Finally, we hit the raised in-tank sumps. They can be very simple, but are usually applied to new tanks where the fabrication is part of the tank manufacturing process.
Raised sumps. These are internal sumps that rise above the bottom of the tank. Again, they are designed to crate a pocket of fuel for the pump (either internal or external) to draw fuel from. The first is the typical raised sump with sloped sides to allow sloshing fuel to ride over the edge, yet be retained at the other side. This variety requires movement and sloshing to pump most of the fuel from the tank.
Down-sides are difficulty to add to a classic tank, and while it can get most of the fuel, it can leave a gallon or more unused before the pickup is un-covered. This is especially true if there is not enough movement and sloshing to maintain fuel to the pickup.
Labyrinth sump. At the bottom-center of this set of graphics is the labyrinth sump. The labyrinth has the advantage that fuel naturally seeking level height will find it's way into the sump area without any need for sloshing. This is especially useful for smooth cruising or extended idle, where other sumps can run dry with a couple gallons still in the tank. When cornering or accelerating, the fuel is relatively trapped in the maze.
The benefits to the labyrinth and similar types is the ability to get more fuel from the tank with it's self-filling design. Like most systems, it has drawbacks, and the primary one in this design is the difficulty installing into exiting tanks.
Raised box sumps. The lasts example is the raised box sump. Note the example has trap doors that act as one-way fuel gates. When fuel sloshes it will push past the trap door and into the sump, where it is trapped inside the one-way doors. While a creative solution that can be a bit easier to install; the box sump must be very well made for the trap doors to seal reasonably well. Even then, sediments, gums and varnishes may render the doors inoperative or poorly sealing, making it effectively useless.
Worse yet are copy-cat box sumps that look good, but have no trap doors, gates, reed valves or other means of filling. While the simplest and cheapest of this variety, they cannot fill except by sloshing, and with steep sides are prone to leave several gallons in the tank. This make them like a module or surge tank with no means to fill when fuel is low. Just as bad are ones that have holes or slots for filling, but the fuel can run out as easily as it gets in, again making it effectively useless.
David
Raised sumps keep a clean tank bottom, but (depending on type) are generally difficult to install or expensive to fabricate for reliable operation. Beware of fakes:
