EFI Surge Tanks

The fuel system on the typical classic car consists of an unbaffled fuel tank, some metal fuel line, a mechanical fuel pump, a filter, and the carburetor. The fuel pump moves fuel to the carburetor, which then meters it out into the air being drawn into the engine. The carburetor keeps a quantity of fuel stored inside it in case there is an interruption in the fuel supply. An interruption could be caused by the fuel level in the tank being low and the car going around a turn, when the fuel could slosh away from the pickup tube.

The EFI system, however, can not tolerate any interruption in fuel delivery. The system has to maintain a stable supply of pressurized fuel at the injectors or the engine will not run properly. Factory fuel-injected cars prevent this problem with baffling inside the tank that prevents the fuel from sloshing away from the pickup.

Unfortunately, the fuel tanks in most carbureted cars do not have such baffling. There are a few options for dealing with the problem. Some people make sure they keep the tank full enough so that the pickup will not be uncovered. Another option is to buy or build a custom fuel tank with baffling. A different solution is to use a surge tank. Other names for it are: a header tank, accumulator, swirl pot, and probably a few others.

The surge tank solution is often easier to build and less expensive than a custom main fuel tank. It can be as simple as a piece of pipe with a few fittings on it, or complex enough to contain a high pressure fuel pump.

The surge tank acts like the float bowls of a carburetor. It is a small storage of fuel for the engine to run on when the main tank pickup is uncovered.

Here is a diagram of a system utilizing a surge tank:



There is a low pressure pump that pulls fuel from the main tank and feeds the surge tank. When the surge tank is full, fuel will flow back to the main tank from the top of the surge tank. The high pressure fuel pump pulls fuel from the bottom of the surge tank and feeds the engine. The fuel pressure regulator (usually located on the fuel rail) returns extra fuel from the engine which is also used to supply fuel to the surge tank.

It's important to note that the surge tank can not be under pressure. A pressurized surge tank would cause problems with the fuel pressure regulator by pressurizing the fuel return line.

Here are a few more links with information about surge tanks:

http://www.midnightdsigns.com/james/FuelSystem.htm (cache)
http://bcbroncos.com/store/product_info.php?cPath=131_108&products_id=119 (cache)
http://www.rosehillperformanceparts.com/H%20Tank%20info.htm (cache)
http://www.awildchild.com/accum1.htm (cache)
http://www.geocities.com/hrayhouston/antisurgetank.html (cache)
http://toyotaperformance.com/surge_tank.htm (cache)
http://www.sr510.com/fuel.php (cache)
http://sdsefi.com/techsurge.htm (cache)
http://www.rosehillperformanceparts.com/H%20Tank%20info.htm (cache)
http://www.awildchild.com/accum1.htm (cache)
A Google search will show there are MANY more pages on this topic out there.

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Here is our first surge tank idea:








This is a mockup. We estimate the volume of the tank to be around 60 cubic inches, which is around a liter. The pump and top part are from the donor Mustang. The tank would have two fittings on the side, one for feed from the primary pump and one for return to the tank. The fittings on the top part are for high pressure to the engine and return from the engine.

We decided not to finish this tank due to space issues. There just wasn't a good place to mount it under the back of the car that wasn't too close to an exhaust pipe.

Here was our second idea:



It's about the size of a soda can and would have fit nicely under the back of the car above the e-brake cable. There are two connections at the top; for the lift (low pressure) pump and the return to the tank. Two connections at the bottom, for the feed to the high pressure pump and for the engine return. It cost about $8 to build.

However, with this tank finding a spot for the secondary fuel filter became a problem. There just wasn't a good place for it except along the subframe connector and if the filter was over 2" in diameter then the filter would be the lowest part of the car.

Our third tank uses a spin-on fuel filter as the fuel reservoir. It's big and bulky, but it combines the surge tank and a 10 micron fuel filter into one unit.

There are a couple of options for the filter. A shorter one is available if space is at a premium:





As oriented in the image below, the fuel comes in on the right side from the low pressure pump. The rear fitting on the left side is the feed for the high pressure pump. The tee fitting has the return from the engine coming in on the left and the return to the tank going out the top.



Installed in the Mustang: