twin fuel pumps in the trunk

[geneulm]

Ok. Everybody has seen the rsr and st pictures w the cool twin fuel pumps in the trunk. Question: how do these things lift the fuel above the tank? Is there another pump in the stock location pushing it up to them?

Thought it would be a nice setup for my twin plug mfi 2.8. But in addition to looking wicked it should actually work! (I'm using the two mercedes type mfi pumps.

Thanks in advance.

[edmayo]

Nope, no third pump, just the two you see. I know, just doesn't seem correct does it!

[geneulm]

It sure doesn't. It just doesn't make sense that it would work?

[Mr9146]

You're piggybacking two very strong pumps. Plenty of suction when they're working together.

[mike curnow]

I wouldn't think that two steel roller pumps would create any suction at all, let alone dry, with a 2 foot head? But I guess they do. I wonder if you are supposed to prime them initally?

[John Z Goriup]

Ok. Everybody has seen the rsr and st pictures w the cool twin fuel pumps in the trunk. Question: how do these things lift the fuel above the tank? Is there another pump in the stock location pushing it up to them?

I can't remember when I first heard that old wife's tale about sucking fuel up to the pumps we've all been listening to for a long time ( I think LBJ might have been President ), and it's being repeated again and again in this thread. With engine shut off and pumps not running, the level of fuel in the fuel line(s), ( from the bottom of the tank to the pump(s) will be exactly the same level as in the tank - i.e. full tank = flooded pumps.

Photo #1 shows a '74 RSR. I even show the tank / pump relationship from two perspectives - showing the layout from above, with the pumps well below the top of the tank.
Photo #2 - same shorthood RSR, almost from the front, where one cannot even see the pumps because they are hidden behind the tank, proving they are below full tank fuel level.
Photo #3 shows a '73 RSR, also with plastic tank and twin fuel pumps - and the pumps are again well below the fuel level in a full tank...as it would be at the beginning of a race.

Photos #4 & 5 ( not taken by me ) - even Flunder's ST recreation with its 100 liter steel tank.........a similar relationship as above is evident: pump fuel intakes at or very near the level of the top of the tank.

Sorry, gents,...... I look, and I look again, but I just don't & can't see that problematic 2' head everyone's talking & worrying about.............. because it isn't there.

Now, as mentioned above, these were / are all race cars ( or very faithful recreations of werks race cars ), therefore the pumps were mounted in a clean, cool, calm and protected location near the tank, one that could be accessed by the mechanics during refueling if they required service and as long as the hood was up, without having to go under the car, and more importantly, because in a race the cars would usually start their races full of fuel ( i.e. the top of the fuel level in the tank was well above the pumps, so they really only pushed the fuel to the rear and didn't have to suck fuel up to the pump level. That only happens when the fuel level in the tank drops below the pump fuel-intake level, but since the motor is running as long as the car is out on track, fuel flow to the pumps is not interrupted and the pump(s) are still only pushing fuel to the engine compartment.

Soooooo,.........if you are going to modify your street driven 911 with its stock tank, whose top is virtually level with the "upper" trunk floor, and which is not always full, you will have created a situation where your new, wicked-looking twin fuel-pump installation may become a liability, because now you have introduced an arrangement where the pumps have to lift a column of fuel upon starting up when the fuel level in the tank is below the pump intakes..

I submit you rethink the pump location to get them as low as possible ( maybe in the "smuggler's bin" ) and approach it from a "package" point of view to ensure that the tank works in concert with your new twin fuel-pump location, and for sure, new, missile grade, fully functional and tested check valves would be mandatory to prevent the dreaded head which will occur frequently on a street driven car with a constantly variable fuel level in the tank. As long as there are no leaks, the pumps are in good condition , correctly plumbed and everything else works nominally, there should be no problem if part of the time the pumps have to "suck" fuel up a few inches from a tank approaching empty.

Best of luck.

[geneulm]

Hmm.

I also have a pair of mfi pumps (which may be better) and a 110ltr tank in the car.

Smugglers bin location and check valves still seem like good ideas to make things last a bit longer

Thanks!

[Longballa]

I had a small 17 gallon fuel cell in my RSR with twin pumps mounted higher than the tank and it always worked fine. It tells me there must be some suction going on.