Anyone converting a car from a carburettor to fuel injection (EFI) might well find the need for an surge tank to prevent fuel starvation when running at a low fuel level or under hard cornering. Stock fuel tanks in older cars don’t have any kind of in built design to prevent this situation other than simple baffles, but how does it occur and what’s so different with fuel injection? The fuel pump in a car equipped with a carburettor pumps the fuel low pressure and at a low volume from the tank to the carb. Should the fuel ever slosh away from the pickup pipe inside the tank, uncovering it and allowing air to be drawn in by the pump, the engine will happily continue running regardless, as a carburettor has its own built-in reservoir of fuel in the float chamber. By contrast, an EFI pump runs at very high pressure and the fuel is circulated to the fuel rail and back very rapidly, so should the pickup become uncovered momentarily, the pump could literally draw in so much air that it would purge all of the fuel from the entire system in seconds. Suddenly the injectors would be getting no fuel at all and the engine could even cut out. This situation is particularly risky with a turbo engine, where a sudden lean mixture condition at speed could result in serious engine damage. So having established that an surge tank is a good idea, how does the system work and how do we go about making and installing one? Read on….
The idea with an surge tank is to provide a permanent reservoir of fuel to feed the EFI pump, which won’t be affected by hard cornering, acceleration or low fuel levels in the main tank. The type of tank required is usually tall and narrow, with the outlet to the EFI pump near the bottom. The surge tank can sometimes be fed from the main tank, just by gravity alone, if it is mounted well below the main tank. More often than not, it’ll be fed by a normal low pressure electric fuel pump, as used on many older cars equipped with a carburettor. The low pressure pump pulls fuel from the main tank and feeds it into the surge tank nearby, via an inlet near the bottom. The top of the surge tank has a return hose to the main tank, which makes sure that there is no air trapped in the surge tank so it will always be full of fuel right to the top. If the fuel in the main tank surges away then the low pressure pump will draw in a tiny bit of air, but it will immediately rise to the top and be purged from the surge tank, thus not affecting the EFI fuel supply. The diagram above illustrates the principle of the system.
You can of course buy a surge tank or have one made for you but a specialist fabricator, but if you have a welder and are a bit DIY minded, they why not make your own? The following method was used to construct a fuel surge tank to use in my 1971 Datsun 510 when I converted to to run a fuel injected engine (a Nissan KA24DE). The main component, the tank, is made from a small disposable CO2 gas welding bottle. These small gas welding bottles can be bought quite cheaply, so even if you don’t use them on your own welder it’s not expensive to buy one purely for this purpose. The first thing to do is make certain it’s empty because you’re going to be cutting it up!! Do so by screwing on a regulator and opening it up to vent the bottle. The valve at the top of the bottle can be unscrewed and removed once it’s empty. You will need to do this to allow for the fitment of a fuel return outlet union in the top.
Once your tank is emptied, grind or sand the paint and any labels from the lower section so you can mark it to cut off the bottom. To mark a line straight around the bottom, try wrapping a sheet of A4 paper around the bottle and using one edge as a guide to mark it. Once you’ve got it marked, cut off the end with either a small angle grinder and cutting disc or if you need some exercise, use a hacksaw. Once cut, use a file to de-burr and flatten the open end. How far from the end you cut depends on how much room you have for the surge tank and the size of the bottle you are using. Make it as tall as you can.
The next job is to drill some holes in the bottle. The exact position of the holes is not critical but they want to be arranged more or less as shown in the photo. The side with two holes is for the high pressure EFI circuit and the single hole is the feed from the main tank (the return is out of the top). Over these holes you will need to weld some nuts into which you can screw the hose unions. The size of these depends on the unions you are using. I used connectors which came from various old carburettors and intake manifolds, all of which had BSP threads. I chose some nuts with a bore smaller than necessary, then drilled and tapped them to suit the unions I had. Chamfer the back of the nuts so you can get good penetration when welding them on, then clamp them in place and weld them using plenty of power. These need to be fuel tight, so it’s essential to get good clean welds. Once the nuts are welded, run a tap through them once more to make certain the threads are clean. It’s also a good idea to check the thread in the top of the tank where the original valve was. If you are lucky, it will be the right size for your union, if not you may have to tap it out. If it’s too large for your fitting, then weld on a nut as with the side unions. It is wise to tap all of the mounts in the surge tank before welding on the base, otherwise you’ll end up with metal filings and swarf inside the tank.
Next stage is to clean out the inside of the tank thoroughly and then a base for it. The base is made from a piece of flat steel plate around 3 or 4mm thick. Make it big enough to accommodate an 8mm bolt in each corner, remembering to allow for the bead of weld that will hold it on. Once cut, drilled and prepared, weld it to the main tank with plenty of power. It helps if you chamfer the bottom edge of the tank a little to get good penetration. Again these welds need to be fuel tight so clean metal and smooth welds are essential.
Now that the tank is made, all that’s needed are some pipe unions. The unions I used, as I mentioned earlier, came from carburettors and intakes. The two larger unions are brake vacuum servo connectors taken from intake manifold from old Nissan and Datsun engines. The small unions are fuel pipe connectors from carburettors from the same engines. These unions have tapered threads so you can wind them in really tight to get a seal. Remember that any jointing compound you use will need to be fuel resistant. I used some PTFE tape. I have had no problems at all with any of them leaking so far.
The last job is to paint it then install the tank in the boot of the car. Use a reinforcement plate or at least some large washers on the mounting bolts under the boot floor. In the 510 I mounted it so that it was above the rear chassis leg, placing the mounting bolts either side of the leg under the floor. Be sure to connect the system with fuel hose suitable for high pressure use. I used a generic low pressure pump purchased from my local motor factor and the high pressure pump is a Bosch unit from a Vauxhall Carlton. The blue cylinder is a special pre-filter for EFI made by Systec, which has a very high flow rate suitable for being placed before the EFI pump. This serves to protect the EFI pump from any tiny particles of dirt or rust from the main tank. Even the tiniest piece of junk will jam this type of EFI pump solid.
If your fuel tank only has one outlet and no return, then the return from the surge tank can be run back into the main tank via to original outlet and a feed for the surge tank can be taken out of the tank drain plug, by replacing the plug with an outlet fitting. If you only have one outlet and no drain plug, then you may have to remove the fuel tank and modify it for a second connector.
This system can be adapted for different vehicles in many ways. For example, I used a similar principle to fuel inject a Nissan Prairie, but on that application the surge tank had to go under the floor. The tank was made much shorter with the mounts placed at the top. It worked as well on the Prairie as the system shown above. The System in my 510 has been run without the slightest hint of fuel starvation on a race track with the main tank nearly down to empty. So there you have it, a DIY surge tank system made for peanuts!