On the other hand, in a dry sump oiling system, the pan itself contains almost no oil at all. It functions more as a drip pan than a traditional oil pan. As such, a mixture of oil and air is scavenged from the oil pan through rubber or steel braided hoses by an externally mounted pump, which is belt-driven by the crankshaft. The oil is then directed into the top of an external supply tank typically mounted in the engine compartment. As oil scavenged out of the pan trickles down the supply tank, internal baffles separate the air from the oil. The oil pump then draws oil from the outlet at the bottom of the supply tank, and sends pressurized oil back into the block. Unlike in a wet sump oiling system, a dry sump pump is really two pumps in one, as it both sucks oil out of the pan and sends pressurized oil back into the block’s oil galleys.
One of the great advantages of a dry sump system is that it can be set up in multiple stages depending on the requirements of a specific application. A stage refers to how many scavenge and pressure circuits a dry sump pump has, and anywhere from one to six stages is common. For instance, in a typical four-stage dry sump system, there are three scavenge pickup points in the oil pan in addition to the pressure stage on the pump. Additional pickup points allow scavenging oil more evenly off of multiple areas of the motor. In a multi-stage dry sump system, for example, oil can be scavenged from the front and back of the motor, as well as the lifter valley.
Storing oil in a separate auxiliary oil tank instead of the pan yields several obvious advantages. First off, it ensures that the pump will always have a steady supply of oil regardless of the g-forces at work, and the result is rock solid oil pressure. Secondly, a dry sump system’s oil supply tank can hold anywhere between 4 and 20 quarts of oil. The increased capacity cools oil temperatures and further stabilizes pressure, which is usually adjustable from 40 to 60 psi using an external regulator. Furthermore, since dry sump pans do not store any oil, they are very low profile in design. This allows lowering an engine farther down into the chassis for improved handling and braking.
Although dry sump systems were originally designed for improved oil control, they can yield dividends in horsepower as well. In a wet sump system, the crankshaft counterweights and rods must whip through the oil, and the resulting increase in windage and drag reduces horsepower output. Conversely, with no oil to contend with in a dry sump pan, windage is virtually eliminated. Likewise, in addition to scavenging oil out of the pan, a dry sump pump can function as a vacuum pump as well. This ensures that there is less pressure inside the crankcase than above the pistons, which improves ring seal, reduces blow-by, and increases horsepower. Negative crankcase pressure also allows the rotating assembly to accelerate more quickly, as there is less resistance acting upon the pistons as they travel down the cylinder bores. The affect is similar to having a lightweight rotating assembly, and the benefits of increased engine vacuum are more pronounced in long-stroke, high-rpm motors that are more susceptible to parasitic horsepower loss due to windage.