Reply to "WET SUMP VS DRY SUMP - OIL"

Hi Jack

Welcome aboard.

There are two major divisions in engine lubrication systems: wet and dry sump. "Sump" simply means the chamber in the bottom of any engine that collects the motor oil for recirculation. In a typical automobile engine the oil pan is the sump.

Wet sump lubrication means the oil pan serves double duty; it collects motor oil and serves as an oil storage location too. Since the oil pan "stores" the oil, it will be full of oil (i.e. WET) when the motor is runing. The oil pump "sucks" the oil from the oil pan and pumps it back through the engine.

With a dry-sump lubrication system the oil pan is single duty, it is only used to collect the oil. Oil collected in the oil pan is removed from the pan by a scavenging pump which pumps the oil into an external storage tank. Because the oil pan is not used to store oil, it is referred to as "DRY". The scavenging pump can have one or several stages, depending upon how many places it is connected to the oil pan. A secondary pump, called a pressurizing pump, "sucks" the oil from the external storage tank and pumps it back through the engine.

A wet sump lubrication system is normally entirely internal to the engine. The advantages are low cost, ease of assembly (important for mass produced automobiles), compactness and less weight than a dry sump system.

A dry-sump system is conversely more complicated, more expensive, heavier, and takes up more room in an engine compartment. There are multiple stages to the oil pump, or possibly two pumps, and an external storage tank. Then there's the external plumbing between the pumps, the oil pan, the storage tank and back to the engine. There are more chances for failure of the system, and more places for leaks to occur.

However the expense and complexity of a dry sump system are off-set by its advantages.

As a seat of the pants approximation, anything above about 1 G Force will cause the oil in a wet sump oil pan to climb the sides of the pan. When this happens, the oil pump pick-up located in the bottom of the oil pan is uncovered and sucks air. The motor will lose oil pressure and damage occurs. The external storage tank of a dry sump system is tall and narrow, the tank's oil pressurizing pump connection is not uncovered by high G Forces. So a dry sump system becomes a necessity when a vehicle encounters high G-Forces.

Most street cars cannot corner at 1 G because most street tires cannot grip the road at such a high G force, and most street car chassis are too high above the road to ever negotiate a corner at 1 G no matter how good the tires are. So the complexity and expense of a dry sump system is not needed for street cars. But on a race car that is lowered to the ground and equipped with race tires, the need for a dry sump system will be dependent upon the types of tracks and the speeds encountered. For instance, Patrick mentioned that a dry sump system is a necessity on the big endurance racing circuits in Europe, with their long high speed corners. But sportsman dirt track racers usually don't need a dry sump system because the turns are too tight, the corner speeds are too slow.

Besides that necessity, a dry sump lubrication system has six other advantages over a wet sump system. (1) Because there is only a minimum of oil in the pan, power robbing windage (oil clinging to or splashing against the rotating assemblies of the engine) is greatly reduced. (2) The crankcase can be operated under a vacuum that further increases horsepower by improving ring seal. (3) Increased oil capacity because of the external storage tank. (4) The external plumbing makes it easier to add remote oil coolers. Oil cooling is a necessity in racing. (5) Since the oil pan doesn’t store oil, it can be quite shallow to allow for lower engine placement in the chassis, this lowers the cars center of gravity and improves the handling of the chassis. (6) The oil can be collected under a vacuum which removes the air from the oil. As the oil drains into the oil pan it encounters moving parts which adds air to the droplets of oil and whips the oil into a froth. Oil full of air cannot lubricate very well. Eliminating aeration of the oil in this way is very important to the life of a high rpm race motor.


-G