The Haynes book suggests a swirl pot on the bottom of the fuel tank if using fuel injection. I can see how it's constructed, but I can't judge how big it needs to be. I was thinking about 4 inches in diameter, about 2 inches deep? Is that enough or does it need to be bigger?

Alternatively, considering most standard tin tops don't really have a similar thing attached to the bottom of their tank, is it something I need to bother with?

I think you have to factor for how long you are going to be running (or think) full throttle with cornering/accelerating conditions that may cause the pump to run dry plus a small reserve.

Modern cars seem to use an intank pump that has an integral swirl pot. I use a VW pump in my tank, and the whole thing fills itself with fuel almost instantly, even if the tank is almost dry (tested on the bench in an inch of fuel. And I can't work out how it does it....)

You really do not want an injection system running dry. Unlike carbs, if no fuel then no pressure then no injection, so instant power cut when corneringis a real possibility.

I would suggest deeper than 2", but just a gut feeling. And even then I would want the fuel return feeding into the swirl pot. Injection pumps can shift fuel very fast, but most is returned to the tank.

Alternatively fit a pump like my VW one with a built in swirl pot...

Its always a good idea to make a swirl pot deeper than it is wide, i.e maybe 3" wide but 6" deep, idea being, under cornering the fuel will go up the wall of the swirl pot, the taller and narrower the pot is, the less chance you have of starving the fuel supply to the injection pump, the fuel injection pump supply should be at the lowest point of the pot, in the center ideally, also, put the return pipe from the injectors into the top of the swirl pot and an overflow/ return from the pot to the tank.

Cliftyhanger is right - all modern cars have a 'swirl-pot' but it's integrated into the fuel pump assembly that sits inside the fuel tank.

They look like this....

I would make a couple additions to clive7883 excellent design suggestions for the swirl spot. You should consider adding a drain plug and two or three hinged gates to the walls of the swirl pot sides, during the fabrication process. davew823

i have a LP pump that feeds an external swirl pot and then a HP pump to take it to the fuel rail from there.
Works well for my car!

Thanks for the input everyone. It hadn't occurred to me that it's not just the amount of fuel that will be burnt, it's the fuel that will flow round the system.

What I have at the moment is basically a completely enclosed tank. What I'm planning to do is cut a hole about an inch smaller diameter than the swirl pot, the idea being that there is a lip that should slow down the sloshing. Taller rather than wider sounds like a plan, I'll have to see how much space underneath I can use. The standard fuel tank placement on a Haynes doesn't seem to give much clearance underneath (the swirl pot will stick out from the bottom of the tank rather than being inside it).

The MX5 donor fuel pump is just a couple of pipes, a pump and a plastic filter, so nothing coming from there.