might not notice. The stopwatch would.
BTW, Porsche used 24 lb weights in the front bumper of the early 911 to cure handling issues, and I can certainly notice when I'm running even 3 gallons light in fuel at the autocross. So yea, "someone" could notice.
Again, making a car faster is the sum of many, many small decisions. Change any one part or fastener to titanium and you'll never notice. Why even bother? But change them all and suddenly you've droped 400 pounds and can run away and hide at the track.

Originally Posted by winders

If there were a competitive advantage to using the fitted cell, I am confident that the folks at JWE would be using the fitted cell.

Everyone has a budget. They likely feel that money is better spent elsewhere, which again is a perfectly valid argument. That's a different question, however.

[Mahler9th]

You are welcome John, and I hope to see you at one of our upcoming events at Sears Point. The extra detail is meant for posterity, not for an experienced PRC racer like you!

Someday there might be an interesting conversation between a racer with a small external surge tank and a club racing steward (NASA, SCCA or PCA for example). My claim is that the surge is part of the lines....

Lots of interesting conversations happen all the time. I once had a steward tell me to turn one of the 12 screws holding my polycarb rear window in place 1/2 a turn lest the window fly off. Of course this pales in comparison to the crazy stuff they see and stop before it can become an issue on track. Thank heavens for the stewards!

As to the cell "type" discussion, I don't think the rectangular cell has to sit two inches higher than the shaped cell. Mine sits much lower than that. I am sure one could calculate the CG difference with 5 gallons of fuel but why bother? The other inherent tradeoffs of my old 911 set up make the issue trivial.

Of course, as has been mentioned, the real reasons to get the shaped cells are:

1. Save on fab cost and time.
2. Have what Fuel Safe (and maybe ATL) may claim is a superior bladder (material/design/construction), as compared to, for example, the bladder in my less-expensive cell.
3. Have 17 gallon capacity, whereas I only have 15.

With many of these things, we struggle to understand what makes a difference, but often we do not define what we mean by difference. At the extreme ends are practical difference and difference in race outcome. At the other extreme is theortical physics. I tend to worry about the former since I drive a really old car.

I can assure you that the type of cell someone runs will not make a difference in the outcome of a PRC race.

I have seen folks debating $10k shocks and all sorts of other things that are better in theory. I have friends that spend a lot of time thinking about these things. And they have not yet raced.

I have seen a lowly (and low tech) air-cooled 911 with a 100 bhp/liter engine and plastic bushings, wooden floorboards, carpeting, turn signals and other "no-nos" routinely beat modern factory GT3 Cups and R/ RS/RSR cars with good drivers.

By the way, my car has plastic and rubber bushings. There is not 1/10 of a second a lap there.

I am sure the OP has much food for thought. It would be nice if there were a few more pictures, so perhaps when I take out my cell to clean out all of the rubber debris around it I will snap a few more and post them.

[Mahler9th]

One more by the way deal...

With a set up like mine or John's, it is important to make sure that the second pump (say a Bosch 044) does not overwhelm the ability of the first (low pressure) pump to keep pace. One result of screwing this up could be an inconsistent stream of fuel to the injectors.

Ask me how I know.